Metabolism of amino acids - Flashcards

Dietary aspects

• Focus: overview of amino acid metabolism starting from dietary protein to downstream fates of N- and C-atoms, and the vitamins involved in amino acid metabolism.
• Major flow: Dietary protein → Digestion → Absorption → Circulating amino acids → Proteins and small nitrogenous compounds → Carbon skeletons → Excretory products (urea, uric acid, creatinine, etc.) → Glucose or energy via carbon skeletons; nitrogen disposal via urea and other excretory products.
• Key categories related to nitrogen and carbon from amino acids:
  • Nitrogen-containing products: urea, ammonia, purines, pyrimidines, haem, creatinine, uric acid, free amino acids.
  • Carbon skeleton fate: glucogenic or ketogenic intermediates feeding into glucose, glycogen, fatty acids, ketone bodies, or TCA cycle intermediates.
• Kwashiorkor (illustrative nutritional disorder):
  • Clinical features: ↓ body weight, oedema (due to hypoalbuminemia), diarrhoea (intestinal epithelium and pancreatic enzymes reduced → steatorrhoea), skin atrophy.
• Essential vs nonessential amino acids:
  • Essential amino acids: His, Lys, Trp, Phe, Leu, Ile, Val, Met, Thr.
  • Semiessential amino acids: Arg, Tyr, Cys (need to be considered for special states).
  • General protein adequacy: ideal protein ~0.8 g per kg body weight; higher needs in children, pregnancy/lactation, postoperative state (2–3×).
  • Complete proteins: a set of amino acids that collectively meet essential amino acid requirements (listed here as “Complete proteins: 6” in slides).
• Nitrogen balance concept:
  • Involves the amino acid pool, protein synthesis and degradation, and nitrogen excretion (urea, ammonia, uric acid, creatinine, free amino acids in urine).
  • Typical daily intake vs excretion balance (illustrative figures shown on slides): approximately 16% nitrogen in dietary protein; excretory routes include urine, feces, etc.
  • The idea: nitrogen consumed per day equals nitrogen excreted per day; a positive balance supports growth and tissue accretion, a negative balance occurs in catabolic states.
• Special aspects of vegetarian diet (nutritional challenges):
  • Energy content: 30–50 kcal/100 g (vs 150–300 kcal/100 g in mixed diets).
  • Protein content: 1–2 g/100 g (vs 15–20 g/100 g in mixed diets).
  • Protein quality and digestibility: generally lower.
  • Limiting amino acids: legumes often low in methionine; cereals often low in lysine; digestion/absorption patterns can affect adequacy in adults, pregnancy, and childhood (birth weight, growth).
• Nature article note (contextual): meat consumption historically supported brain growth; in modern diets, meat may be less necessary if energy and nitrogen balance are met through varied sources.
• Takeaway on vegetarian diets: careful planning required to ensure adequate methionine, lysine, and overall essential amino acid supply, especially for vulnerable groups; consider complementary proteins to improve amino acid balance.

Protein digestion and absorption

• Digestive overview: dietary proteins are digested to liberate amino acids and small peptides, absorbed by enterocytes, and released to the bloodstream for tissue uptake.
• Digestive pathway (simplified): Dietary proteins → Peptide hydrolysis in stomach and small intestine → Absorption of free amino acids and small peptides → Circulation of amino acids → Cellular uptake for protein synthesis or energy via nitrogen and carbon skeleton disposal.
• Peptide bond hydrolysis (general):
  • Peptide bonds are hydrolyzed by proteases to form shorter peptides and amino acids, enabling intestinal absorption.
• Digestive enzymes and active sites:
  • Serine proteases (trypsin, chymotrypsin, elastase): active site composed of Ser, His, Asp.
  • Metalloproteases: require a metal ion (e.g., Zn2+); example: metalloprotease family includes enzymes like carboxypeptidase A.
  • Carboxyl proteases (carboxypeptidase A): exopeptidases that cleave at the carboxyl terminus.
  • Pepsin (an aspartic protease) acts in the stomach; optimum activity at very low pH (acidic environment).
• Stomach acid and protein denaturation:
  • Parietal cells secrete HCl (pH ~1–2.5) to denature proteins, assist digestion, antimicrobial action, and intrinsic factor production for vitamin B12 absorption.
• Pepsin activation and specificity:
  • Pepsinogen (zymogen) is activated by low pH; active pepsin cleaves peptide bonds adjacent to aromatic amino acids.
  • Activation involves a conformational change and removal of an activation peptide.
• Duodenum and pancreas in digestion:
  • Pancreas secretes bicarbonate (HCO3-) to neutralize chyme (pH ~7–8) and zymogens:
  • Trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase A.
  • Activation cascade:
  • Trypsinogen is activated to trypsin by enteropeptidase (aka enterokinase) in the duodenum.
  • Trypsin then activates other pancreatic zymogens (and itself from trypsinogen).
  • Enzymatic groups and substrate specificity in the small intestine:
  • Trypsin: endopeptidase with preference for Lys/Arg.
  • Chymotrypsin: endopeptidase with aromatic amino acids preference.
  • Elastase: endopeptidase with small neutral amino acids (Gly, Ala, Ile, Ser).
  • Carboxypeptidase A: exopeptidase at the C-terminus.
• Absorption mechanisms:
  • Free amino acids are absorbed into enterocytes from the lumen via various Na+-dependent and Na+-independent transporters.
  • Dipeptides and tripeptides can be absorbed and subsequently hydrolyzed to amino acids inside enterocytes.
  • Basolateral export into blood via Na+/K+ ATPase maintains electrochemical gradients for amino acid transport.
  • Transport into tissues relies on distinct transporter patterns (e.g., some amino acids are preferentially taken up by liver, others by muscle/brain).
• Enterocyte metabolism:
  • Glutamate can be oxidatively deaminated by glutamate dehydrogenase (GDH), generating α-ketoglutarate and NH4+; NAD(P)+ is reduced to NAD(P)H in the process.
  • Glutamate dehydrogenase reaction is a key link between amino acid catabolism and carbon skeleton entry into the TCA cycle.
• Chinese restaurant syndrome (educational note): oxidative deamination of glutamate can produce symptoms such as headache, sweating, and nausea in some contexts; this is an illustrative caution rather than a universal effect.

Fate of the nitrogen atoms (N-atom) from amino acids

• Absorptive vs fasting state (key dichotomy):
  • Absorptive state: amino acids are used for protein synthesis; carbon skeletons directed toward fatty acids, triglycerides, glycogen, or glucose; nitrogen disposed via urea cycle and other nitrogenous waste.
  • Fasting state: amino acid carbon skeletons used for energy production, gluconeogenesis, or conversion to glucose; nitrogen disposed via urea cycle and related pathways.
  • Core motifs: C skeletons form glucose, glycogen, fatty acids, or ketone bodies; nitrogen is disposed as urea, uric acid, creatinine, or free amino acids.
• Direct deamination vs transdeamination vs indirect deamination:
  • Direct deamination: amino acid loses amino group directly to form a corresponding α-keto acid and NH4+ (ammonium).
  • Transdeamination: sequential process where amino group is transferred to another acceptor (often glutamate via transaminases) and NH4+ is released subsequently via deamination.
  • Indirect deamination involves the urea cycle and/or nucleotide cycles (purine nucleotide cycle) contributing to nitrogen disposal.
• Direct deamination details (examples):
  • Glutamate dehydrogenase (GDH) reaction:
    $ext{Glutamate} + ext{NAD(P)}^+ <br /> ightarrow ext{α-ketoglutarate} + NH_4^+ + ext{NAD(P)H}.$
  • Glutaminase (glutaminase) converts glutamine to glutamate and NH3:
    $ext{Glutamine} + H<em>2O ightarrow ext{Glutamate} + NH</em>3.$
  • Asparaginase converts asparagine to aspartate and NH3:
    ext{Asparagine} + H2O ightarrow ext{Aspartate} + NH3.$n- Direct deamination: serine and threonine can be deaminated to pyruvate and 2-ketobutyrate, respectively (serine/threonine dehydratases) with removal of amino groups.
  • Serine dehydratase:
    ext{Serine}
    ightarrow ext{Pyruvate} + NH_3.$</li> <li>Threonine dehydratase: <br />$ ext{Threonine}
    ightarrow 2 ext{-ketobutyrate} + NH_3.$</li> <li>Histidine ammonia-lyase: <br />$ ext{Histidine}
    ightarrow ext{Urocanate} + NH_3.$</li> <li>Glycine cleavage system: glycine is cleaved to yield CO2, NH3, and reduced THF derivatives; this is a key one-carbon unit donor pathway.</li></ul></li> <li>D-amino acid oxidases (in tissue contexts): oxidize D-amino acids to amino acid-derived products, generating NH4+ and reactive oxygen species in some contexts; observed in peripheral tissues and glands.</li> <li>Transamination overview (PLP cofactor):<ul> <li>Core reaction transfers amino groups between amino acids and α-keto acids (predominantly α-ketoglutarate) via a pyridoxal phosphate (PLP) cofactor.</li> <li>General transamination: <br />$ ext{Amino acid} + ext{α-ketoglutarate}
    ightleftharpoons ext{α-keto acid} + ext{glutamate}.$</li> <li>Major aminotransferases: AST (GOT) and ALT (GPT) catalyze transamination between specific amino acids and α-ketoglutarate, producing corresponding α-keto acids and glutamate.</li> <li>Typical substrates: Aspartate and alanine are commonly transaminated to oxaloacetate and pyruvate, respectively, with glutamate acting as the amino group donor/acceptor.</li></ul></li> <li>Indirect deamination and the purine nucleotide cycle:<ul> <li>The purine nucleotide cycle contributes to nitrogen disposal in muscle by converting AMP to inosine monophosphate (IMP) and feeding fumarate into the TCA cycle, enabling amino group removal and energy balance during high activity.</li> <li>Key steps include AMP deaminase activity and aspartate involvement; this cycle operates in parallel to the urea cycle for efficient nitrogen management during exercise.</li></ul></li> </ul> <h3 id="ureacycleornithinecycleandnitrogendisposal">Urea cycle (ornithine cycle) and nitrogen disposal</h3> <ul> <li>Core net reaction (liver-focused):$NH3 + HCO3^- + 3 \,ATP + 2 H2O + \,Asp ightarrow ext{urea} + 2 \,ADP + 2 \,Pi + AMP + PPi + \,fumarate.$<ul> <li>The entire enzyme system of the urea cycle is expressed in the liver only, and enzymes are localized in both the cytosol and mitochondria.</li></ul></li> <li>Key enzymes and organelles:<ul> <li>CPS I (carbamoyl phosphate synthetase I) in mitochondria; combines NH3 with bicarbonate to form carbamoyl phosphate.</li> <li>OTC (ornithine transcarbamylase) in mitochondria; transfers the carbamoyl group to ornithine to form citrulline.</li> <li>ASS (arginosuccinate synthetase) in cytosol; condenses citrulline with aspartate to form argininosuccinate.</li> <li>ASL (argininosuccinate lyase) in cytosol; cleaves argininosuccinate to arginine and fumarate.</li> <li>ARG (arginase) in liver cytosol; hydrolyzes arginine to urea and ornithine, completing the cycle.</li></ul></li> <li>Link to the citric acid cycle:<ul> <li>Fumarate produced by ASL feeds into the TCA cycle, while malate generated in the cycle can re-enter the malate–aspartate shuttle, contributing to energy metabolism.</li> <li>The urea cycle is thus connected to TCA cycle intermediates (malate, oxaloacetate).</li></ul></li> <li>Transport and compartmentalization:<ul> <li>The CPS I step occurs in the mitochondria; subsequent steps occur in the cytosol, illustrating the intra-cellular coordination of the cycle.</li></ul></li> <li>Regulation of the urea cycle:<ul> <li>High urea concentrations can inhibit ASL; transcription is upregulated by high protein intake, fasting, glucocorticoids, and elevated cAMP (glucagon).</li> <li>Allosteric activator: N-acetyl-glutamate (NAG) activates CPS I when arginine is available; NAG formation is regulated by N-acetylglutamate synthase in mitochondria.</li> <li>Post-translational regulation: sirtuins (NAD+-dependent deacetylases) can modulate CPS I and OTC through acetylation/deacetylation.</li> <li>Acidosis can alter enzyme activity and overall cycle dynamics; fasting vs fed state modulates transcriptional and enzymatic activity.</li></ul></li> <li>N-acetylglutamate (NAG) and regulatory logic:<ul> <li>NAG is formed when acetyl-CoA combines with glutamate (via N-acetylglutamate synthase) to activate CPS I.</li> <li>In fed state, NAG production and CPS I activity increase for amino nitrogen disposal; in fasting state, regulation shifts based on energy and hormonal signals.</li> <li>Arginine serves as a precursor for NAG synthesis and can modulate CPS I activity indirectly.</li></ul></li> <li>Glutamine synthetase (GS) and nitrogen handling outside the liver:<ul> <li>GS catalyzes the ATP-dependent amidation of glutamate to form glutamine, which serves as a non-toxic transporter of ammonia in the blood.</li> <li>Net reaction: <br />$ ext{Glutamate} + NH_3 + ATP
    ightarrow ext{Glutamine} + ADP + Pi.$</li></ul></li> <li>Anatomical and zonation considerations in liver:<ul> <li>Periportal vs pericentral hepatocytes show distinct metabolic profiles with different enzyme expression (e.g., CPS I, glutamine synthetase, gluconeogenic and amino acid-processing enzymes).</li></ul></li> <li>Acidosis and urea cycle adaptation in liver:<ul> <li>Acidosis can decrease CPS I activity indirectly via altered NAG production and transporter regulation; nitrogen disposal adapts to maintain pH balance.</li></ul></li> </ul> <h3 id="catabolismofaminoacidsfateofthecarbonskeletoncskeleton">Catabolism of amino acids: fate of the carbon skeleton (C-skeleton)</h3> <ul> <li>Overall idea:<ul> <li>The carbon skeletons (minus the amino group) are fate-determined into glucose, glycogen, fatty acids, ketone bodies, or TCA cycle intermediates depending on tissue needs and hormonal signals.</li></ul></li> <li>Ketogenic vs glucogenic pathways (general concept):<ul> <li>Some amino acids feed carbon into acetyl-CoA or acetoacetyl-CoA (ketogenic).</li> <li>Others feed into pyruvate, oxaloacetate, α-ketoglutarate, succinyl-CoA, malate, or fumarate (glucogenic and/or ketogenic).</li> <li>In total, the catabolic routes feed into the TCA cycle and downstream energy-producing pathways or gluconeogenesis for maintenance of blood glucose.</li></ul></li> <li>Representative catabolic fates (examples, non-exhaustive):<ul> <li>Pyruvate entry point: Several amino acids (e.g., alanine, serine, cysteine) can be converted to pyruvate.</li> <li>Oxaloacetate entry: Aspartate and others can feed oxaloacetate via transamination or deamination.</li> <li>α-Ketoglutarate entry: Glutamate, glutamine, and others provide carbon to α-ketoglutarate.</li> <li>Succinyl-CoA entry: Val, Leu, Ile, and others contribute to succinyl-CoA in various steps.</li> <li>Fumarate entry: Argininosuccinate breakdown and other routes yield fumarate.</li> <li>Malate, citrate, and other TCA intermediates connect amino acid catabolism to energy production and gluconeogenesis.</li></ul></li> <li>Specific amino acids and their catabolic routes (highlights):<ul> <li>Phenylalanine and tyrosine: phenylalanine hydroxylation to tyrosine links to downstream metabolism.</li> <li>Branched-chain amino acids (BCAAs: Leu, Ile, Val): catabolized primarily in muscle, brain, and adipose tissue (not in the liver). Initial transamination by branched-chain aminotransferase yields corresponding keto acids; subsequent decarboxylation by branched-chain α-keto acid dehydrogenase complex (BCKD) leads to acyl-CoA derivatives feeding into energy pathways.</li> <li>Methionine, threonine, lysine, tryptophan, phenylalanine, tyrosine, valine, leucine, isoleucine, etc., feed into various TCA intermediates or acetyl-CoA units depending on their specific catabolic steps.</li></ul></li> <li>Phenylalanine catabolism and phenylketonuria (PKU):<ul> <li>Phenylalanine → Tyrosine by phenylalanine hydroxylase (requires tetrahydrobiopterin, THB).</li> <li>PKU results from deficiency of phenylalanine hydroxylase, leading to elevated phenylalanine and altered neurotransmitter synthesis; newborn screening detects PKU; clinical implications include potential disturbances in blood-brain barrier transport of neutral amino acids, impaired protein and neurotransmitter synthesis, and myelin sheath damage.</li></ul></li> <li>Branched-chain amino acids (BCAAs):<ul> <li>Initial transamination occurs in muscle, brain, and adipose tissue (not in liver) to generate corresponding α-keto acids.</li> <li>Dehydrogenation by the BCKD complex occurs in mitochondria; genes and enzymes involved include E1, E2, E3 subunits.</li> <li>Metabolic integration with energy pathways and nucleotide synthesis in proliferating cells (glutamine and α-ketoglutarate interplay) is noted in advanced contexts.</li></ul></li> <li>Glutaminolysis and proliferating cells:<ul> <li>Glutamine-derived carbon and nitrogen support nucleotide synthesis and protein synthesis in rapidly dividing cells; transformation of glutamate and α-ketoglutarate links to NADH/NADPH balance and TCA cycle fluxes.</li></ul></li> <li>Key intermediary conversions (conceptual):<ul> <li>Propionyl-CoA to succinyl-CoA (catabolism of odd-chain fatty acids, certain amino acids: Val, Ile, Met, Thr; propionate from isoleucine and valine; odd-chain fatty acids).</li></ul></li> <li>Complete oxidation of C-skeletons is a coordinated network involving malate, oxaloacetate, pyruvate, acetyl-CoA, ketone bodies, and TCA cycle intermediates to generate energy or to supply precursors for other biosynthetic pathways.</li> </ul> <h3 id="vitaminsinthemetabolismofaminoacids">Vitamins in the metabolism of amino acids</h3> <ul> <li>Overview:<ul> <li>Vitamins act as coenzymes or cofactors in amino acid catabolism, neurotransmitter synthesis, one-carbon metabolism, and methylation cycles.</li></ul></li> <li>Vitamin B1 (thiamine) – as thiamine pyrophosphate (TPP):<ul> <li>Roles include function in branched-chain α-keto acid dehydrogenase, pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and transketolase.</li></ul></li> <li>Vitamin B6 – pyridoxal phosphate (PLP):<ul> <li>Central cofactor for transamination, serine dehydratase, serine hydroxymethyltransferase, glycine cleavage system, decarboxylation (neurotransmitter synthesis), glycogen phosphorylase, serine palmitoyltransferase (sphingolipid synthesis), δ-aminolevulinate synthase (heme synthesis).</li></ul></li> <li>Vitamin B7 – biotin:<ul> <li>Propionyl-CoA carboxylase, pyruvate carboxylase, acetyl-CoA carboxylase.</li></ul></li> <li>Vitamin B9 – folic acid (THF derivatives) and one-carbon metabolism:<ul> <li>Transfer and interconversion of one-carbon units (methyl, methylene, formyl, formimino) across THF derivatives (N5–methyl-THF, N5,N10–methylene-THF, N5,N10–formyl-THF, etc.).</li></ul></li> <li>Vitamin B12 – cobalamin:<ul> <li>Coenzyme in propionyl-CoA to succinyl-CoA conversion and methylmalonyl-CoA mutase pathways; also involved in homocysteine methylation to methionine via methionine synthase.</li></ul></li> <li>One-carbon metabolism and folate cycle:<ul> <li>THF derivatives donate and accept one-carbon units for purine bases and thymidylate synthesis; interplay with the SAM cycle for methylation reactions.</li></ul></li> <li>Methionine cycle and SAM:<ul> <li>SAM (S-adenosylmethionine) donates methyl groups; after transfer, it becomes SAH (S-adenosylhomocysteine), which is hydrolyzed to homocysteine; homocysteine can be remethylated to methionine or enter the transsulfuration pathway.</li></ul></li> <li>Propionyl-CoA metabolism and B12/biotin role:<ul> <li>Propionyl-CoA is converted to methylmalonyl-CoA (biotin-dependent carboxylase) and then to succinyl-CoA (B12-dependent mutase).</li></ul></li> <li>Vitamin C and collagen/hydroxyproline synthesis:<ul> <li>Hydroxyproline and hydroxylysine synthesis requires prolyl-4-hydroxylase which uses α-ketoglutarate, O2, Fe2+, and vitamin C as cofactors.</li></ul></li> <li>Vitamin K and y-carboxyglutamate:<ul> <li>Gamma-carboxylation of glutamate residues in certain coagulation factors requires vitamin K; this post-translational modification is essential for proper clotting factor activity.</li></ul></li> <li>Synthesis of hydroxyproline, hydroxylysine, and y-carboxyglutamate:<ul> <li>Hydroxyproline/hydroxylysine: collagen cross-linking; vitamin C and iron are required.</li> <li>γ-carboxyglutamate: coagulation factors via vitamin K-dependent carboxylation.</li></ul></li> </ul> <h3 id="transaminationandaminogrouptransfersmechanisticdetail">Transamination and amino group transfers (mechanistic detail)</h3> <ul> <li>Central role of pyridoxal phosphate (PLP):<ul> <li>PLP acts as a cofactor that transiently accepts amino groups during transamination, decarboxylation, racemization, and various other reactions.</li></ul></li> <li>Mechanism (conceptual):<ul> <li>An amino group is transferred from an amino acid to an α-keto acid (commonly α-ketoglutarate) via a PLP-mediated Schiff base intermediate, forming a new amino acid and a new α-keto acid.</li></ul></li> <li>Enzymes involved (major):<ul> <li>AST (aspartate aminotransferase, GOT) and ALT (alanine aminotransferase, GPT) are the principal transaminases used diagnostically to assess tissue injury (liver).</li></ul></li> <li>Net representation: <br />$ ext{Amino acid} + ext{α-ketoglutarate}
    ightarrow ext{α-keto acid} + ext{glutamate}.$</li> <li>Link to energy and nitrogen metabolism:<ul> <li>Transamination balances amino acid pools and connects amino acid catabolism to the TCA cycle via α-ketoglutarate and glutamate.</li></ul></li> </ul> <h3 id="directandindirectdeaminationandnitrogendisposalpathways">Direct and indirect deamination, and nitrogen disposal pathways</h3> <ul> <li>Direct deamination (examples):<ul> <li>Glutamate dehydrogenase (GDH) reaction (oxidative deamination):<br />$ ext{Glutamate} + ext{NAD(P)}^+
    ightarrow ext{α-ketoglutarate} + NH_4^+ + ext{NAD(P)H}.$</li> <li>Glutaminase reaction (glutamine → glutamate):<br />$ ext{Glutamine} + H2O ightarrow ext{Glutamate} + NH3.$</li> <li>Asparaginase reaction (asparagine → aspartate):<br />$ ext{Asparagine} + H2O ightarrow ext{Aspartate} + NH3.$</li> <li>Serine dehydratase: <br />$ ext{Serine}
    ightarrow ext{Pyruvate} + NH_3.$</li> <li>Threonine dehydratase: <br />$ ext{Threonine}
    ightarrow 2 ext{-ketobutyrate} + NH_3.$</li> <li>Histidine ammonia-lyase: <br />$ ext{Histidine}
    ightarrow ext{Urocanate} + NH_3.$</li></ul></li> <li>Glycine cleavage system (one-carbon metabolism):<ul> <li>Glycine is cleaved to CO2, NH3, and a one-carbon donor (N5,N10-methylene-THF) with concurrent generation of reduced cofactors.</li></ul></li> <li>Indirect deamination via the urea cycle and purine nucleotide cycle:<ul> <li>Ammonia produced in tissues is transported to the liver for disposal primarily as urea; ammonia can also be channeled through the purine nucleotide cycle in muscle to yield fumarate and aid energy metabolism during exercise.</li></ul></li> <li>Transport forms of nitrogen to liver:<ul> <li>Peripheral tissues export nitrogen mainly as glutamine and alanine to the liver, where nitrogen disposal via the urea cycle occurs.</li></ul></li> </ul> <h3 id="sourceandhandlingofammonia">Source and handling of ammonia</h3> <ul> <li>Sources of ammonia:<ul> <li>From GI tract: ammonia can be produced via digestion and bacterial processing and delivered to liver for disposal (urea cycle in liver).</li> <li>From tissues: amino acid deamination and transamination yield ammonia, which is incorporated into urea for excretion or into glutamine for transport.</li></ul></li> <li>Transport of nitrogen to liver:<ul> <li>Glutamine and alanine serve as major nitrogen carriers from tissues to liver for urea synthesis.</li></ul></li> <li>Glutamine recycling and glutaminase activity in kidney and intestine:<ul> <li>In extrahepatic tissues, glutamine acts as a non-toxic transporter; in kidney and intestine, glutaminase activity contributes to ammonia production for excretion and acid-base balance.</li></ul></li> </ul> <h3 id="glutaminesynthetasegsandextrahepaticammoniahandling">Glutamine synthetase (GS) and extrahepatic ammonia handling</h3> <ul> <li>Role of GS:<ul> <li>Converts glutamate to glutamine using NH3 and ATP, aiding transport of ammonia in a non-toxic form through the bloodstream.</li> <li>Reaction:$ ext{Glutamate} + NH_3 + ATP
    ightarrow ext{Glutamine} + ADP + Pi.$</li></ul></li> <li>Role in tissue nitrogen economy:<ul> <li>GS helps maintain nitrogen balance in tissues that export ammonia load to the liver for disposal via the urea cycle.</li></ul></li> </ul> <h3 id="regulationandintegrationofaminoacidmetabolism">Regulation and integration of amino acid metabolism</h3> <ul> <li>Key regulatory themes in nitrogen handling:<ul> <li>High protein intake upregulates urea cycle gene transcription; fasting or hormonal signals (glucagon, cortisol) can modulate CPS I activity and NAG synthesis.</li> <li>N-acetylglutamate (NAG) as the essential allosteric activator of CPS I; NAG synthesis is regulated by mitochondrial enzymes and fed vs fasting states.</li> <li>Acetylation/deacetylation dynamics (sirtuins) influence CPS I and OTC activities; acidosis can alter enzyme kinetics and expression.</li></ul></li> <li>Liver zonation and metabolic flexibility:<ul> <li>Periportal vs pericentral hepatocytes show differential expression of CPS I, glutamine synthetase, and other amino-acid-processing enzymes, providing spatial regulation of nitrogen metabolism within the liver.</li></ul></li> <li>Acidosis effects on amino acid metabolism in liver:<ul> <li>Reduced hepatic uptake of amino acids; increased NH3/NH4+ in plasma; decreased glutaminase activity; altered Km for NAG synthase; changes in hepatic ammonia handling and urea production.</li></ul></li> <li>Fate of glutamine in the kidney and blood:<ul> <li>Glutamine from blood is taken up by proximal tubule cells; in kidney, glutaminase and GDH activity leads to glutamate and NH3 generation; NH3 contributes to acid-base balance (urine acidification) and ammonia excretion; gluconeogenesis can also occur in kidney for glucose production.</li></ul></li> </ul> <h3 id="ureasalvageandintestinalnitrogenhandling">Urea salvage and intestinal nitrogen handling</h3> <ul> <li>Urea salvage (recycling):<ul> <li>Ornithine cycle can operate partially in intestinal tissues to generate urea or ammonia that can be recycled or excreted; microbial proteins in gut can also contribute to nitrogen flow.</li> <li>Conceptual diagram: liver and intestine exchange nitrogenous compounds, supporting whole-body nitrogen economy.</li></ul></li> <li>Clinical relevance of salvage pathways:<ul> <li>Alterations in intestinal nitrogen handling or gut microbiota composition can impact systemic nitrogen balance and urea production.</li></ul></li> </ul> <h3 id="catabolismofaminoacidsoverviewofthecskeletonfatesynthesisanddegradationpathways">Catabolism of amino acids: overview of the C-skeleton fate (synthesis and degradation pathways)</h3> <ul> <li>Main idea:<ul> <li>Degraded amino acids yield carbon skeletons that feed into central metabolic pathways (glycolysis, gluconeogenesis, TCA cycle) or into lipid synthesis; they also feed into ketone body production under certain conditions.</li></ul></li> <li>Key metabolic fates/entries:<ul> <li>Pyruvate entry: some amino acids (e.g., alanine, serine, cysteine) feed into pyruvate and then to glucose or lactate.</li> <li>Oxaloacetate entry: aspartate, asparagine, and others contribute to oxaloacetate for gluconeogenesis and TCA flux.</li> <li>α-Ketoglutarate entry: glutamate and glutamine contribute to α-ketoglutarate for the TCA cycle.</li> <li>Succinyl-CoA entry: certain amino acids contribute to succinyl-CoA for TCA/input anaplerotic reactions.</li> <li>Fumarate and malate entries: contribute to the TCA cycle and gluconeogenesis.</li> <li>Acetyl-CoA and acetoacetyl-CoA entries: ketogenic amino acids contribute to ketone body formation and lipid synthesis.</li></ul></li> <li>Specific pathways by amino acid group (highlights):<ul> <li>Phenylalanine → Tyrosine; phenylalanine hydroxylase requires tetrahydrobiopterin; PKU consequences reviewed earlier.</li> <li>BCAAs (Val, Leu, Ile): largely degraded in muscle; initial transamination to corresponding α-keto acids, followed by decarboxylation and subsequent metabolism to acetyl-CoA or succinyl-CoA by BCKD complex.</li> <li>Proline, arginine, glutamine, glutamate, methionine, lysine, tryptophan, threonine, serine, glycine and others feed into various TCA intermediates or specialized pathways (one-carbon metabolism, purine synthesis, etc.).</li></ul></li> <li>Special pathways and cross-links:<ul> <li>Glutaminolysis in proliferating cells integrates amino acid catabolism with nucleotide synthesis and NADH/NADPH balance to support rapid cell growth.</li></ul></li> </ul> <h3 id="phenylalanineandtyrosinemetabolismpkuphenylketonuria">Phenylalanine and tyrosine metabolism; PKU (phenylketonuria)</h3> <ul> <li>Phenylalanine → Tyrosine:<ul> <li>Enzyme: phenylalanine hydroxylase; cofactor THB (tetrahydrobiopterin); oxygen is required.</li> <li>Product: tyrosine; by-products include water and dihydropteridine derivatives (DHB).</li></ul></li> <li>PKU (phenylketonuria):<ul> <li>Result of deficient phenylalanine hydroxylase activity, leading to accumulation of phenylalanine and altered synthesis of phenylalanine-derived products.</li> <li>Clinical concerns: impaired transport of neutral amino acids across the blood–brain barrier; disturbances in protein synthesis and neurotransmitter synthesis; potential myelin sheath damage.</li> <li>Newborn screening is critical for early intervention and management.</li></ul></li> </ul> <h3 id="branchedchainaminoacidsbcaasandskeletalmusclemetabolism">Branched-chain amino acids (BCAAs) and skeletal muscle metabolism</h3> <ul> <li>Tissue specificity:<ul> <li>Transamination of BCAAs occurs predominantly in muscle, brain, and adipose tissue; liver does not significantly metabolize BCAAs.</li></ul></li> <li>Enzymatic steps:<ul> <li>Transamination by branched-chain aminotransferase yields α-keto acids.</li> <li>Branched-chain α-keto acid dehydrogenase complex (BCKD) decarboxylates the α-keto acids to produce acyl-CoA derivatives that feed into energy metabolism.</li></ul></li> <li>Metabolic integration in muscle:<ul> <li>Muscle metabolism of BCAAs links to glucose production and energy generation (via pyruvate, acetyl-CoA, oxaloacetate, citrate) and to purine nucleotide cycles.</li></ul></li> <li>Proliferating cells and glutaminolysis context:<ul> <li>In rapidly dividing cells, glutamine and BCAA–derived carbon contribute to nucleotide synthesis, TCA flux, and energy metabolism.</li></ul></li> </ul> <h3 id="glutaminolysisandproliferatingcellscontextualoverview">Glutaminolysis and proliferating cells (contextual overview)</h3> <ul> <li>Glutamine-as-a-carbon donor in proliferating cells:<ul> <li>Glutamate and α-ketoglutarate link to ATP generation and the synthesis of nucleotides and amino acids.</li> <li>Interplay with NADH and NADPH balance influences redox state and biosynthetic capacity.</li></ul></li> <li>Central pathways (conceptual):<ul> <li>Glutamate ↔ α-ketoglutarate interconversion, transaminations, and deaminations support nucleotide and amino acid synthesis during cell proliferation.</li></ul></li> </ul> <h3 id="vitaminsinaminoacidmetabolismexpandedview">Vitamins in amino acid metabolism (expanded view)</h3> <ul> <li>B1 (thiamine) – active as TPP:<ul> <li>Enzymes: branched-chain α-ketoacid dehydrogenase, pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, transketolase.</li></ul></li> <li>B6 – PLP (pyridoxal phosphate):<ul> <li>Central cofactor for transamination, serine dehydratase, serine hydroxymethyltransferase, glycine cleavage system, decarboxylases, glycogen phosphorylase, sphingolipid synthesis, δ-aminolevulinate synthase.</li></ul></li> <li>B7 – biotin:<ul> <li>Carboxylases: propionyl-CoA carboxylase, pyruvate carboxylase, acetyl-CoA carboxylase.</li></ul></li> <li>B9 – folic acid (THF) and one-carbon metabolism:<ul> <li>THF derivatives accept/deliver one-carbon units for purine/pyrimidine synthesis and amino acid interconversions.</li></ul></li> <li>B12 – cobalamin:<ul> <li>Propionyl-CoA to succinyl-CoA (biotin and B12 dependent steps); methylmalonyl-CoA mutase; methionine synthase in the methionine cycle.</li></ul></li> <li>Methionine cycle and SAM (S-adenosylmethionine):<ul> <li>SAM donates methyl groups in methylation reactions; forms SAH; hydrolysis yields homocysteine; remethylation to methionine or entry into transsulfuration to cysteine.</li></ul></li> <li>Propionyl-CoA and metabolism:<ul> <li>Biotin-dependent propionyl-CoA carboxylase and B12-dependent methylmalonyl-CoA mutase convert propionyl-CoA to succinyl-CoA, integrating amino acid catabolism with the TCA cycle.</li></ul></li> <li>Synthesis of hydroxyproline/hydroxylysine (collagen cross-linking):<ul> <li>Prolyl-4-hydroxylase requires α-ketoglutarate, O2, vitamin C, and Fe2+; adds hydroxyl groups to proline and lysine.</li></ul></li> <li>Gamma-carboxylation of glutamate (y-carboxyglutamate):<ul> <li>Vitamin K-dependent carboxylation essential for coagulation factors; enables calcium binding.</li></ul></li> </ul> <h3 id="onecarbonmetabolismandfolatecycledetails">One-carbon metabolism and folate cycle details</h3> <ul> <li>Folate derivatives and one-carbon transfers:<ul> <li>THF derivatives carry one-carbon units in various oxidation states: methyl, methylene, formyl, formimino, N5-formimino-THF, N5,N10-methylene-THF, N5,N10-methenyl-THF, N5,N10-formyl-THF.</li></ul></li> <li>Sources and destinations:<ul> <li>Serine, glycine, histidine, tryptophan, and α-ketoglutarate contribute one-carbon units into the folate pool; formate can also feed into THF derivatives.</li></ul></li> <li>The SAM cycle and methylation potential:<ul> <li>Methionine → SAM → SAH → homocysteine; homocysteine can be remethylated to methionine or enter the transsulfuration pathway to cysteine.</li></ul></li> <li>Propionyl-CoA and folate interplay:<ul> <li>Propionyl-CoA metabolism connects to odd-chain fatty acids, certain amino acids, and one-carbon unit flux through folate and B12-dependent steps.</li></ul></li> </ul> <h3 id="synthesisofcollagenandbonerelatedposttranslationalmodifications">Synthesis of collagen and bone-related post-translational modifications</h3> <ul> <li>Hydroxyproline and hydroxylysine synthesis:<ul> <li>Prolyl-4-hydroxylase uses α-ketoglutarate, O2, ascorbate (Vitamin C), and Fe2+ to hydroxylate proline to hydroxyproline in collagen.</li> <li>Hydroxylysine formation is similarly enzyme-catalyzed and contributes to cross-linking of collagen.</li></ul></li> <li>y-Carboxyglutamate formation:<ul> <li>Gamma-carboxylation of glutamate residues in certain coagulation factors requires vitamin K; this post-translational modification enables proper calcium binding and clotting activity.</li></ul></li> </ul> <h3 id="practicalimplicationsandclinicalnotes">Practical implications and clinical notes</h3> <ul> <li>Phenylketonuria (PKU) screening and management:<ul> <li>Early detection allows dietary management to limit phenylalanine intake and support normal development.</li></ul></li> <li>Vegetarian and vegan diets:<ul> <li>Attention to methionine, lysine, and overall essential amino acid balance; plan complementary protein sources to meet daily requirements.</li></ul></li> <li>Urea cycle regulation in disease states:<ul> <li>Defects in CPS I, OTC, ASL, or ARG can lead to hyperammonemia with neurotoxic consequences; understanding regulation helps anticipate metabolic disturbances.</li></ul></li> <li>Exercise and amino acid transport:<ul> <li>Transport across the blood-brain barrier (LAT1) and uptake by liver vs muscle modulates neurotransmitter availability and energy balance; exercise can shift the uptake of branched-chain amino acids and aromatic amino acids.</li></ul></li> <li>The broader context:<ul> <li>Amino acid metabolism integrates with energy metabolism (glycolysis, TCA cycle, fatty acid synthesis), one-carbon metabolism (folate and SAM cycles), redox biology (NADH/NADPH balance), and hormone signaling (glucagon, insulin) to maintain homeostasis across fed, fasting, and stressed states.</li></ul></li> <li>Ethical/philosophical angle (contextual):<ul> <li>Diet composition (animal vs plant-based) has implications for energy availability, brain development (historical meat consumption vs modern dietary diversity), and public health nutrition policies.</li></ul></li> </ul> <blockquote> <p>Equations and key reactions (for quick reference)</p> <ul> <li>Glutamate dehydrogenase (oxidative deamination):<br />$ ext{Glutamate} + ext{NAD(P)}^+
    ightarrow ext{α-ketoglutarate} + NH_4^+ + ext{NAD(P)H}.$</li> <li>Glutaminase (glutamine → glutamate):<br />$ ext{Glutamine} + H2O ightarrow ext{Glutamate} + NH3.$</li> <li>Asparaginase (asparagine → aspartate):<br />$ ext{Asparagine} + H2O ightarrow ext{Aspartate} + NH3.$</li> <li>Serine dehydratase: <br />$ ext{Serine}
    ightarrow ext{Pyruvate} + NH_3.$</li> <li>Threonine dehydratase: <br />$ ext{Threonine}
    ightarrow 2 ext{-ketobutyrate} + NH_3.$</li> <li>Transamination (general):<br />$ ext{Amino acid} + ext{α-ketoglutarate}
    ightleftharpoons ext{α-keto acid} + ext{glutamate}.$</li> <li>Urea cycle (net):<br />$NH3 + HCO3^- + 3ATP
    ightarrow ext{urea} + 2ADP + 2Pi + AMP + PP_i + ext{fumarate}.$</li> <li>Glutamine synthetase: <br />$ ext{Glutamate} + NH_3 + ATP
    ightarrow ext{Glutamine} + ADP + Pi.$</li> <li>Phenylalanine hydroxylase (phenylalanine → tyrosine):<br />$ ext{Phenylalanine} + O2 + THB ightarrow ext{Tyrosine} + DHB + H2O.$$
  • Prolyl-4-hydroxylase (collagen synthesis):
    requires α-ketoglutarate, O2, Vitamin C, Fe2+; converts proline to hydroxyproline.
  • Note: The above notes capture the major and many minor concepts presented in the transcript, including regulation, tissue-specific aspects, and clinical correlations. If you want, I can tailor these notes to focus more on chemistry, physiology, or clinical correlations for your exam prep, or condense into a more concise study sheet with only the most likely exam topics.