Amino Acid Metabolism: Key Concepts for Exam

The Amino Acid Pool and Balance

  • Inputs maintain the amino acid pool: protein turnover (damaged/defective/excess proteins), dietary proteins, amino acid biosynthesis.
  • Outputs: protein synthesis, synthesis of nitrogenous compounds, and catabolism to glucose, ketone bodies, fatty acids, or CO2/H2O with ATP production.
  • No long-term amino acid storage depot; in normal physiology, amino acids must be obtained from diet.
  • Humans synthesize only a subset of amino acids; others are essential and must be supplied by diet.

Protein Digestion and Absorption

  • Daily dietary protein requirement: roughly
    70−100 g70-100\,g to maintain the amino acid pool.
  • Proteolysis: proteases cleave peptide bonds via hydrolysis; digestion begins in the stomach and completes in the small intestine.
  • Proteases are secreted as inactive zymogens and activated when needed (example: pepsinogen → pepsin via autocatalysis at low pH).

Digestive Pathway Overview

  • Mouth: chewing increases surface area and triggers neural responses that prepare the stomach.
  • Stomach: gastric juice (up to ~1 L) with HCl denatures proteins and pepsin begins proteolysis.
  • Small intestine: pancreatic proteases (trypsin, chymotrypsin, elastase) and carboxypeptidases; aminopeptidases further digest; di-/tripeptides absorbed or further digested to amino acids for entry into the portal circulation to liver.

Nitrogen Balance and Ammonium Handling

  • Ammonium (NH4+) is toxic; ammonia (NH3) is minimized at physiological pH; most free ammonium is rapidly disposed.
  • Two components of amino acids:
    • Amino group ( nitrogen)
    • Carbon skeleton (a-keto acids) that can feed energy production or gluconeogenesis.
  • Nitrogen disposal via urea cycle; carbon skeletons funnel into energy metabolism or glucose production.

Transamination and Amino Group Handling

  • Step 1: Transamination funnels amino groups to glutamate using aminotransferases and pyridoxal phosphate (PLP):
    extaminoacid+α−ketoglutarate⇌α−keto acid+glutamateext{amino acid} + \alpha{-}\text{ketoglutarate} \rightleftharpoons \alpha{-}\text{keto acid} + \text{glutamate}
  • Step 2: Oxidative deamination in liver mitochondria:
    glutamate+NAD+  or  NADP+→α−ketoglutarate+NH4++NADH/NADPH\text{glutamate} + \mathrm{NAD^+} \;\text{or} \; \mathrm{NADP^+} \rightarrow \alpha{-}\text{ketoglutarate} + NH_4^+ + \mathrm{NADH/NADPH}
  • ALT and AST are diagnostic markers of tissue damage in blood.

Nitrogen Removal from Peripheral Tissues

  • Peripheral tissues export amino groups primarily as:
    • Glutamine (via glutamine synthetase; liver has glutaminase to release NH4+).
    • Alanine (via alanine aminotransferase in muscle; glucose-alanine cycle).
  • Glucose-Alanine Cycle: muscle ⇄ liver
    • Alanine carries amino groups from muscle to liver; in liver ALT transfers NH2 to \alpha{-}ketoglutarate forming glutamate; glutamate dehydrogenase releases NH4+ for urea production; pyruvate from this process progresses to gluconeogenesis and glucose is sent back to muscle.

The Urea Cycle: Overview and Localization

  • Starts in mitochondria and completes in cytosol; ammonium is produced in mitochondria and entered into the cycle.
  • Rate-limiting step: Carbamoyl phosphate synthetase I (CPS I); activated by N-acetylglutamate (NAG).
  • NAG synthesis: from glutamate + acetyl-CoA via N-acetylglutamate synthase; stimulated by arginine.
  • Part 1 (mitochondria): Carbamoyl phosphate + ornithine → citrulline (ornithine transcarbamoylase); citrulline moves to cytosol.
  • Part 1 note: one amino group in urea comes from aspartate (formed in mitochondria via transamination of glutamate).
  • Part 2 (cytosol): Citrulline + aspartate → argininosuccinate (argininosuccinate synthetase, uses ATP); argininosuccinase → fumarate + arginine; fumarate re-enters CAC; arginine → urea + ornithine (arginase); ornithine re-enters mitochondria.
  • Enzymes argininosuccinate synthetase, argininosuccinase, and arginase can form a complex to keep intermediates high and flux efficient, while limiting side reactions.

Regulation and Interconnections

  • Urea cycle linked to CAC via fumarate and aspartate.
  • Gluconeogenic and ketogenic fate of amino acids:
    • Glucogenic amino acids yield pyruvate or CAC intermediates for glucose synthesis.
    • Ketogenic amino acids yield acetyl-CoA and/or acetoacetate for ketone bodies or lipids.
    • Some amino acids (e.g., tryptophan, tyrosine, threonine, phenylalanine, isoleucine) are both glucogenic and ketogenic.
  • Energy cost: roughly 3 ATP3\,ATP (4 high-energy bonds) per urea formed.
  • Overall urea cycle equation (simplified):
    Aspartate+NH<em>4++CO</em>2+3 ATP→urea+fumarate+2 ADP+AMP+2 Pi\text{Aspartate} + NH<em>4^+ + CO</em>2 + 3\,ATP \rightarrow \text{urea} + \text{fumarate} + 2\,ADP + AMP + 2\,P_i

Nitrogen Disposal Pathways Across Species

  • Ammoniotelic (e.g., fish): excrete NH4+ directly through gills.
  • Ureotelic (mammals): convert NH4+ to urea for urinary excretion.
  • Uricotelic (birds/reptiles): excrete uric acid to conserve water.

Ammonium and Hyperammonemia

  • Sources of free ammonium: transdeamination, serine/threonine dehydratases, glutaminase, kidney glutaminase, degradation of nitrogenous compounds.
  • Keeping ammonium low: urea synthesis, glutamate/glutamine synthesis in tissues, glutamine transport to liver.
  • Hyperammonemia CNS effects: brain swelling due to glutamine in astrocytes, reduced glutamate and GABA; tremors, slurred speech, coma.
  • Causes: acquired (liver damage) or congenital (urea cycle enzyme defects).
  • Treatments: e.g., phenylbutyrate binds glutamine to form phenylacetylglutamine for urinary excretion.

Carbon Skeleton Fate: Glucogenic vs Ketogenic

  • After transamination, carbon skeletons (a-keto acids) feed into:
    • Gluconeogenesis to glucose (e.g., alanine → pyruvate → glucose).
    • CAC intermediates for energy production.
    • Ketone bodies or fatty acids via acetyl-CoA or acetoacetate.
  • Important categories:
    • Glucogenic amino acids
    • Ketogenic amino acids
    • Some amino acids are both (e.g., tryptophan, tyrosine, threonine, phenylalanine, isoleucine).

Porphyrins, Heme, and Derived Molecules

  • Porphyrins (e.g., heme) derived from glycine + succinyl-CoA via ALA synthase; steps lead to porphyrin ring formation.
  • Porphyrias arise from defects in specific steps, causing accumulation of intermediates with distinct symptoms.

Catecholamines, Histamine, Serotonin, and Creatine

  • Catecholamines: tyrosine → DOPA → dopamine → norepinephrine → epinephrine.
  • Histamine: decarboxylation of histidine.
  • Serotonin and melatonin: tryptophan → serotonin; serotonin can be converted to melatonin.
  • Creatine synthesis: glycine + arginine + methionine → guanidinoacetate → creatine.

MSG Sensitivity Assertion

  • MSG is glutamate with Na+; most dietary glutamate is metabolized in the gut to α-ketoglutarate; clinical evidence for MSG sensitivity is not definitive; content of glutamate in body and foods is substantial and well-handled by metabolism.

Amino Acid Catabolic Diseases (Highlights)

  • Phenylketonuria (PKU): phenylalanine hydroxylase deficiency; accumulation of phenylalanine and phenylpyruvate/phenylacetate/phenyllactate; managed by low-phenylalanine diet and avoiding aspartame.
  • Maple syrup urine disease: branched-chain 2-oxoacid dehydrogenase deficiency; sweet-smelling urine; CNS impact.
  • Other examples: albinism (tyrosine metabolism), alkaptonuria (homogentisate dioxygenase deficiency), argininemias, citrullinemias, carbamoyl phosphate synthetase I deficiency, homocystinuria, among others; CNS symptoms are common.

Summary Points

  • Amino acid pool is balanced by intake and degradation; no storage depot, so daily intake is essential.
  • Protein digestion converts proteins to amino acids for absorption; enzymes are activated from zymogens.
  • Nitrogen balance is managed via transamination, oxidative deamination, and the urea cycle; peripheral tissues export nitrogen as glutamine or alanine.
  • Urea cycle links to CAC; CPS I regulation by N-acetylglutamate and arginine; energy cost per urea formed is significant.
  • Amino acid carbon skeletons can become glucose or ketone bodies; classification into glucogenic vs ketogenic guides metabolic fate.
  • Amino acids are precursors to important biomolecules (heme, catecholamines, histamine, serotonin, creatine).
  • Abnormal amino acid catabolism leads to inherited metabolic diseases with CNS involvement; dietary management and targeted therapies are used.