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What is the coenzyme involved in transaminase reactions?

Pyridoxal phosphate (PLP), derived from vitamin B6.

Why is PLP required for transamination?

PLP temporarily carries the amino group as an aminotransferase transfers it from an amino acid to an α-keto acid.

What is the function of N-acetylglutamate (NAG)?

It is an essential allosteric activator of carbamoyl phosphate synthetase I (CPS I).

How does N-acetylglutamate regulate the urea cycle?

NAG activates CPS I, the mitochondrial enzyme that initiates the urea cycle by producing carbamoyl phosphate.

What processes are associated with the glucose-alanine cycle?

Transamination in muscle, alanine transport to liver, urea formation, and gluconeogenesis.

How does the glucose-alanine cycle connect muscle and liver metabolism?

Muscle transfers amino nitrogen to pyruvate to form alanine, which travels to the liver where its nitrogen enters the urea cycle and its carbon skeleton is converted to glucose.

What is the function of glutamate dehydrogenase in amino acid metabolism?

It catalyzes oxidative deamination of glutamate to α-ketoglutarate + NH₄⁺.

Why is glutamate dehydrogenase important in amino acid catabolism?

Transamination funnels amino groups onto glutamate, then glutamate dehydrogenase releases them as NH₄⁺ for disposal through the urea cycle.

How is the energy demand of the urea cycle compensated?

Fumarate produced by the urea cycle enters the TCA cycle and helps generate NADH/ATP.

Why is the energetic cost of the urea cycle lower than it initially appears?

Urea synthesis consumes 4 high-energy phosphate bonds, but fumarate can generate NADH through its conversion to malate and oxaloacetate, partially compensating for this cost.

What compounds are excreted for the elimination of nitrogen in mammals?

Primarily urea, with smaller amounts of NH₄⁺ and uric acid.

Why do mammals convert ammonia primarily into urea?

Free ammonia is highly toxic, so mammals convert nitrogen into urea, a much less toxic, water-soluble compound that can be excreted in urine.

What is phenylketonuria (PKU)?

A disorder usually caused by phenylalanine hydroxylase deficiency, preventing normal conversion of phenylalanine to tyrosine.

What metabolic change occurs in phenylketonuria?

Failure to convert phenylalanine → tyrosine causes phenylalanine and phenylketones to accumulate, making tyrosine conditionally essential.

How does the urea cycle support the Krebs/TCA cycle?

It produces fumarate, which can enter the TCA cycle.

How are the urea cycle and TCA cycle metabolically connected?

Urea-cycle fumarate → malate → oxaloacetate, while oxaloacetate can form aspartate that returns nitrogen to the urea cycle.

Which compound directly donates a nitrogen atom for urea formation during the urea cycle?

Aspartate donates one of urea's two nitrogen atoms.

Where do the two nitrogen atoms of urea come from?

One comes from free NH₄⁺ through carbamoyl phosphate, and the other comes directly from aspartate.

What is the metabolic defect in maple syrup urine disease (MSUD)?

Deficiency of branched-chain α-keto acid dehydrogenase (BCKDH).

Why do branched-chain amino acids accumulate in maple syrup urine disease?

Defective BCKDH prevents normal degradation of the branched-chain amino acids leucine, isoleucine, and valine.

Where does urea synthesis take place in mammals?

In the liver, with reactions occurring in both the mitochondrial matrix and cytosol.

How is the urea cycle compartmentalized within hepatocytes?

The first two reactions occur in the mitochondrial matrix, while the remaining reactions occur in the cytosol.

Catabolism of which amino acids yields pyruvate?

Alanine, serine, glycine, cysteine, threonine, and tryptophan.

What does it mean for an amino acid to be glucogenic through pyruvate?

Its carbon skeleton can be degraded to pyruvate, which can then contribute to gluconeogenesis.

What is the first reaction and cofactor for many amino acids during amino acid catabolism?

Transamination using pyridoxal phosphate (PLP).

What happens during the initial transamination step of amino acid catabolism?

An amino acid transfers its amino group, usually to α-ketoglutarate, producing glutamate + the corresponding α-keto acid, using PLP.

Which amino acid transports amino groups from muscle to the liver in a nontoxic form?

Alanine.

How does alanine safely transport nitrogen from muscle to liver?

Muscle transfers an amino group to pyruvate to produce alanine, which carries both the amino nitrogen and pyruvate carbon skeleton to the liver.

What is the function of tetrahydrofolate (THF) and its derivatives in metabolism?

Transfer of one-carbon units in different oxidation states.

Why is tetrahydrofolate important in biosynthesis?

THF carries one-carbon units used in pathways including purine, thymidylate, methionine, glycine, and serine metabolism.

Which enzyme is critical for activating pancreatic zymogens during protein digestion in the small intestine?

Enteropeptidase (enterokinase).

How does enteropeptidase initiate pancreatic protease activation?

Enteropeptidase converts trypsinogen → trypsin, and trypsin then activates additional trypsinogen and other pancreatic zymogens.

What substrate is used to synthesize methionine by methionine synthase?

Homocysteine.

What reaction does methionine synthase catalyze?

Homocysteine → methionine using a methyl group from 5-methyl-THF and vitamin B12 as a cofactor.

How many ATP are used in urea formation?

3 ATP, equivalent to 4 high-energy phosphate bonds.

Why does urea synthesis use 3 ATP but 4 high-energy phosphate bonds?

Two ATP become ADP, while one ATP becomes AMP + PPi, so the cycle consumes four phosphoanhydride bonds.

Which two amino acids are exclusively ketogenic?

Leucine and lysine.

What does it mean for an amino acid to be exclusively ketogenic?

Its carbon skeleton produces acetyl-CoA and/or acetoacetate but cannot provide net carbon for glucose synthesis. Only leucine and lysine are exclusively ketogenic.

What coenzyme is required for all transamination reactions?

Pyridoxal phosphate (PLP; vitamin B6 derivative).

What role does pyridoxal phosphate play in aminotransferase reactions?

PLP forms a temporary intermediate with the substrate and accepts/transfers the amino group during transamination.

What signals stimulate the secretion of gastrin and secretin?

Gastrin: peptides/amino acids and stomach distension; secretin: acidic chyme in the duodenum.

How do gastrin and secretin respond differently to digestion?

Gastrin responds mainly to food and distension in the stomach and stimulates gastric acid, whereas secretin responds to low duodenal pH and stimulates bicarbonate secretion.

Metabolism of which amino acid is associated with serotonin production?

Tryptophan.

How is serotonin synthesized from an amino acid?

Tryptophan → 5-hydroxytryptophan → serotonin (5-HT) through hydroxylation followed by decarboxylation.

Which amino acid is used to make phosphocreatine?

Creatine is synthesized from glycine, arginine, and methionine; arginine provides the guanidino group.

How is phosphocreatine produced from amino acid metabolism?

Arginine + glycine form the creatine precursor, SAM derived from methionine provides a methyl group, and creatine is then phosphorylated to phosphocreatine.

Which compound is required for the synthesis of methionine from homocysteine?

5-Methyltetrahydrofolate (5-methyl-THF) provides the methyl group, with vitamin B12 required by methionine synthase.

How are folate and vitamin B12 involved in methionine synthesis?

5-Methyl-THF transfers its methyl group through B12 to homocysteine, producing methionine and regenerating THF.

What are the major bile pigments?

Bilirubin and biliverdin.

Where do bile pigments come from?

Heme degradation produces biliverdin, which is reduced to bilirubin for transport to and processing by the liver.

What is glutathione?

A tripeptide antioxidant composed of glutamate, cysteine, and glycine (γ-Glu-Cys-Gly).

What is the major biochemical function of glutathione?

Reduced glutathione (GSH) protects cells against oxidative damage by donating electrons to reduce reactive oxygen species and peroxides.

Which amino acid is used to make the plant hormone indole-3-acetate (auxin)?

Tryptophan.

How is tryptophan related to auxin biosynthesis?

The major natural auxin, indole-3-acetic acid (IAA), can be synthesized through pathways derived from tryptophan.

What is the precursor to phenylalanine in its biosynthetic pathway?

Prephenate is the immediate pathway precursor, derived from chorismate.

How is phenylalanine synthesized from the shikimate pathway?

Chorismate → prephenate → phenylalanine, with prephenate serving as the branch-point precursor for phenylalanine and tyrosine.

Which reaction step in proline synthesis is nonenzymatic?

Glutamate-γ-semialdehyde spontaneously cyclizes to Δ¹-pyrroline-5-carboxylate (P5C).

How is proline synthesized from glutamate?

Glutamate is converted to glutamate-γ-semialdehyde, which spontaneously cyclizes to P5C, followed by enzymatic reduction to proline.

What enzymes are involved in the assimilation of inorganic nitrogen into organic molecules?

Glutamine synthetase (GS) and glutamate synthase (GOGAT) are the major pathway.

How is inorganic ammonium incorporated into amino acid metabolism?

GS incorporates NH₄⁺ into glutamate to form glutamine, then GOGAT transfers glutamine's amide nitrogen to α-ketoglutarate to form glutamate.

How is nitrogen fixation regulated?

Primarily by oxygen and fixed-nitrogen availability, including regulation of nitrogenase expression/activity.

Why must nitrogen fixation be tightly regulated?

Nitrogenase requires large amounts of ATP and is inactivated by O₂, so cells activate nitrogen fixation mainly when usable nitrogen is scarce and protect nitrogenase from oxygen.

How do glutamine synthetase and glutamate synthase work together?

GS makes glutamine from glutamate + NH₄⁺; GOGAT transfers its amide nitrogen to α-ketoglutarate to make two glutamates.

What is the net effect of the GS-GOGAT pathway?

It incorporates NH₄⁺ into glutamate, the major amino-group donor for synthesis of many other amino acids.

Which amino acid gives rise to the biological messenger nitric oxide (NO)?

Arginine.

How is nitric oxide synthesized from arginine?

Nitric oxide synthase (NOS) converts arginine to citrulline + NO, producing the signaling molecule nitric oxide.

Biosynthesis of which amino acid is crippled if a cell cannot obtain tetrahydrofolate?

Methionine, because 5-methyl-THF supplies the methyl group for remethylation of homocysteine.

Why does tetrahydrofolate deficiency interfere with methionine synthesis?

5-Methyl-THF is the one-carbon donor used by methionine synthase to convert homocysteine into methionine.

What reversible covalent modification regulates bacterial glutamine synthetase?

Adenylylation/deadenylylation.

How does adenylylation regulate glutamine synthetase?

Adenylylation generally decreases GS activity, whereas deadenylylation increases its activity in response to cellular nitrogen status.

Which amino acid is used to make epinephrine and norepinephrine?

Tyrosine.

How are catecholamines synthesized from tyrosine?

Tyrosine → L-DOPA → dopamine → norepinephrine → epinephrine.

What is shikimate, and where does it occur?

An intermediate in the shikimate pathway of plants, fungi, and many microorganisms used to synthesize aromatic amino acids.

What is the significance of the shikimate pathway?

It produces chorismate, the common precursor for the aromatic amino acids phenylalanine, tyrosine, and tryptophan; animals lack this pathway.

What is chorismate, and where does it occur?

The branch-point product of the shikimate pathway in plants, fungi, and microorganisms.

Why is chorismate important for amino acid biosynthesis?

Chorismate is the common metabolic precursor that branches toward synthesis of phenylalanine, tyrosine, and tryptophan.

Which amino acid provides the nitrogen atom in the indole ring of tryptophan?

Glutamine.

How does glutamine contribute nitrogen to tryptophan biosynthesis?

Anthranilate synthase uses glutamine as an amino-group donor during conversion of chorismate to anthranilate, ultimately supplying the indole nitrogen of tryptophan.

What is the anammox reaction?

Anaerobic oxidation of NH₄⁺ using NO₂⁻ to produce N₂ gas and water.

What is the biological significance of anammox?

Anaerobic ammonium-oxidizing bacteria combine NH₄⁺ + NO₂⁻ → N₂ + H₂O, removing biologically available nitrogen from ecosystems.

What enzymes are involved in incorporating ammonium into metabolism?

Glutamine synthetase, glutamate synthase, and glutamate dehydrogenase.

What are the major routes for incorporating NH₄⁺ into organic metabolism?

The high-affinity route is GS-GOGAT, while glutamate dehydrogenase can directly incorporate NH₄⁺ into α-ketoglutarate when ammonium concentrations are high.

Which cofactor or functional group in the nitrate reductase electron-transfer chain has the highest reduction potential?

The molybdenum cofactor (MoCo) nitrate-reduction center has the highest reduction potential.

How do electrons flow through the nitrate reductase electron-transfer chain?

Electrons flow toward progressively higher reduction potentials, typically NAD(P)H → FAD → heme/cytochrome b → Mo cofactor → nitrate, where nitrate is reduced to nitrite.