Comprehensive Biochemistry and Pathophysiology of Amino Acid Metabolism and the Urea Cycle

Vocabulary practice flashcards covering amino acid classification, protein digestion, transamination, the urea cycle, genetic disorders of amino acid metabolism (PKU, MSUD, cystinuria), one-carbon transfers, and neurotransmitter biosynthesis.

Zero Nitrogen Balance

A metabolic state where nitrogen intake equals nitrogen excretion ($\text{anabolism} = \text{catabolism}$), characteristic of a mature, healthy individual.

Positive Nitrogen Balance

A metabolic state where nitrogen intake exceeds nitrogen excretion ($\text{anabolism} > \text{catabolism}$), occurring during periods of growth, pregnancy, or recovery from starvation and wasting diseases.

Negative Nitrogen Balance

A fragile metabolic state where nitrogen excretion exceeds nitrogen intake ($\text{catabolism} > \text{anabolism}$), observed during starvation, acute injury or infection, senescence, essential amino acid deficiencies, and chronic diseases like diabetes mellitus.

Sarcopenia

The progressive loss of skeletal muscle tissue and strength associated with aging that begins around age 30, which can be largely mitigated or reversed through exercise.

Essential Amino Acids

Amino acids whose carbon skeletons cannot be synthesized by the human body (or not synthesized fast enough to meet demand) and therefore must be supplied through dietary protein.

Conditionally Essential Amino Acids

Amino acids that can be synthesized from essential amino acid precursors but cannot be produced from scratch (such as tyrosine derived from phenylalanine, and cysteine derived from methionine).

Complementary Food Combinations

Dietary combinations of foods that balance out amino acid deficiencies (e.g., pairing legumes that are limited in methionine and cysteine with grains, nuts, or seeds that are high in them).

Pepsin

The primary gastric endopeptidase secreted by stomach chief cells as the inactive zymogen pepsinogen, which is autocatalytically activated by the low $\text{pH}$ environment maintained by parietal cell $\text{HCl}$ secretion.

Cystinuria

The most common genetic defect of amino acid transport (incidence of $1$ in $7000$), characterized by impaired renal and intestinal resorption of dibasic amino acids (cystine, ornithine, arginine, and lysine), resulting in insolubility-induced cystine kidney stones.

Transamination

The enzymatic transfer of an $\alpha$-amino group from an amino acid to an $\alpha$-keto acid (typically $\alpha$-ketoglutarate to yield L-glutamate), catalyzed by pyridoxal phosphate-dependent aminotransferases.

Pyridoxal Phosphate (PLP)

The active cofactor derived from vitamin $\text{B}_6$ covalently linked to aminotransferases, which temporarily accepts an amino group from an amino acid donor to become pyridoxamine phosphate (PMP) before transferring it to an $\alpha$-keto acid acceptor.

Aspartate Aminotransferase (AST)

An aminotransferase highly expressed in hepatocytes and muscle tissue that catalyzes the interconversion of aspartate and $\alpha$-ketoglutarate to oxaloacetate and glutamate; serves as a serum diagnostic marker for hepatic or muscular injury (normal range: $0 - 30\,\text{units/L}$).

Alanine Aminotransferase (ALT)

An aminotransferase that catalyzes the reversible transamination reaction between alanine and $\alpha$-ketoglutarate to form pyruvate and glutamate.

Glutamate Dehydrogenase

A mitochondrial matrix enzyme that catalyzes the reversible oxidative deamination of L-glutamate into $\alpha$-ketoglutarate and free $\text{NH}_4^+$, allosterically inhibited by high levels of ATP and GTP and stimulated by ADP and GDP.

Urea

The non-toxic, water-soluble end product of amino acid nitrogen disposal synthesized solely in the liver, accounting for approximately $90\%$ of urinary nitrogen.

Carbamoyl Phosphate Synthetase I (CPS I)

The mitochondrial enzyme that catalyzes the first and rate-limiting step of the urea cycle by condensing $\text{NH}_4^+$ and $\text{HCO}_3^-$ into carbamoyl phosphate using $2\,\text{ATP}$, strictly requiring $N$-acetylglutamate for activation.

N-Acetylglutamate (NAG)

The obligate allosteric activator of carbamoyl phosphate synthetase I, synthesized from acetyl-CoA and glutamate by $N$-acetylglutamate synthase in an activation step stimulated by arginine.

Ornithine Transcarbamoylase (OTC)

A mitochondrial urea cycle enzyme that catalyzes the reaction between carbamoyl phosphate and L-ornithine to produce L-citrulline.

Argininosuccinate Synthetase

A cytosolic urea cycle enzyme that condenses L-citrulline with L-aspartate to synthesize argininosuccinate at the expense of converting ATP to AMP and $\text{PP}_i$.

Argininosuccinase (Argininosuccinate Lyase)

A cytosolic urea cycle enzyme that cleaves argininosuccinate into L-arginine and fumarate.

Arginase

The cytosolic enzyme catalyzing the final step of the urea cycle, cleaving L-arginine to release free urea and regenerate L-ornithine for transport back into the mitochondria.

Aspartate-Argininosuccinate Shunt

The metabolic pathway linking the urea cycle and citric acid cycle, wherein fumarate released by argininosuccinase is converted to malate, brought into the mitochondria or converted to oxaloacetate, and transaminated to regenerate aspartate.

Hyperammonemia

A toxic accumulation of ammonia in the blood caused by hepatic disease or genetic enzyme deficiencies in the urea cycle, frequently causing central nervous system toxicity, flapping tremor (asterixis), and hepatic encephalopathy.

Sodium Benzoate

A therapeutic compound administered in hyperammonemia that conjugates with glycine to form hippurate, which is subsequently excreted in urine to eliminate nitrogen.

Phenylbutyrate

A therapeutic prodrug converted into phenylacetate, which conjugates with glutamine to generate phenylacetylglutamine for urinary excretion as an alternative route for nitrogen removal.

Glucogenic Amino Acids

Amino acids that are degraded to citric acid cycle intermediates (pyruvate, $\alpha$-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate) whose carbon skeletons can serve as precursors for glucose synthesis.

Ketogenic Amino Acids

Amino acids degraded directly to acetyl-CoA or acetoacetyl-CoA, which can produce ketone bodies (leucine and lysine are purely ketogenic; isoleucine, phenylalanine, tryptophan, tyrosine, and threonine are both ketogenic and glucogenic).

Phenylketonuria (PKU)

An inherited defect in phenylalanine catabolism caused primarily ($99\%$ of cases) by a mutation in phenylalanine hydroxylase or ($1\%$ of cases) dihydrobiopterin reductase, resulting in blood phenylalanine levels reaching up to $1000\,\mu\text{M}$ (or $\text{mM}$ scale), formation of phenylpyruvate/phenylacetate/phenyllactate, and central nervous system damage if left untreated.

Tetrahydrobiopterin ($\text{BH}_4$)

The unconjugated pterin cofactor required by phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase, which is oxidized to dihydrobiopterin ($\text{BH}_2$) and regenerated by dihydrobiopterin reductase using NADH.

Maple Syrup Urine Disease (MSUD)

A genetic metabolic disorder with an approximate incidence of $1$ in $185,000$ live births (or $< 0.4$ per $100,000$) caused by a defect in the branched-chain $\alpha$-keto acid dehydrogenase complex, causing elevated plasma and urinary levels of leucine, isoleucine, valine, and their corresponding $\alpha$-keto acids.

Branched-Chain $\alpha$-Keto Acid Dehydrogenase (BCKAD) Complex

A multienzyme complex comprising three catalytic and two regulatory subunits that performs oxidative decarboxylation on the $\alpha$-keto acid intermediates of valine, isoleucine, and leucine into acyl-CoA derivatives.

Tetrahydrofolate ($\text{H}_4\text{folate}$ / FH4)

A versatile coenzyme composed of 6-methylpterin, $p$-aminobenzoate, and glutamate that carries one-carbon fragments at oxidation levels from methyl to methenyl on positions $\text{N}^5$, $\text{N}^{10}$, or both.

S-Adenosylmethionine (AdoMet / SAM)

A principal methyl donor synthesized from methionine and ATP by methionine adenosyltransferase (MAT), which transfers methyl groups to DNA, RNA, proteins, and precursors of epinephrine.

$\gamma$-Aminobutyrate (GABA)

An inhibitory neurotransmitter in the central nervous system synthesized through the irreversible decarboxylation of L-glutamate by the enzyme glutamate decarboxylase in the presence of PLP.

Serotonin

A neurotransmitter synthesized from tryptophan via a two-step sequence: hydroxylation by tryptophan hydroxylase (utilizing $\text{BH}_4$) to form 5-hydroxytryptophan, followed by decarboxylation by aromatic amino acid decarboxylase (utilizing PLP).

General Structure of an $\alpha$-Amino Acid

A biochemical monomer consisting of a central $\alpha$-carbon covalently linked to an amino group ($-\text{NH}_3^+$), a carboxylate group ($-\text{COO}^-$), a hydrogen atom, and a distinguishing side-chain ($\text{R}$ group).