10. Amino Acid Metabolism

Transamination and the PLP Mechanism

Transamination is the reversible transfer of an amino group from an amino acid to an $\alpha$-keto acid, catalyzed by aminotransferases (transaminases). The reaction funnels nitrogen from multiple amino acids into glutamate, which then serves as the nitrogen donor for the urea cycle. Alexander Braunstein and Maria Kritzmann discovered this reaction in 1937 in pigeon breast muscle.

Pyridoxal Phosphate (PLP) Mechanism

All aminotransferases require pyridoxal phosphate (PLP), the active form of vitamin B$_6$, as a cofactor. PLP forms a Schiff base (internal aldimine) with a lysine residue in the enzyme active site. The mechanism proceeds through a ping-pong (double displacement) kinetic pattern:

Clinical Significance: AST and ALT

Two aminotransferases are of paramount clinical importance:

• Alanine Aminotransferase (ALT/GPT): predominantly liver-specific; catalyzes alanine + $\alpha$-ketoglutarate $\rightleftharpoons$ pyruvate + glutamate
• Aspartate Aminotransferase (AST/GOT): found in liver, heart, muscle, kidney; catalyzes aspartate + $\alpha$-ketoglutarate $\rightleftharpoons$ oxaloacetate + glutamate

The De Ritis ratio (AST/ALT) helps distinguish the etiology of liver disease: a ratio >2 suggests alcoholic hepatitis, while <1 suggests viral hepatitis (where ALT elevation predominates due to hepatocyte-specific release).

Derivation 1: Urea Cycle Energetics

The urea cycle, discovered by Hans Krebs and Kurt Henseleit in 1932 (the first cyclic metabolic pathway ever described), converts toxic ammonia into urea for excretion. It consists of 5 enzymatic reactions spanning the mitochondrial matrix and cytoplasm.

The Five Reactions

Overall Stoichiometry

Counting high-energy phosphate bonds: 2 ATP (CPS-I) + 2 equivalents (ASS: ATP $\rightarrow$ AMP + PP$_i$) = 4 ATP equivalents per urea. However, the fumarate produced can be recycled via fumarase to malate, then oxidized by malate dehydrogenase to generate NADH (2.5 ATP), partially offsetting the cost.

Ammonia Transport and Toxicity

Free ammonia (NH$_3$/NH$_4^+$) is highly toxic to the central nervous system. Normal blood ammonia is <50 $\mu$M; levels above 100 $\mu$M cause confusion, and >200 $\mu$M can cause coma and death. The toxicity mechanisms include:

• Glutamate depletion: excess NH$_4^+$ drives glutamate $\rightarrow$ glutamine (via glutamine synthetase in astrocytes), depleting the excitatory neurotransmitter pool and causing osmotic swelling
• $\alpha$-Ketoglutarate depletion: reductive amination depletes this TCA cycle intermediate, impairing mitochondrial energy production
• pH disruption: NH$_3$ + H$^+$ $\rightleftharpoons$ NH$_4^+$; ammonia acts as a pH buffer, disrupting transmembrane proton gradients

Safe Nitrogen Transport: Glutamine and Alanine

To avoid ammonia toxicity during inter-organ nitrogen transport, amino groups are carried as:

Derivation 2: Carbon Skeleton Fates — Glucogenic vs Ketogenic

After removal of the amino group, the remaining carbon skeleton ($\alpha$-keto acid) is directed to one of seven entry points in central metabolism:

The mnemonic for purely ketogenic amino acids is simple: "Leu and Lys lie only in the ketone lane". Of the 20 standard amino acids, 13 are purely glucogenic, 2 are purely ketogenic, and 5 (Ile, Phe, Thr, Trp, Tyr) are both. A glucogenic amino acid can produce net glucose via gluconeogenesis; a ketogenic amino acid cannot because there is no net pathway from acetyl-CoA to glucose in animals:

This is because the two carbons of acetyl-CoA entering the TCA cycle are lost as CO$_2$ (at the isocitrate dehydrogenase and $\alpha$-ketoglutarate dehydrogenase steps) before reaching oxaloacetate, the gluconeogenic precursor.

One-Carbon Metabolism: The Methionine Cycle and Folate

One-carbon metabolism links amino acid metabolism to nucleotide synthesis, epigenetics (DNA methylation), and redox balance. The central metabolite is S-adenosylmethionine (SAM), the universal methyl donor for >100 methyltransferase reactions.

The Methionine Cycle

Alternatively, homocysteine is irreversibly committed to the transsulfuration pathway (requiring vitamin B$_6$):

Hyperhomocysteinemia: A Cardiovascular Risk Factor

Elevated plasma homocysteine (>15 $\mu$mol/L) is an independent risk factor for cardiovascular disease, venous thromboembolism, and neural tube defects. Causes include:

• MTHFR C677T polymorphism: ~10% of population is homozygous (TT), reducing enzyme activity by ~70%; the most common genetic cause of mild hyperhomocysteinemia
• Vitamin B$_{12}$ deficiency: impairs methionine synthase, trapping folate as N$^5$-methyl-THF (the "methyl trap" hypothesis)
• Folate deficiency: insufficient N$^5$-methyl-THF for homocysteine remethylation
• CBS deficiency (homocystinuria): severe elevation (>100 $\mu$mol/L), causing premature atherosclerosis and thrombosis

The connection between folate, B$_{12}$, and homocysteine explains why periconceptional folic acid supplementation (400 $\mu$g/day) reduces neural tube defects by ~70% — one of the most successful public health interventions in biochemistry. Many countries now mandate folic acid fortification of grain products.

Organ-Specific Amino Acid Metabolism

Different organs have different capacities and preferences for amino acid catabolism, reflecting their unique metabolic roles:

Derivation 3: Glutamate Dehydrogenase and Nitrogen Balance

Glutamate dehydrogenase (GDH) catalyzes the reversible oxidative deamination of glutamate, releasing free NH$_4^+$ for the urea cycle:

The equilibrium constant relates the reactant and product concentrations:

This very small $K_{eq}$ means the reaction strongly favors glutamate formation. In the liver (where [NAD$^+$]/[NADH] is high in the mitochondrial matrix), the reaction is driven in the oxidative deamination direction. GDH is allosterically regulated: GTP inhibits (high energy) and ADP activates (low energy), linking nitrogen disposal to the cell's energy status.

Nitrogen Balance

In a healthy adult, nitrogen intake (dietary protein) equals nitrogen excretion (primarily as urea). The nitrogen balance equation is:

The factor 6.25 reflects that protein is ~16% nitrogen by mass. Positive nitrogen balance indicates growth or recovery; negative balance indicates muscle wasting (catabolism exceeds anabolism).

Derivation 4: Inborn Errors of Amino Acid Metabolism

Archibald Garrod first articulated the concept of "inborn errors of metabolism" in 1908, proposing that inherited enzyme deficiencies cause accumulation of metabolic intermediates. Amino acid disorders exemplify this principle.

Phenylketonuria (PKU)

PKU (incidence ~1:10,000) results from deficiency of phenylalanine hydroxylase (PAH), which requires the cofactor tetrahydrobiopterin (BH$_4$):

When PAH is deficient, phenylalanine accumulates (>1200 $\mu$mol/L vs normal <120 $\mu$mol/L) and is shunted into alternative pathways: transamination to phenylpyruvate, reduction to phenyllactate, and decarboxylation to phenylacetate. These metabolites are excreted in urine (the "mousy odor") and cause intellectual disability if untreated.

Maple Syrup Urine Disease (MSUD)

MSUD (~1:185,000) results from deficiency of the branched-chain $\alpha$-keto acid dehydrogenase complex (BCKDH), which normally decarboxylates the $\alpha$-keto acids of leucine, isoleucine, and valine:

Accumulation of branched-chain amino acids and their keto acids causes neurological damage. The urine has a characteristic sweet, maple syrup-like odor from the $\alpha$-keto acids.

Homocystinuria

Most commonly caused by cystathionine $\beta$-synthase (CBS) deficiency (~1:200,000), which normally catalyzes the condensation of homocysteine with serine:

Elevated homocysteine promotes thrombosis (by damaging endothelium), lens dislocation (downward, in contrast to Marfan syndrome where the lens dislocates upward), and intellectual disability. Some patients respond to high-dose vitamin B$_6$ (pyridoxine), which stabilizes the mutant CBS enzyme.

Major Amino Acid Degradation Pathways

The 20 amino acids are degraded by pathways of varying complexity. Here we examine the major routes connecting amino acid catabolism to central metabolic intermediates.

Pyruvate Family (Ala, Cys, Gly, Ser, Thr, Trp)

These amino acids converge on pyruvate, which can enter the TCA cycle via pyruvate dehydrogenase or be used for gluconeogenesis. The simplest pathway is alanine aminotransferase:

Serine dehydratase directly converts serine to pyruvate (PLP-dependent, $\beta$-elimination):

Glycine is catabolized primarily by the glycine cleavage system (GCS) in liver mitochondria, a multienzyme complex that oxidatively decarboxylates glycine to CO$_2$, NH$_4^+$, and a one-carbon unit (N$^5$,N$^{10}$-methylene-THF). GCS deficiency causes nonketotic hyperglycinemia — a devastating neonatal disorder characterized by seizures, apnea, and intellectual disability due to glycine's action as an excitatory co-agonist at NMDA receptors.

$\alpha$-Ketoglutarate Family (Arg, Glu, Gln, His, Pro)

These amino acids are converted to glutamate, then oxidatively deaminated by GDH to $\alpha$-ketoglutarate. The histidine pathway is notable because it involves elimination of ammonia to form urocanate, followed by hydration and ring-opening to form N-formiminoglutamate (FIGLU). Transfer of the formimino group to THF yields glutamate. FIGLU accumulation in urine is a diagnostic marker for folate deficiency (the FIGLU test).

Succinyl-CoA Family (Ile, Met, Thr, Val)

The branched-chain amino acids isoleucine and valine (along with odd-chain fatty acid-derived propionyl-CoA) converge on methylmalonyl-CoA, which requires vitamin B$_{12}$ for conversion to succinyl-CoA. Methionine degradation produces homocysteine (via SAM $\rightarrow$ SAH) and then either enters the transsulfuration pathway (to cysteine) or is remethylated to methionine. The propionyl-CoA/methylmalonyl-CoA pathway is shared with odd-chain fatty acid oxidation:

Phenylalanine and Tyrosine Degradation

Phenylalanine is first hydroxylated to tyrosine (PAH), then tyrosine is degraded through a 5-step pathway producing fumarate (glucogenic) and acetoacetate (ketogenic), making these amino acids both glucogenic and ketogenic:

Deficiency of homogentisate 1,2-dioxygenase (HGD) causes alkaptonuria — the first genetic disease recognized by Garrod (1902). Homogentisic acid accumulates, polymerizes in connective tissues (ochronosis, dark coloration), and is excreted in urine (turns dark on standing). It is generally a benign condition but can cause arthropathy in later life.

Derivation 5: The Glucose-Alanine Cycle

During fasting and exercise, muscle protein is catabolized and the amino groups must be transported to the liver for urea synthesis. Direct transport of NH$_4^+$ in the blood would be toxic (normal blood [NH$_4^+$] < 50 $\mu$M). Two safe carrier molecules are used: glutamine (from most tissues) and alanine (from muscle).

The Cycle Mechanism

In muscle, pyruvate (from glycolysis) accepts the amino group from glutamate via ALT:

Alanine is released into the blood and taken up by the liver, where the reverse transamination occurs:

The pyruvate in the liver is used for gluconeogenesis, and the resulting glucose is sent back to muscle. The glutamate is oxidatively deaminated by GDH, feeding NH$_4^+$ into the urea cycle. The net effect is an inter-organ transfer of nitrogen (muscle $\rightarrow$ liver) and carbon (liver $\rightarrow$ muscle), similar to the Cori cycle but involving amino groups.

Glutamine as a Nitrogen Carrier

While the glucose-alanine cycle operates between muscle and liver, most other tissues use glutamine as their primary nitrogen carrier. Glutamine synthetase (brain, muscle, lung) converts glutamate + NH$_4^+$ to glutamine, which is transported to the liver or kidney. In the liver, glutaminase releases NH$_4^+$ for the urea cycle. In the kidney, glutaminase releases NH$_4^+$ for urinary excretion during acidosis — an important mechanism for acid-base homeostasis:

During metabolic acidosis (diabetic ketoacidosis, lactic acidosis), renal glutaminase expression is upregulated 5-10 fold, increasing NH$_4^+$ excretion and HCO$_3^-$ generation — a crucial compensatory mechanism. The glutamine-derived $\alpha$-ketoglutarate can also be used for gluconeogenesis, contributing up to 50% of hepatic glucose output during prolonged fasting.

Energetic Comparison with the Cori Cycle

Like the Cori cycle, the glucose-alanine cycle has a net energy cost borne by the liver:

Additionally, the liver expends 4 ATP equivalents per urea molecule to dispose of the two nitrogen atoms. The total liver cost is 6 (gluconeogenesis) + 4 (urea cycle) = 10 ATP per cycle.

Applications in Medicine and Research

Historical Context

Hans Krebs proposed the urea cycle in 1932 using liver slices from rats — one year before his discovery of the citric acid cycle. This was a landmark in metabolic biochemistry: the first demonstration that a biosynthetic pathway could operate cyclically, with ornithine serving as the catalytic carrier molecule (analogous to oxaloacetate in the TCA cycle). Krebs received the Nobel Prize in 1953 for the citric acid cycle, but the urea cycle was arguably his more innovative contribution.

Archibald Garrod's 1908 insight that alkaptonuria represented a "chemical individuality" due to a specific enzyme deficiency was decades ahead of its time — predating the one gene-one enzyme hypothesis of Beadle and Tatum (1941) by 33 years. His four original inborn errors (alkaptonuria, albinism, cystinuria, pentosuria) remain textbook examples today. The expansion of newborn screening from PKU alone (Guthrie, 1963) to ~50 conditions via tandem MS/MS represents one of the most impactful applications of biochemistry to public health.

Amino Acid Metabolism in Cancer

Cancer cells have altered amino acid metabolism to support their proliferative demands:

• Glutamine addiction: many tumors consume glutamine at rates far exceeding normal cells (MYC-driven upregulation of glutaminase and glutamine transporters). Glutamine provides nitrogen for nucleotide synthesis, carbon for the TCA cycle (anaplerosis), and reducing equivalents (NADPH via malic enzyme).
• Serine/glycine biosynthesis: PHGDH (phosphoglycerate dehydrogenase) is amplified in breast and melanoma cancers; diverts glycolytic intermediates into serine synthesis for one-carbon units needed in nucleotide and methylation reactions.
• Asparagine dependence: acute lymphoblastic leukemia (ALL) cells cannot synthesize asparagine (low asparagine synthetase). L-asparaginase (from E. coli or Erwinia) depletes plasma asparagine, selectively killing ALL cells — a cornerstone of ALL chemotherapy since the 1960s.
• Arginine deprivation: some tumors (hepatocellular carcinoma, melanoma) lack argininosuccinate synthetase and depend on exogenous arginine. ADI-PEG 20 (pegylated arginine deiminase) depletes plasma arginine as a targeted therapy.

Python Simulations

Branched-Chain Amino Acid Metabolism

The branched-chain amino acids (BCAAs) — leucine, isoleucine, and valine — constitute ~35% of essential amino acids in muscle protein. Unlike most amino acids, BCAAs are primarily catabolized in skeletal muscle (not liver), because the first enzyme in their degradation (branched-chain aminotransferase, BCAT) is highly expressed in muscle but low in liver.

The Two-Step Initial Catabolism

The branched-chain $\alpha$-keto acid dehydrogenase complex (BCKDH) is the rate-limiting step and is structurally analogous to pyruvate dehydrogenase and $\alpha$-ketoglutarate dehydrogenase (all three use TPP, lipoamide, FAD, NAD$^+$, and CoA). BCKDH is regulated by phosphorylation: a specific BCKDH kinase inactivates it (phosphorylation of E1$\alpha$), and a BCKDH phosphatase activates it.

Leucine: A Unique Signaling Amino Acid

Beyond its role as a protein building block, leucine is a potent activator of mTORC1 (mechanistic target of rapamycin complex 1), the master regulator of protein synthesis and cell growth. Leucine activates mTORC1 through the Sestrin2/GATOR2 pathway:

This explains why leucine-rich foods (whey protein, eggs) are particularly effective at stimulating muscle protein synthesis — a key consideration for athletes and for preventing sarcopenia in the elderly. The recommended leucine threshold for maximal mTORC1 activation is ~2-3 g per meal.

BCAAs in Disease

Elevated plasma BCAAs are a biomarker for insulin resistance and type 2 diabetes risk (discovered in metabolomics studies). The mechanism is debated, but may involve BCAA-derived metabolites (3-hydroxyisobutyrate) promoting lipid accumulation in muscle, impairing insulin signaling. Conversely, BCAA supplementation is used to treat hepatic encephalopathy (competes with aromatic amino acids for brain uptake, normalizing the Fischer ratio).

Amino Acids as Neurotransmitter Precursors

Several amino acids serve as precursors for neurotransmitters and other bioactive amines, connecting amino acid metabolism directly to brain function and psychiatric disease:

Note the recurring importance of PLP (vitamin B$_6$) and tetrahydrobiopterin (BH$_4$) in these pathways. B$_6$ deficiency can cause seizures (impaired GABA synthesis) and peripheral neuropathy. BH$_4$ deficiency mimics PKU and can cause severe neurological disease even with normal PAH levels (because BH$_4$ is also required for Tyr and Trp hydroxylases).