Amino acid and protein metabolism

In short

Amino acid and protein metabolism covers how dietary and tissue proteins are digested and degraded to amino acids, how the amino (nitrogen) group is removed and disposed of as urea, and how the remaining carbon skeletons enter energy metabolism or build glucose, ketone bodies and specialised products such as heme, neurotransmitters, creatine and nucleotide bases. It matters because, unlike carbohydrate and fat, amino acids carry nitrogen that is toxic as ammonia, so nitrogen disposal is as important as energy yield. The topic is central to B.Sc/M.Sc Biochemistry papers, CSIR-UGC NET, GATE Biotechnology and Life Sciences (XL), ICMR/DBT-JRF and clinical lab entrance tests. Indian employers examine it in diagnostic roles (liver function tests, newborn screening), nutrition and food-protein work (FSSAI protein quality claims), and biopharma where amino acid feeds and protein degradation control fermentation yield.

Explained

Protein digestion, absorption and turnover

• Digestion: gastric HCl denatures protein and activates pepsinogen → pepsin (endopeptidase, acid pH optimum). Pancreatic trypsin (activated by enteropeptidase from trypsinogen, then autocatalytically), chymotrypsin, elastase and carboxypeptidases A/B complete it; intestinal aminopeptidases and dipeptidases finish at the brush border.
• Specificity: trypsin cleaves after Lys/Arg; chymotrypsin after aromatic residues (Phe, Tyr, Trp); elastase after small residues (Ala, Gly, Ser); carboxypeptidase A removes non-basic, B removes basic C-terminal residues.
• Absorption: Na⁺-dependent amino acid transporters and the PepT1 H⁺-coupled di/tripeptide transporter in enterocytes; transport into tissues partly via the γ-glutamyl cycle (glutathione-dependent). Defects: cystinuria (dibasic transporter), Hartnup disease (neutral amino acids, pellagra-like signs).
• Turnover: adult body protein ≈ 10-12 kg, of which 300-400 g turns over daily; only ~50-70 g/day is dietary, the rest is reused. Half-lives range from minutes (ornithine decarboxylase) to months (haemoglobin, collagen).
• Degradation routes: (i) ubiquitin-proteasome system — ATP-dependent, E1 activating, E2 conjugating, E3 ligase enzymes tag Lys residues with polyubiquitin; the 26S proteasome (20S core + 19S caps) degrades it. Selectivity follows the N-end rule and PEST sequences. (ii) Lysosomal/autophagic degradation — non-selective, for extracellular and long-lived proteins. (iii) Calcium-activated calpains, and caspases in apoptosis.
• Amino acid pool: free amino acids (~100 g) fed by diet, tissue breakdown and *de novo* synthesis; drained by protein synthesis, nitrogen excretion and special products. There is no storage form of protein — all body protein is functional.
• Nitrogen balance: intake − excretion. Positive in growth, pregnancy, recovery; negative in trauma, infection, starvation, and whenever one essential amino acid is limiting.
• Essential (indispensable): Val, Leu, Ile, Thr, Met, Phe, Trp, Lys, His (+ Arg semi-essential in growth). Conditionally essential: Cys (from Met), Tyr (from Phe), Arg, Gln, Gly, Pro. ICMR-NIN RDA 2020 recommends ~0.83 g/kg/day good-quality protein for Indian adults; cereal-pulse combinations (rice-dal, khichdi) give mutual supplementation because cereals lack Lys and pulses lack Met.

Removal of the amino group: transamination and deamination

• Transamination: α-amino group transferred to α-ketoglutarate, forming glutamate + a new keto acid. Enzymes: aminotransferases (transaminases), coenzyme pyridoxal phosphate (PLP, vitamin B6) working by a ping-pong bi-bi mechanism through a pyridoxamine phosphate intermediate and Schiff-base (aldimine/ketimine) chemistry.
• Key pairs: ALT (SGPT): alanine + α-KG ⇌ pyruvate + glutamate; AST (SGOT): aspartate + α-KG ⇌ oxaloacetate + glutamate. ΔG ≈ 0, so fully reversible and also used for amino acid synthesis.
• Lys, Thr, Pro and Hyp are not transaminated.
• Clinical: serum ALT/AST are routine liver function tests; ALT is more liver-specific, AST also from heart and muscle. AST/ALT (De Ritis) ratio > 2 suggests alcoholic liver disease; < 1 typical of viral hepatitis and NAFLD.
• Oxidative deamination: glutamate dehydrogenase (GDH) in the mitochondrial matrix, the only enzyme using NAD⁺ or NADP⁺: glutamate + H₂O + NAD(P)⁺ → α-KG + NH₄⁺ + NAD(P)H. Allosterically inhibited by GTP/ATP, activated by ADP/GDP — so high energy charge slows nitrogen release.
• Together, transdeamination = transamination funnelling nitrogen into glutamate, then GDH releasing free ammonia for urea synthesis.
• Non-oxidative deamination: serine and threonine dehydratases (PLP), histidase, cysteine desulphydrase.
• Oxidases: L-amino acid oxidase (FMN, minor), D-amino acid oxidase (FAD, liver/kidney, handles D-isomers from bacteria and diet), both producing H₂O₂ destroyed by catalase.
• Decarboxylation (PLP): gives amines — histidine → histamine, glutamate → GABA, tryptophan → serotonin (via 5-HTP), tyrosine → dopamine → noradrenaline → adrenaline, ornithine → putrescine → spermidine/spermine (polyamines).
• Transport of nitrogen to liver: glutamine (glutamine synthetase in muscle/brain; glutaminase in liver and kidney) and the glucose-alanine (Cahill) cycle — muscle pyruvate is transaminated to alanine, carried to liver, reconverted to pyruvate for gluconeogenesis, nitrogen entering urea.

Urea cycle and ammonia toxicity

• Site: liver; first two steps mitochondrial, rest cytosolic. Discovered by Krebs and Henseleit (1932) — the first cyclic metabolic pathway described.
• Steps: 1) CPS-I: NH₄⁺ + CO₂ (as HCO₃⁻) + 2 ATP → carbamoyl phosphate (rate-limiting; obligate activator N-acetylglutamate, made by NAG synthase, stimulated by arginine). 2) OTC: carbamoyl phosphate + ornithine → citrulline (exported by ORNT1). 3) Argininosuccinate synthetase: citrulline + aspartate + ATP → argininosuccinate + AMP + PPi (two ATP equivalents). 4) Argininosuccinase (lyase) → arginine + fumarate. 5) Arginase-1 → urea + ornithine (regenerated).
• Stoichiometry: 2 NH₃ + CO₂ + 3 ATP → urea + 2 ADP + AMP + 2 Pi + PPi + H₂O; 4 high-energy phosphate bonds consumed. One nitrogen comes from free ammonia, the other from aspartate.
• Aspartate-argininosuccinate shunt links the urea cycle to the TCA cycle: fumarate → malate → oxaloacetate → (AST) aspartate, which re-enters the cycle.
• Regulation: short-term by N-acetylglutamate and substrate supply; long-term by induction of all five enzymes on a high-protein diet or in starvation (when tissue protein is catabolised).
• Hyperammonaemia: acquired (cirrhosis, portosystemic shunting, Reye syndrome) or inherited (urea cycle disorders, autosomal recessive except X-linked OTC deficiency, the commonest). Signs: vomiting, lethargy, tremor, slurred speech, cerebral oedema, coma.
• Mechanism of toxicity: ammonia drives glutamate → glutamine in astrocytes, depleting glutamate/α-KG (TCA cycle anaplerosis loss) and causing osmotic astrocyte swelling. CPS-I deficiency and OTC deficiency give low citrulline; OTC deficiency alone raises urinary orotic acid (carbamoyl phosphate spills into pyrimidine synthesis). Citrullinaemia (ASS), argininosuccinic aciduria (ASL), argininaemia (arginase).
• Management principles: restrict protein, give sodium benzoate and phenylbutyrate (conjugate glycine and glutamine for excretion), arginine/citrulline supplementation, dialysis in crisis. Treated in India at metabolic units of tertiary centres (AIIMS, SGPGI, CMC Vellore, NIMHANS); several states run newborn screening programmes under RBSK (Rashtriya Bal Swasthya Karyakram).
• Other nitrogen end-products: ureotelic (mammals — urea), ammonotelic (bony fish), uricotelic (birds, reptiles — uric acid).

Fate of carbon skeletons

• All 20 amino acid skeletons funnel into seven intermediates: pyruvate, acetyl-CoA, acetoacetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate.
• Purely ketogenic: Leu and Lys. Both gluco- and ketogenic: Phe, Tyr, Trp, Ile, Thr. All the rest are glucogenic — which is why body protein sustains gluconeogenesis in starvation.
• Entry points: → pyruvate (Ala, Ser, Cys, Gly, Thr, Trp); → α-KG (Glu, Gln, Pro, Arg, His); → succinyl-CoA (Met, Val, Ile, Thr); → fumarate (Phe, Tyr, Asp); → oxaloacetate (Asp, Asn); → acetyl-CoA/acetoacetate (Leu, Lys, Phe, Tyr, Trp, Ile).
• Branched-chain amino acids (BCAA — Val, Leu, Ile): mainly catabolised in muscle, not liver. Steps: BCAA aminotransferase → branched-chain α-keto acid dehydrogenase (BCKD) complex (same design as PDH: TPP, lipoate, FAD, NAD⁺, CoA) → dehydrogenation. BCKD defect = maple syrup urine disease (sweet-smelling urine, ketoacidosis, neurological damage; thiamine-responsive variants exist).
• Phenylalanine/tyrosine: Phe → Tyr by phenylalanine hydroxylase (PAH), needing tetrahydrobiopterin (BH4) and O₂; Tyr → homogentisate → maleylacetoacetate → fumarylacetoacetate → fumarate + acetoacetate.
  • PKU: PAH (or BH4 synthesis) defect; phenylalanine accumulates, phenylpyruvate/phenylacetate excreted (musty odour), intellectual disability, hypopigmentation. Treated by a low-Phe diet; aspartame must be avoided. Reported Indian incidence is low (roughly 1 in 18,000-60,000 in screening studies, far below the ~1 in 10,000 of European populations) — figures vary by region and study.
  • Alkaptonuria (homogentisate oxidase): urine darkens on standing, ochronosis, arthritis. Tyrosinaemia type I (fumarylacetoacetate hydrolase): liver and kidney damage. Albinism (tyrosinase): melanin not formed.
• Methionine and one-carbon metabolism: Met + ATP → SAM (S-adenosylmethionine), the universal methyl donor → SAH → homocysteine. Homocysteine is either remethylated (methionine synthase, needs methyl-B12 and N⁵-methyl-THF) or goes down transsulphuration (cystathionine β-synthase, PLP → cystathionine → cysteine). Defects of CBS, or B12/folate deficiency, cause homocystinuria/hyperhomocysteinaemia — a thrombosis and cardiovascular risk marker; B12 deficiency is common in Indian vegetarian diets.
• Folate one-carbon pool: THF carries formyl, methylene and methyl groups for purine, thymidylate, methionine and glycine/serine interconversion (serine hydroxymethyltransferase).
• Glycine: degraded by the glycine cleavage system; defect = non-ketotic hyperglycinaemia. Glycine also forms glyoxylate → oxalate (primary hyperoxaluria).
• Tryptophan: kynurenine pathway yields NAD⁺ precursor (60 mg Trp ≈ 1 mg niacin), serotonin → melatonin; maize-based diets low in Trp contribute to pellagra. Hartnup disease gives similar signs.
• Histidine → urocanate → formiminoglutamate → glutamate (FIGLU excretion is a folate-deficiency test).

Amino acid biosynthesis and specialised products

• Non-essential amino acids come from intermediates: Ala ← pyruvate; Asp ← oxaloacetate; Glu ← α-KG; Gln ← Glu + NH₃ (glutamine synthetase); Asn ← Asp + Gln; Ser ← 3-phosphoglycerate; Gly ← Ser; Cys ← Ser + homocysteine sulphur; Pro and Arg ← Glu/ornithine; Tyr ← Phe.
• Nitrogen fixation/assimilation (in plants and microbes): GS-GOGAT (glutamine synthetase-glutamate synthase) at low ammonia, GDH at high ammonia.
• Heme: glycine + succinyl-CoA → δ-aminolaevulinic acid (ALA synthase, PLP, rate-limiting, repressed by heme) → porphobilinogen → ... → protoporphyrin IX → heme (ferrochelatase). Defects = porphyrias (acute intermittent porphyria — PBG deaminase; porphyria cutanea tarda). Lead inhibits ALA dehydratase and ferrochelatase — basis of occupational lead-poisoning tests (ALA in urine, ZPP in blood).
• Heme catabolism: heme → biliverdin (heme oxygenase, releases CO) → bilirubin → albumin-bound (unconjugated) → glucuronidated by UGT1A1 → excreted. Raised unconjugated bilirubin = haemolysis, neonatal jaundice, Gilbert/Crigler-Najjar; raised conjugated = obstruction, hepatocellular disease.
• Creatine: Gly + Arg → guanidinoacetate (kidney) → creatine (liver, SAM methylation) → stored in muscle as creatine phosphate → spontaneous conversion to creatinine, excreted at a rate proportional to muscle mass (basis of creatinine clearance and eGFR).
• Other derivatives: glutathione (Glu-Cys-Gly, antioxidant); nitric oxide from arginine (NOS); thyroxine from tyrosine iodination; melanin from tyrosine; purines from Gly, Gln, Asp; pyrimidines from Gln and Asp; taurine from cysteine; carnitine from Lys + Met; histamine, GABA, catecholamines, serotonin.
• Protein synthesis link: amino acids are activated as aminoacyl-tRNAs; mTORC1 senses leucine and arginine and switches on translation, while fasting and glucocorticoids drive muscle proteolysis — the balance that defines muscle mass.

Clinical, nutritional and regulatory context in India

• Protein quality: measured by PDCAAS (protein digestibility-corrected amino acid score) and the newer DIAAS; FSSAI labelling rules govern "high protein"/"source of protein" claims and the use of RDA-based % contribution on packaged food.
• Protein-energy malnutrition: kwashiorkor (adequate calories, protein deficient — oedema, fatty liver, low albumin) and marasmus (overall energy deficit — severe wasting, no oedema). Managed under ICDS, POSHAN Abhiyaan and WHO/IAP severe-acute-malnutrition protocols using F-75/F-100 formulations.
• Key lab tests: serum total protein and albumin/globulin ratio, ALT/AST/ALP/GGT, blood urea and BUN, serum creatinine and eGFR, serum ammonia, urine amino acid chromatography, tandem mass spectrometry newborn screening, serum homocysteine, bilirubin fractions.
• Laboratory quality is accredited to NABL under ISO 15189; newborn screening and metabolic-disorder care are supported through RBSK and the Rare Diseases policy (NPRD, 2021, with centres of excellence).
• Biosafety and genetic-material work follow DBT/RCGM rules under the Environment (Protection) Act, 1986 Rules, 1989; clinical research under NDCT Rules, 2019 and ICMR ethical guidelines.

Key points

• Transamination - reversible PLP-dependent transfer of an amino group to α-ketoglutarate, making glutamate.
• Glutamate dehydrogenase - the hinge enzyme; only one that releases free ammonia using NAD⁺ or NADP⁺.
• Glutamine and alanine - the two non-toxic carriers that move nitrogen from muscle and brain to liver.
• Urea cycle - five steps; 2 NH₃ + CO₂ + 3 ATP → urea; costs 4 high-energy bonds; CPS-I is rate-limiting.
• N-acetylglutamate - obligatory allosteric activator of CPS-I; the switch that couples protein load to urea output.
• OTC deficiency - commonest urea cycle defect, X-linked, raised urinary orotic acid with hyperammonaemia.
• Ketogenic only - leucine and lysine; all others are at least partly glucogenic.
• BCKD complex - degrades branched-chain keto acids in muscle; defect gives maple syrup urine disease.
• Phenylalanine hydroxylase + BH4 - converts Phe to Tyr; defect gives PKU, treated by low-Phe diet.
• SAM - universal methyl donor from methionine; its cycle links B12, folate and homocysteine.
• Transsulphuration - CBS (PLP) route from homocysteine to cysteine; blocked in homocystinuria.
• ALA synthase - rate-limiting step of heme synthesis (Gly + succinyl-CoA); inhibited end-product by heme, blocked downstream by lead.
• Creatinine - non-enzymatic product of creatine phosphate, excreted in proportion to muscle mass; used for eGFR.
• Ubiquitin-proteasome system - ATP- and ubiquitin-dependent selective degradation of intracellular proteins.
• Nitrogen balance - intake minus loss; negative whenever any single essential amino acid is limiting.
• Mutual supplementation - cereal (Lys-poor) with pulse (Met-poor) gives a complete amino acid pattern.
• AST/ALT ratio - > 2 suggests alcoholic liver disease; < 1 is typical of viral hepatitis.
• Ammonia toxicity - astrocyte glutamine accumulation plus α-ketoglutarate depletion causing cerebral oedema.

Formulas and examples

Example 1 - Nitrogen balance of an adult. A 60 kg man eats 75 g protein/day; urinary N = 10.5 g, faecal N = 1.4 g, skin and other losses = 0.5 g.

• N intake = 75 / 6.25 = 12.0 g N/day
• Total N loss = 10.5 + 1.4 + 0.5 = 12.4 g N/day
• Balance = 12.0 − 12.4 = −0.4 g N/day → mildly negative; intake 75 g is above the 0.83 × 60 ≈ 49.8 g/day RDA, so the shortfall points to poor protein quality or increased catabolism rather than quantity.

Example 2 - Urea and BUN conversion. A patient's blood urea is 90 mg/dL (reference ~15-40 mg/dL).

• BUN = 90 × 28/60 = 42 mg/dL (reference ~7-19 mg/dL).
• Serum creatinine 1.2 mg/dL → BUN/creatinine = 42 / 1.2 = 35, well above the usual ~10-20, suggesting a pre-renal cause (dehydration or GI bleed) or a high protein load rather than intrinsic renal failure.

Example 3 - ATP cost of disposing of amino nitrogen. 12 g of urea nitrogen is excreted in a day.

• Urea excreted = 12 × 60/28 = 25.7 g = 25.7 / 60 = 0.429 mol.
• Each urea costs 4 high-energy phosphate bonds → 0.429 × 4 = 1.71 mol ATP equivalents.
• At ~30.5 kJ/mol, energy spent on nitrogen disposal ≈ 1.71 × 30.5 ≈ 52 kJ/day (~12 kcal) — small against a 2,000 kcal intake, which is why urea synthesis is affordable but still shows why the liver, not muscle, bears the cost.

Example 4 - Complete oxidation of alanine (ATP yield). Alanine → (ALT) pyruvate + glutamate; nitrogen goes to urea.

• Pyruvate → acetyl-CoA (NADH), acetyl-CoA through TCA: pyruvate oxidation yields 12.5 ATP (modern P/O values: NADH = 2.5, FADH₂ = 1.5).
• Glutamate → α-KG by GDH yields 1 NADH = 2.5 ATP; α-KG re-used as acceptor, so not counted again.
• Urea share for one nitrogen = 4/2 = 2 ATP spent.
• Net ≈ 12.5 + 2.5 − 2 = 13 ATP per alanine (against 32 for glucose) — amino acids are workable but costlier fuels.

Example 5 - Protein quality by PDCAAS. A wheat protein has 25 mg lysine/g protein; the reference pattern needs 45 mg/g lysine; true digestibility 0.88 (illustrative values).

• Amino acid score = 25 / 45 = 0.556; PDCAAS = 0.556 × 0.88 = 0.49 (truncated to 1.00 if above).
• Blending with a pulse supplying 68 mg lysine/g protein in a 1:1 protein ratio gives (25 + 68)/2 = 46.5 mg/g → score = 46.5/45 = 1.03 → truncated to 1.00; with digestibility 0.86 → PDCAAS ≈ 0.86, showing how a rice/roti-dal meal rescues quality.

Nitrogen flow:  tissue protein → amino acids → (transamination) → glutamate
                glutamate → (GDH) → NH4+ ─┐
                muscle N → Gln / Ala ─────┴→ LIVER urea cycle → urea → kidney → urine
Carbon flow:    carbon skeleton → 7 entry points → TCA / gluconeogenesis / ketone bodies

Where it is used

• Diagnostic laboratories and hospitals (Dr Lal PathLabs, SRL, Metropolis, Thyrocare, AIIMS/PGI labs): LFT panels, urea-creatinine, ammonia, homocysteine, amino acid chromatography and tandem-MS newborn screening — roles as biochemist, lab technologist, quality officer under NABL/ISO 15189.
• Pharma and biopharma (Biocon, Serum Institute, Bharat Biotech, Syngene, Dr Reddy's): amino acid and yeast-extract feed design in fermentation, controlling protease and proteasome-mediated degradation of recombinant proteins, media optimisation for monoclonal antibodies.
• Nutrition, food and nutraceuticals (Amul, Nestlé India, ITC, Hindustan Unilever, whey and plant-protein brands): protein quality scoring, fortification, lysine/methionine supplementation, FSSAI-compliant protein claims.
• Animal feed and agribusiness (Godrej Agrovet, Suguna, Venky's): synthetic lysine, methionine and threonine addition to poultry feed — straight application of limiting-amino-acid theory.
• Clinical nutrition and public health: PEM management under ICDS/POSHAN Abhiyaan, renal and hepatic diets (protein restriction in hepatic encephalopathy and CKD), sports nutrition and BCAA/leucine use.
• Research and academia: CSIR labs (CCMB, IGIB, NIN Hyderabad), proteomics and protein-degradation research, inborn-error-of-metabolism genetics, drug targets such as proteasome inhibitors (bortezomib) in myeloma.

Interview questions

• Q: Why must nitrogen be removed before an amino acid can be used as fuel? A: The carbon skeleton enters the TCA cycle or gluconeogenesis, but the amino group would release toxic ammonia, so it is first transferred to glutamate and then disposed of as urea.
• Q: Which enzyme is rate-limiting in the urea cycle and how is it controlled? A: Carbamoyl phosphate synthetase I, which requires N-acetylglutamate as an obligatory allosteric activator; NAG rises with arginine and a high protein load.
• Q: How many ATP are spent per urea molecule? A: Three ATP are used but four high-energy phosphate bonds, because argininosuccinate synthetase cleaves ATP to AMP + PPi.
• Q: Why does OTC deficiency raise urinary orotic acid while CPS-I deficiency does not? A: In OTC deficiency carbamoyl phosphate accumulates and leaks to the cytosol into pyrimidine synthesis; in CPS-I deficiency carbamoyl phosphate is never formed.
• Q: Which amino acids are purely ketogenic? A: Leucine and lysine; all others yield at least some glucogenic intermediate.
• Q: Why is ALT more useful than AST for liver damage? A: ALT is largely confined to hepatocyte cytosol, whereas AST is also abundant in heart, skeletal muscle and red cells, so AST rises in non-hepatic injury too.
• Q: What is the role of PLP in amino acid metabolism? A: It is the coenzyme for transaminations, decarboxylations, deaminations, transsulphuration and ALA synthase, working through Schiff-base intermediates — hence the broad effects of B6 deficiency.
• Q: How do muscle and liver cooperate in the glucose-alanine cycle? A: Muscle transaminates pyruvate to alanine and exports it; the liver converts alanine back to pyruvate for gluconeogenesis and routes the nitrogen into urea.
• Q: Why does ammonia cause brain damage? A: Astrocytes convert glutamate to glutamine, which accumulates osmotically and swells the cell, while α-ketoglutarate withdrawal starves the TCA cycle and glutamate/GABA transmission is disturbed.
• Q: How does the body choose which protein to degrade? A: Mainly by ubiquitin tagging: E3 ligases recognise degradation signals such as N-terminal residues (N-end rule), PEST sequences, phosphorylation and misfolding, then the 26S proteasome degrades the tagged protein using ATP.
• Q: Why is a rice-dal combination nutritionally better than either alone? A: Cereals are limiting in lysine and pulses in methionine, so together they complete the essential amino acid pattern and raise PDCAAS.
• Q: Tyrosine is called non-essential, yet a PKU patient needs it in the diet. Why? A: Tyrosine is made only from phenylalanine by phenylalanine hydroxylase; if that enzyme is defective, tyrosine becomes dietarily essential.

Common mistakes

• Saying transamination releases ammonia — it only transfers the amino group; free ammonia comes from glutamate dehydrogenase and non-oxidative deaminases.
• Quoting the urea cycle as costing 3 ATP without noting that 4 high-energy bonds are used, and missing that only the first two steps are mitochondrial.
• Forgetting that the second urea nitrogen comes from aspartate, not from a second free ammonia molecule.
• Listing lysine or threonine as transaminated, or calling all amino acids glucogenic (leucine and lysine are not).
• Confusing blood urea with BUN (factor 0.467) or mixing mg/dL with mmol/L in renal calculations.
• Mixing up the inborn errors — PKU with alkaptonuria, or MSUD with methylmalonic acidaemia — and their marker metabolites.
• Assuming excess dietary protein is stored; there is no protein store, so surplus nitrogen becomes urea and the carbon becomes glucose or fat.

Related topics

Carbohydrate metabolism · Lipid metabolism · Nucleotide metabolism · Hormones and signal transduction · Enzymes and enzyme kinetics · Vitamins and coenzymes