Biochemistry, Bioenergetics & Inborn Errors of Metabolism

Every metabolic pathway on USMLE and COMLEX examinations is anchored by a single irreversible rate-limiting enzyme. Glycolysis is governed by PFK-1; gluconeogenesis is governed by fructose-1,6-bisphosphatase (and requires four irreversible bypass enzymes: pyruvate carboxylase in mitochondria requiring biotin B7, PEP carboxykinase in cytosol, fructose-1,6-bisphosphatase in cytosol, and glucose-6-phosphatase in the endoplasmic reticulum of liver and kidney—absent in skeletal muscle). The TCA cycle is regulated by isocitrate dehydrogenase (activated by ADP, inhibited by ATP and NADH), while the HMP shunt (pentose phosphate pathway) is regulated by glucose-6-phosphate dehydrogenase (G6PD), which generates NADPH for glutathione reduction, reductive biosynthesis of fatty acids and steroids, cytochrome P450 metabolism, and the phagocytic respiratory burst.

Oxidative phosphorylation across the inner mitochondrial membrane couples electron transfer through Complexes I–IV to proton pumping into the intermembrane space, driving Complex V (ATP synthase). Complex I (NADH dehydrogenase) is inhibited by rotenone, MPP+, and barbiturates; Complex II (succinate dehydrogenase, the only TCA enzyme embedded in the inner mitochondrial membrane, passing electrons via FADH2 without pumping protons) is unobstructed by rotenone; Complex III (cytochrome bc1) is inhibited by antimycin A; and Complex IV (cytochrome c oxidase) is inhibited by cyanide, carbon monoxide, andazide. Cyanide binds ferric iron (Fe3+) in Complex IV, halting electron transport and producing bright red venous blood with severe lactic acidosis (treated with hydroxocobalamin or nitrites plus sodium thiosulfate). Uncouplers such as 2,4-dinitrophenol, toxic salicylate (aspirin) overdose, and thermogenin (UCP-1) in brown adipose tissue permeabilize the inner mitochondrial membrane to protons, collapsing the proton gradient so oxygen consumption and heat production surge while ATP synthesis plummets.

Type I (Von Gierke disease) results from glucose-6-phosphatase (GSD Ia) or glucose-6-phosphate translocase (GSD Ib, which adds neutropenia) deficiency in the liver and kidney; patients present at 3–4 months of age with severe fasting hypoglycemia, hepatorenomegaly, doll-like facies, severe lactic acidosis, hypertriglyceridemia, and hyperuricemia (gout), managed with frequent oral cornstarch and avoidance of fructose and galactose. Type II (Pompe disease) is the only lysosomal GSD, caused by acid alpha-1,4-glucosidase (acid maltase) deficiency; it presents in infancy with hypertrophic cardiomyopathy, macroglossia, profound hypotonia, and early death (“Pompe trashes the Pump”), with normal blood glucose. Type III (Cori disease) is caused by debranching enzyme (alpha-1,6-glucosidase) deficiency, leading to accumulation of cytosolic limit dextrin-like structures, milder fasting hypoglycemia and hepatomegaly, and normal blood lactate because gluconeogenesis remains intact. Type V (McArdle disease) results from skeletal muscle glycogen phosphorylase (myophosphorylase) deficiency; adolescents present with painful muscle cramps, rhabdomyolysis, myoglobinuria, and a flat venous lactate curve during ischemic forearm exercise, coupled with a characteristic “second-wind” phenomenon after 10 minutes as muscular blood flow delivers free fatty acids.

Essential fructosuria (fructokinase deficiency) is an asymptomatic, benign condition in which fructose spills into the urine. In contrast, Hereditary Fructose Intolerance (aldolase B deficiency) manifests after weaning when infants first ingest fruit, juice, or honey (sucrose = glucose + fructose); intracellular trapping of fructose-1-phosphate depletes ATP and inorganic phosphate, inhibiting both glycogenolysis and gluconeogenesis to cause severe hypoglycemia, vomiting, jaundice, cirrhosis, and proximal renal tubular acidosis (Fanconi syndrome). Similarly, galactokinase deficiency causes mild pediatric cataracts (due to aldose reductase converting galactose to galactitol in the lens) and failure to track objects or develop a social smile, whereas Classic Galactosemia (galactose-1-phosphate uridyltransferase [GALT] deficiency) presents in the first days of life upon initiating breast milk or lactose-containing formula with vomiting, hepatomegaly, jaundice, bilateral oil-drop cataracts, intellectual disability, and fatal neonatal Escherichia coli sepsis; immediate discontinuation of all milk and lactose products (switching to soy formula) is mandatory.

Gaucher disease is the most common lysosomal storage disorder, caused by beta-glucocerebrosidase (glucocerebrosidase) deficiency and accumulation of glucocerebroside; patients present with hepatosplenomegaly, pancytopenia, avascular necrosis of the femur, bone crises, and hallmark Gaucher cells (lipid-laden macrophages resembling crumpled tissue paper). Tay-Sachs disease (hexosaminidase A deficiency, GM2 ganglioside accumulation) and Niemann-Pick disease (sphingomyelinase deficiency, sphingomyelin accumulation) both present in Ashkenazi Jewish infants with progressive neurodegeneration and a macular cherry-red spot; however, Tay-Sachs has no hepatosplenomegaly and shows onion-skin lysosomes on electron microscopy, whereas Niemann-Pick features prominent hepatosplenomegaly and lipid-laden foam cells (zebra bodies). Fabry disease (X-linked alpha-galactosidase A deficiency, ceramide trihexoside accumulation) presents in childhood/adolescence with episodic burning neuropathic pain in the hands and feet (acroparesthesias), hypohidrosis, and clusters of reddish-purple angiokeratomas in the bathing-trunk distribution, progressing in adulthood to proteinuric renal failure and restrictive/hypertrophic cardiomyopathy. Krabbe disease (galactocerebrosidase deficiency) causes optic atrophy, peripheral neuropathy, developmental regression, and multinucleated globoid cells, while Metachromatic leukodystrophy (arylsulfatase A deficiency) causes cerebroside sulfate accumulation with central and peripheral demyelination, ataxia, and dementia.

Mucopolysaccharidoses impair the degradation of heparan sulfate and dermatan sulfate. Hurler syndrome (MPS I) is an autosomal recessive deficiency of alpha-L-iduronidase presenting with developmental delay, gargoylism (coarse facial features, frontal bossing), airway obstruction, hepatosplenomegaly, dysostosis multiplex, and prominent corneal clouding. Hunter syndrome (MPS II) is an X-linked recessive deficiency of iduronate-2-sulfatase presenting with milder developmental delay, aggressive behavioral hyperactivity, hepatosplenomegaly, and carpal tunnel syndrome, with no corneal clouding (remembered as “Hunters see clearly to aim for the X”).

Phenylketonuria (PKU) stems from autosomal recessive deficiency of phenylalanine hydroxylase (or its cofactor tetrahydrobiopterin [BH4], regenerated by dihydropteridine reductase), preventing conversion of phenylalanine to tyrosine (making tyrosine an essential dietary amino acid). Accumulated phenylketones produce a musty body odor, intellectual disability, microcephaly, seizures, and fair skin/eczema (due to competitive inhibition of tyrosinase); BH4 deficiency additionally depletes serotonin and catecholamines, requiring BH4 and neurotransmitter precursor supplementation alongside phenylalanine restriction (including avoidance of aspartame). Maple Syrup Urine Disease (MSUD) results from deficiency of the branched-chain alpha-ketoacid dehydrogenase complex (requiring B1, B2, B3, B5, and lipoic acid), blocking degradation of Isoleucine, Leucine, and Valine; neonates develop vomiting, poor feeding, lethargy, dystonia, and burnt-sugar/maple-syrup urine within days of birth (treated with dietary restriction of branched-chain amino acids and high-dose thiamine for thiamine-responsive variants). Alkaptonuria (homogentisate oxidase deficiency) causes benign dark/black urine upon standing, bluish-black ochronotic pigmentation of sclerae and ear cartilage, and debilitating large-joint ochronotic arthropathy. Homocystinuria (most commonly cystathionine synthase deficiency; treated with pyridoxine B6, folate, B12, and cysteine supplementation) presents with marfanoid habitus, downward-and-inward (inferomedial) ectopia lentis, intellectual disability, osteoporosis, and premature arterial/venous thromboembolism. Cystinuria is a defect in the PCT dibasic amino acid transporter (COAL: Cystine, Ornithine, Arginine, Lysine) causing recurrent radiopaque hexagonal cystine kidney stones that precipitate in acidic urine (diagnosed via positive urinary cyanide-nitroprusside test and treated with urinary alkalinization using potassium citrate, high fluid intake, and tiopronin/penicillamine).

Rate-limiting Carbamoyl phosphate synthetase I (CPS-I) requires N-acetylglutamate as an obligate allosteric activator. When hyperammonemia presents in a neonate (tachypnea, lethargy, vomiting, cerebral edema, and low BUN), measuring urinary orotic acid differentiates the two mitochondrial urea cycle defects: CPS-I deficiency (or N-acetylglutamate synthase deficiency) yields low carbamoyl phosphate and therefore normal/low urinary orotic acid, whereas X-linked Ornithine transcarbamylase (OTC) deficiency (the most common urea cycle disorder) causes mitochondrial carbamoyl phosphate to spill into the cytosolic pyrimidine synthesis pathway, producing markedly elevated urinary orotic acid with hyperammonemia (distinguished from Orotic Aciduria [UMP synthase deficiency], which presents with elevated orotic acid, megaloblastic anemia unresponsive to B12/folate, and normal ammonia). Acute hyperammonemia is treated with protein restriction, intravenous sodium benzoate or sodium phenylbutyrate (which scavenge glycine and glutamine for renal excretion), and arginine supplementation.

Lead poisoning inhibits two sulfhydryl-containing heme enzymes—cytosolic ALA dehydratase and mitochondrial ferrochelatase—leading to accumulation of delta-ALA and protoporphyrin IX, microcytic hypochromic sideroblastic anemia with coarse basophilic stippling (due to inhibition of ribonuclease), Burton lines on gingivae, metaphyseal lead lines on radiographs, wrist/foot drop, and abdominal colic. Acute Intermittent Porphyria (AIP) is an autosomal dominant deficiency of porphobilinogen (PBG) deaminase (hydroxymethylbilane synthase); accumulated ALA and PBG cause episodic severe abdominal pain (without peritonitis), autonomic tachycardia/hypertension, peripheral motor neuropathy, psychosis, and urine that turns dark port-wine red upon standing in light, with zero cutaneous photosensitivity. Treatment of acute AIP attacks requires intravenous hemin and high-dose IV glucose to transcriptionally repress hepatic ALA synthase. Conversely, Porphyria Cutanea Tarda (PCT) is the most common porphyria, caused by deficiency of uroporphyrinogen decarboxylase; uroporphyrin accumulates (producing tea-colored urine with coral-pink Wood lamp fluorescence) and causes chronic blistering cutaneous photosensitivity on sun-exposed dorsal hands and face with hypertrichosis, strongly associated with Hepatitis C, HIV, alcohol use, and hereditary hemochromatosis (treated with therapeutic phlebotomy or low-dose hydroxychloroquine).

Lesch-Nyhan syndrome is an X-linked recessive absence of HGPRT; failure to salvage hypoxanthine and guanine shunts PRPP and purine bases into xanthine oxidase catabolism, producing orange uric acid crystals in the diaper, juvenile gouty arthritis, urate nephropathy, choreoathetosis/dystonia, intellectual disability, and compulsive self-mutilating lip and finger biting (managed with allopurinol or febuxostat for hyperuricemia). Adenosine deaminase (ADA) deficiency is an autosomal recessive purine degradation defect in which excess dATP allosterically inhibits ribonucleotide reductase, arresting DNA synthesis in T, B, and NK lymphocytes and causing Severe Combined Immunodeficiency (SCID). In the de novo pyrimidine synthesis pathway, Hereditary Orotic Aciduria results from autosomal recessive deficiency of UMP synthase, presenting in children with urinary orotic acid crystalluria, failure to thrive, and megaloblastic anemia unresponsive to vitamin B12 or folate, with completely normal blood ammonia; treatment is oral uridine supplementation, which bypasses the enzymatic block and restores UTP feedback inhibition on carbamoyl phosphate synthetase II.

Vitamin A (retinol) deficiency causes nyctalopia (night blindness), dry conjunctivae (xerophthalmia), Bitot spots (foamy keratin debris on the bulbar conjunctiva), keratomalacia, and immunosuppression, whereas Vitamin A toxicity causes pseudotumor cerebri (idiopathic intracranial hypertension), alopecia, hepatotoxicity, and severe neural crest craniofacial/cardiac teratogenicity (requiring two forms of contraception before prescribing isotretinoin). Vitamin B1 (thiamine) deficiency impairs PDH, alpha-ketoglutarate dehydrogenase, transketolase, and branched-chain ketoacid dehydrogenase, causing Wernicke-Korsakoff syndrome and wet/dry beriberi. Vitamin B2 (riboflavin) deficiency causes cheilosis, angular stomatitis, glossitis, and corneal vascularization (the 2 C's). Vitamin B3 (niacin) deficiency (resulting from corn-based diets, isoniazid therapy, Hartnup disease [defective renal/GI neutral amino acid TRP transporter], or malignant carcinoid syndrome) causes Pellagra (Diarrhea, Dermatitis with a C3/C4 Casal necklace, Dementia). Vitamin B6 (pyridoxine) deficiency (induced by isoniazid or oral contraceptives) causes convulsions, hyperirritability, peripheral neuropathy, and sideroblastic anemia. Vitamin C (ascorbic acid) deficiency impairs prolyl/lysyl hydroxylase in collagen synthesis, causing Scurvy (swollen bleeding gums, perifollicular hemorrhages, corkscrew hairs, subperiosteal hematomas, and poor wound healing).

Osteogenesis Imperfecta is an autosomal dominant mutation in COL1A1/COL1A2 impairing Type I collagen triple-helix formation; patients present with multiple fractures after minimal trauma (frequently mistaken for non-accidental child abuse), blue sclerae (translucent choroidal veins), conductive hearing loss (otosclerosis-like ossicular fractures), and opalescent dentinogenesis imperfecta. Ehlers-Danlos syndrome encompasses hypermobile (Type V collagen), classical (Type V collagen with hyperextensible skin and atrophic cigarette-paper scars), and life-threatening vascular type (Type III reticular collagen deficiency causing spontaneous aortic dissection, berry aneurysm rupture, and hollow-organ bowel/uterine perforation). Alport syndrome is an X-linked dominant mutation in Type IV basement membrane collagen (COL4A5), presenting with hematuria/glomerulonephritis, sensorineural hearing loss, and anterior lenticonus (“Can't see, can't pee, can't hear a bee”) with characteristic basket-weave splitting of the glomerular basement membrane on electron microscopy. Marfan syndrome is an autosomal dominant mutation in the FBN1 gene encoding fibrillin-1 (which forms the microfibrillar scaffold for elastin), presenting with tall stature, arachnodactyly (positive wrist and thumb signs), pectus excavatum/carinatum, upward-and-outward (superotemporal) ectopia lentis, mitral valve prolapse, and cystic medial necrosis of the ascending aorta predisposing to Stanford Type A aortic dissection.