Cellular Energy & Genetics

Medical Biochemistry & Metabolism

Complete rate-limiting enzyme catalogs, inborn errors of metabolism decoder (GSDs, LSDs, amino acid defects), electron transport chain inhibitors, and clinical vitamin matrices.

Drill 10-Q Biochem Block ← All Basic Sciences

Master Rate-Limiting Enzymes & Allosteric Regulators

The exact biochemical switchpoints targeted on COMLEX Level 1 and USMLE Step 1.

Pathway	Rate-Limiting Enzyme	Allosteric Activators (+)	Allosteric Inhibitors (-)	Cellular Compartment & Notes
Glycolysis	Phosphofructokinase-1 (PFK-1)	AMP, Fructose-2,6-bisphosphate (F-2,6-BP)	ATP, Citrate	Cytosol. Insulin activates PFK-2 → ↑F-2,6-BP → stimulates glycolysis. Glucagon stimulates FBPase-2.
Gluconeogenesis	Fructose-1,6-bisphosphatase-1	Citrate, ATP	AMP, F-2,6-BP	Cytosol (Liver/Kidney). 4 unique bypass enzymes: Pyruvate carboxylase (biotin), PEP carboxykinase, F-1,6-BPase, Glucose-6-phosphatase.
TCA (Krebs) Cycle	Isocitrate Dehydrogenase	ADP, NAD+	ATP, NADH	Mitochondrial matrix. Produces 3 NADH, 1 FADH2, 1 GTP per acetyl-CoA (10 ATP total per acetyl-CoA).
Glycogenesis	Glycogen Synthase	Glucose-6-phosphate, Insulin, Cortisol	Epinephrine, Glucagon (via PKA phosphorylation)	Cytosol. Forms α-1,4 bonds. Branching enzyme forms α-1,6 linkages.
Glycogenolysis	Glycogen Phosphorylase	Epinephrine, Glucagon, AMP, Ca2+ (calmodulin in muscle)	Glucose-6-phosphate, ATP, Insulin	Cytosol. Cleaves α-1,4 bonds yielding glucose-1-phosphate. Deficient in McArdle (muscle) and Hers (liver).
HMP Shunt (Pentose Phosphate)	Glucose-6-Phosphate Dehydrogenase (G6PD)	NADP+	NADPH	Cytosol. Yields NADPH (for glutathione reduction, fatty acid/sterol synthesis) & Ribose-5-P (for nucleotides). G6PD deficiency → Heinz bodies & bite cells under oxidative stress.
Fatty Acid Synthesis	Acetyl-CoA Carboxylase (ACC)	Insulin, Citrate	Glucagon, Palmitoyl-CoA	Cytosol. Requires biotin (B7), ATP, and CO2. Product malonyl-CoA inhibits carnitine palmitoyltransferase-1 (CPT-1) to halt beta-oxidation.
Fatty Acid Beta-Oxidation	Carnitine Acyltransferase I (CPT-1)	Glucagon, low malonyl-CoA	Malonyl-CoA (synthesis intermediate)	Mitochondrial outer membrane → matrix. Carnitine shuttle transports long-chain fatty acids into matrix. Systemic carnitine deficiency → hypoketotic hypoglycemia.
Ketogenesis	HMG-CoA Synthase	Fasting, low oxaloacetate, high acetyl-CoA	Insulin	Liver mitochondria only (RBCs cannot use ketones, lack mitochondria; liver cannot use ketones, lacks thiophorase/SCOT).
Cholesterol Synthesis	HMG-CoA Reductase	Insulin, Thyroxine	Statins (competitive), Glucagon, Cholesterol	Smooth ER. Converts HMG-CoA to mevalonate. Statins competitively inhibit; side effects = myopathy & hepatotoxicity.
Urea Cycle	Carbamoyl Phosphate Synthetase I (CPS I)	N-acetylglutamate (NAG)	Low arginine	Mitochondria (CPS I & OTC); Cytosol (Argininosuccinate synthetase/lyase, Arginase). NAG is an obligate allosteric activator.

Electron Transport Chain Complexes & Classic Poisons

High-yield mechanism correlations tested on toxicological and cellular respiration questions.

Complex I Inhibitors Rotenone • Metformin

Directly halts electron transfer from NADH to CoQ (Ubiquinone), eliminating the proton gradient from Complex I.

Complex III Inhibitors Antimycin A

Blocks electron transfer from Cytochrome b to Cytochrome c1, freezing upstream cytochromes in reduced state.

Complex IV Inhibitors Cyanide • Carbon Monoxide • Azide

Inhibits Cytochrome c oxidase (Fe3+ in Cyanide, Fe2+ in CO). Halts cellular aerobic respiration; severe lactic acidosis with high venous oxygen saturation in cyanide.

ATP Synthase & Uncouplers Oligomycin vs 2,4-DNP / Aspirin

Oligomycin directly blocks F0 proton pore (halts ATP & electron flow). 2,4-DNP & Aspirin dissipate proton gradient as heat → hyperthermia, tachypnea, ↑O2 consumption.

Electron Transport Chain & Oxidative Phosphorylation

Mitochondrial proton electrochemical gradient, Complexes I–V, specific chemical inhibitors, and fatal uncoupling agents.

Mitochondrial Inner Membrane

Complexes & Electron Shuttles

Complex I (NADH Dehydrogenase): Transfers electrons from NADH to Coenzyme Q (Ubiquinone). Pumps 4 H+ into intermembrane space.
Complex II (Succinate Dehydrogenase): Only enzyme shared by TCA cycle and ETC! Transfers electrons from FADH2 to Coenzyme Q. Pumps NO protons.
Complex III (Cytochrome bc1): Transfers electrons from Coenzyme Q to Cytochrome c. Pumps 4 H+.
Complex IV (Cytochrome c Oxidase): Contains copper (Cu) and heme irons (a/a3). Transfers electrons to molecular O2 to form H2O. Pumps 2 H+.
Complex V (ATP Synthase / F0F1): Uses proton-motive force to spin F1 subunit and synthesize ATP from ADP + Pi.

Board Favorite Distinction

ETC Inhibitors vs. Uncoupling Agents

Direct ETC Inhibitors (Blocks Electron Flow)

Halts electron flow → proton gradient collapses → ↓ O2 consumption, ↓ ATP synthesis, lactic acidosis.

Complex I: Rotenone, Amytal, Metformin | Complex III: Antimycin A | Complex IV: Cyanide, CO, Sodium Azide, H2S | Complex V: Oligomycin

Uncoupling Agents (Increases Membrane Permeability)

Increases inner membrane H+ permeability → dissipates proton gradient without passing through ATP synthase → ↑ O2 consumption, ↓ ATP synthesis, energy released as massive heat.

Agents: 2,4-Dinitrophenol (DNP - illegal fat burner), High-dose Aspirin (hyperthermia, tachypnea), Thermogenin (UCP-1 in brown fat of newborns).

Glycogen Storage Diseases (Types I–VI)

Mnemonic: Very Poor Carbohydrate Accumulation Might Hurt

Autosomal Recessive

Type I • Von Gierke Severe Hypoglycemia

Glucose-6-Phosphatase Deficiency

Presentation: Severe fasting hypoglycemia, massive hepatomegaly (normal spleen), doll-like facial appearance, lactic acidosis, hyperuricemia (gout), hyperlipidemia.
Mechanism: Liver cannot dephosphorylate G6P to free glucose in glycogenolysis OR gluconeogenesis.
Treatment: Frequent oral glucose/cornstarch; avoid fructose and galactose.

Type II • Pompe "Trashes the Pump"

Lysosomal α-1,4-Glucosidase (Acid Maltase)

Presentation: Massive hypertrophic cardiomyopathy, profound hypotonia ("floppy infant"), cardiomegaly, macroglossia, early heart failure.
Mechanism: Glycogen accumulation in lysosomes throughout heart, skeletal muscle, and liver. Normal blood glucose.
Board Trap: Confusing with Werdnig-Hoffmann (SMA-1) which lacks cardiomyopathy.

Type III • Cori Debranching Defect

Debranching Enzyme (α-1,6-Glucosidase)

Presentation: Milder fasting hypoglycemia, hepatomegaly, normal blood lactate level (unlike Von Gierke), accumulation of limit dextrin-like structures.
Mechanism: Gluconeogenesis is INTACT (lactate is normal, alanine can still form glucose).

Type IV • Andersen Branching Defect

Branching Enzyme Deficiency

Presentation: Infantile failure to thrive, hepatosplenomegaly, and rapidly progressive hepatic cirrhosis with portal hypertension. Fatal by age 2.
Mechanism: Linear unbranched glycogen chains with poor solubility trigger foreign-body immune scarring in liver.

Type V • McArdle Muscle Cramps

Skeletal Muscle Glycogen Phosphorylase (Myophosphorylase)

Presentation: Painful muscle cramps, myoglobinuria (burgundy/cola urine) following strenuous exercise, second-wind phenomenon (blood flow increases fatty acid delivery).
Lab Finding: Failure of blood lactate to rise following forearm ischemic exercise test; normal liver enzyme function and normal blood glucose.

Type VI • Hers Hepatic Phosphorylase

Hepatic Glycogen Phosphorylase Deficiency

Presentation: Mild fasting hypoglycemia, hepatomegaly, growth retardation in childhood. Normal lactate and uric acid levels. Benign clinical course.

Lysosomal Storage Diseases (Sphingolipidoses & Mucopolysaccharidoses)

Deficient lysosomal hydrolases leading to substrate accumulation.

High-Yield USMLE & COMLEX

Disease	Inheritance	Deficient Enzyme	Accumulated Substrate	Pathognomonic Clinical Triad	Key Pathology Pearl
Gaucher Disease	AR	β-Glucocerebrosidase	Glucocerebroside	Hepatosplenomegaly, pancytopenia, bone crises & avascular necrosis of femur head.	Lipid-laden macrophages resembling "wrinkled/crumpled tissue paper". Most common LSD.
Tay-Sachs Disease	AR	Hexosaminidase A	GM2 Ganglioside	Neurodevelopmental regression, hyperreflexia, cherry-red macular spot, NO hepatomegaly.	Onion-skin lysosomes on electron microscopy. Normal liver/spleen size separates from Niemann-Pick.
Niemann-Pick Disease	AR	Sphingomyelinase	Sphingomyelin	Neurodegeneration, cherry-red macular spot, HEPATOSPLENOMEGALY, hypotonia.	"Foam cells" (lipid-laden histiocytes with vacuolated bubbly cytoplasm).
Fabry Disease	X-Linked Recessive	α-Galactosidase A	Ceramide Trihexoside	Episodic burning peripheral neuropathy, angiokeratomas (bathing-trunk), hypohidrosis, renal/cardiac failure.	Zebra bodies on renal biopsy EM. High risk of early stroke and CAD.
Krabbe Disease	AR	Galactocerebrosidase	Galactocerebroside & Psychosine	Progressive peripheral neuropathy, limb spasticity, optic atrophy, developmental regression.	Globoid multinucleated epithelioid cells in white matter.
Metachromatic Leukodystrophy	AR	Arylsulfatase A	Cerebroside Sulfate	Central and peripheral demyelination with ataxia, hyporeflexia, and dementia.	Brown-metachromatic sulfatide granules staining with cresyl violet.
Hurler Syndrome (MPS I)	AR	α-L-Iduronidase	Heparan & Dermatan Sulfate	Coarse facial features (gargoylism), developmental delay, hepatosplenomegaly, CORNEAL CLOUDING.	Corneal clouding separates from Hunter syndrome.
Hunter Syndrome (MPS II)	X-Linked Recessive	Iduronate-2-Sulfatase	Heparan & Dermatan Sulfate	Aggressive behavioral disturbances, coarse facies, hepatosplenomegaly, NO corneal clouding.	"Hunters see clearly (no clouding) and aim for the X (X-linked)".

Amino Acid Catabolism Disorders

Phenylketonuria (PKU)

Deficiency of Phenylalanine Hydroxylase (PAH) or BH4 cofactor. Phenylalanine cannot convert to Tyrosine. Findings: Musty/mousy body odor, intellectual disability, fair complexion (tyrosine is melanin precursor), seizures, eczema. Tyrosine becomes essential.

Maple Syrup Urine Disease (MSUD)

Deficiency of Branched-Chain α-Ketoacid Dehydrogenase (requires B1, B2, B3, B5, Lipoic acid). Blocked degradation of branched-chain amino acids: Isoleucine, Leucine, Valine (I Love Vermont maple syrup). Findings: Burnt sugar/maple syrup urine smell, vomiting, lethargy, dystonia.

Alkaptonuria

Deficiency of Homogentisate Oxidase in tyrosine degradation. Homogentisic acid accumulates. Findings: Urine turns black when exposed to air; ochronosis (blue-black pigmentation of sclera, ear cartilage); debilitating joint arthropathy in adulthood.

Homocystinuria

Deficiency of Cystathionine β-Synthase (CBS) (requires B6). Findings: Marfanoid habitus, osteoporosis, downward and inward lens ectopia lentis (vs Marfan upward/outward), severe premature thromboembolic events (DVT, stroke, MI).

Cystinuria

Defective renal PCT transporter for dibasic amino acids: Cystine, Ornithine, Lysine, Arginine (COLA). Excess cystine precipitates into radiopaque hexagonal stones. Diagnosis: Urinary sodium cyanide-nitroprusside test → red-purple color.

Urea Cycle & Purine Disorders

Ornithine Transcarbamylase (OTC) Deficiency

Most common urea cycle disorder (X-linked recessive). Excess carbamoyl phosphate shunts into pyrimidine synthesis → elevated orotic acid in blood and urine, severe hyperammonemia, asterixis, cerebral edema. Normal blood urea nitrogen (BUN).

Orotic Aciduria (UMP Synthase Deficiency)

Autosomal recessive defect in uridine monophosphate (UMP) synthase. Findings: Megaloblastic anemia refractory to B12/folate, orotic aciduria, growth failure. NORMAL ammonia levels (distinguishes from OTC deficiency). Treatment: Uridine monophosphate supplementation.

Lesch-Nyhan Syndrome

Defective purine salvage enzyme Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT) (X-linked recessive). Cannot convert hypoxanthine to IMP or guanine to GMP. Findings: HGPRT • Hyperuricemia (orange "sand" uric acid in diapers), Gout, Pissed off (aggressive self-mutilation of lips/fingers), Retardation, Tone dystonia.

Adenosine Deaminase (ADA) Deficiency

Excess dATP accumulation inhibits ribonucleotide reductase → blocks DNA synthesis in proliferating lymphocytes → severe lymphopenia and Severe Combined Immunodeficiency (SCID) (autosomal recessive form).

Familial Dyslipidemias (Fredrickson Types I–V)

Apolipoprotein mutations, elevated lipoprotein fractions, and classic cutaneous xanthoma distributions.

Type & Disorder	Inheritance & Defect	Elevated Lipoprotein	Clinical Manifestations & Board Clues
Type I • Hyperchylomicronemia	AR • Lipoprotein Lipase (LPL) or ApoC-II deficiency	Chylomicrons (Triglycerides > 1,000 mg/dL)	Recurrent acute pancreatitis, eruptive itchy xanthomas on back/buttocks, hepatosplenomegaly, milky/creamy supernatant in standing plasma tube. No increased risk of atherosclerosis!
Type IIa • Familial Hypercholesterolemia	AD • Defective/absent LDL receptors or defective ApoB-100	LDL (Severe hypercholesterolemia: 300-500 mg/dL)	Severe premature coronary artery disease (MI before age 20 in homozygotes), Achilles tendon xanthomas, corneal arcus, xanthelasma around eyelids.
Type III • Dysbetalipoproteinemia	AR • Defective Apolipoprotein E (ApoE-2 homozygosity)	Chylomicron remnants & IDL	Pathognomonic palmar xanthomas (orange/yellow deposits in palmar creases) and tuberoeruptive xanthomas on elbows/knees. Premature CAD and peripheral vascular disease.
Type IV • Hypertriglyceridemia	AD • Hepatic overproduction of VLDL	VLDL (Triglycerides > 200-500 mg/dL)	Associated with insulin resistance, obesity, type 2 diabetes, and acute pancreatitis at TG levels > 1,000 mg/dL. Clear or turbid serum.

ApoA-I

Found on HDL; activates LCAT (lecithin-cholesterol acyltransferase) for reverse cholesterol transport from periphery to liver.

ApoB-48

Mediates chylomicron secretion from intestinal enterocytes into lymphatic lacteals. Absent in Abetalipoproteinemia.

ApoB-100

Found on VLDL, IDL, and LDL; binds the LDL receptor for endocytosis. Derived from same pre-mRNA as B-48 via RNA editing.

ApoC-II

Cofactor for Lipoprotein Lipase (LPL) on endothelial surface of capillaries in adipose and skeletal muscle tissue.

ApoE

Mediates remnant uptake by liver receptors. ApoE-4 allele increases risk of late-onset sporadic Alzheimer disease.

Purine & Pyrimidine Catabolism & Diagnostic Crystals

Salvage pathway mutations, Lesch-Nyhan self-mutilation, ADA-SCID, and polarized light microscopy.

X-Linked Recessive

Lesch-Nyhan Syndrome

Defective HGPRT (Hypoxanthine-Guanine Phosphoribosyltransferase) in purine salvage.

Mechanism: Cannot salvage hypoxanthine/guanine → excess PRPP → ↑ de novo purine synthesis → massive uric acid production.

Mnemonic HGPRT: Hyperuricemia (orange "sand" in diaper), Gout, Pissed off (aggression & compulsive self-mutilation of lips/fingers), Retardation (intellectual disability), dysTonia/choreoathetosis.

Treatment: Allopurinol or Febuxostat (inhibits Xanthine Oxidase).

Autosomal Recessive

Adenosine Deaminase (ADA) Deficiency

Second most common cause of Severe Combined Immunodeficiency (SCID).

Mechanism: ADA converts adenosine → inosine. Deficiency leads to accumulation of dATP, which is allosterically toxic to ribonucleotide reductase.

Clinical: Halts DNA synthesis selectively in rapidly dividing lymphocytes → profound loss of both T-cells and B-cells.

Presents with recurrent viral, fungal, and opportunistic infections in infancy. Absent thymic shadow.

Pyrimidine Synthesis

Hereditary Orotic Aciduria

Deficiency of UMP Synthase (Orotate phosphoribosyltransferase & OMP decarboxylase).

Clinical: Orotic acid in urine, megaloblastic anemia refractory to Vitamin B12 and Folate, failure to thrive, developmental delay.

Distinguish from OTC Deficiency: Orotic aciduria has NORMAL blood ammonia and megaloblastic anemia. OTC deficiency has MASSIVE hyperammonemia with normal hematocrit!

Treatment: Oral Uridine supplementation bypasses UMP synthase.

Electron Microscopy

Type IV Collagen Defect (Alport)

Mutation in COL4A5 (X-linked). Glomerular basement membrane splitting with irregular thinning and thickening giving a "basket-weave" appearance on TEM. Sensorineural hearing loss + nephritis.

Polarized Microscopy

Apple-Green Birefringence (Amyloid)

Extracellular aggregation of insoluble β-pleated sheet fibrils. Congo red stain shows pathognomonic apple-green birefringence under cross-polarized light. AL vs AA amyloidosis.

Non-Enzymatic Glycation

Kimmelstiel-Wilson Nodules

Hyperglycemia causes non-enzymatic glycation of efferent arterioles → hyperfiltration → mesangial expansion with eosinophilic, PAS-positive nodular glomerulosclerosis.

Metabolic Neuro-Pathology

Subacute Combined Degeneration

Vitamin B12 deficiency halts methylmalonyl-CoA mutase → ↑ methylmalonic acid → myelin vacuolization and demyelination of dorsal columns and lateral corticospinal tracts.

Medical Biochemistry & Metabolism

Master Rate-Limiting Enzymes & Allosteric Regulators

Electron Transport Chain Complexes & Classic Poisons

Electron Transport Chain & Oxidative Phosphorylation

Complexes & Electron Shuttles

ETC Inhibitors vs. Uncoupling Agents

Glycogen Storage Diseases (Types I–VI)

Glucose-6-Phosphatase Deficiency

Lysosomal α-1,4-Glucosidase (Acid Maltase)

Debranching Enzyme (α-1,6-Glucosidase)

Branching Enzyme Deficiency

Skeletal Muscle Glycogen Phosphorylase (Myophosphorylase)

Hepatic Glycogen Phosphorylase Deficiency

Lysosomal Storage Diseases (Sphingolipidoses & Mucopolysaccharidoses)

Amino Acid Catabolism Disorders

Urea Cycle & Purine Disorders

Familial Dyslipidemias (Fredrickson Types I–V)

Purine & Pyrimidine Catabolism & Diagnostic Crystals

Lesch-Nyhan Syndrome

Adenosine Deaminase (ADA) Deficiency

Hereditary Orotic Aciduria

Type IV Collagen Defect (Alport)

Apple-Green Birefringence (Amyloid)

Kimmelstiel-Wilson Nodules

Subacute Combined Degeneration

Comprehensive Vitamin Master Reference

Interactive Metabolic Defect Solver