Biochemistry
MBBS Biochemistry question bank: biomolecules, enzymes, metabolism, molecular biology, vitamins, nutrition and clinical biochemistry.
12chapters144questions75High-Yield
Definition
The catabolism of amino acids begins with removal of the amino group by transamination and deamination, leaving a carbon skeleton with a fate.
Transamination
- Transfer of amino group to a keto acid
- Needs pyridoxal phosphate (vitamin B6)
- Enzymes: ALT, AST
- Forms glutamate
Deamination
- Removal of amino group as ammonia
- Oxidative (glutamate dehydrogenase)
- Ammonia → urea cycle
Fate of Carbon Skeleton
- Glucogenic → glucose
- Ketogenic → ketone bodies
- Some are both
The amino group is stripped off; the carbon skeleton is used for fuel or glucose. Type Enzyme Transamination ALT, AST Deamination Glutamate DH Applied
- AST/ALT in liver disease
- Vitamin B6 deficiency
🔑KEY POINTS TO REMEMBER- Transamination (B6, ALT/AST) → glutamate.
- Deamination → ammonia → urea cycle.
- Carbon skeleton: glucogenic / ketogenic.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The urea cycle is the hepatic pathway that converts toxic ammonia into urea for safe excretion.
Steps (5 enzymes)
- Ammonia + CO₂ → carbamoyl phosphate
- + ornithine → citrulline
- + aspartate → argininosuccinate
- → arginine + fumarate
- Arginine → urea + ornithine (regenerated)
Features
- Occurs in the liver (mitochondria + cytoplasm)
- Two nitrogens per urea (ammonia + aspartate)
- Rate-limiting: carbamoyl phosphate synthetase I
Ammonia is packaged into urea across a cyclic set of five enzymes. Feature Detail Site Liver Product Urea N sources Ammonia + aspartate Applied
- Urea cycle disorders → hyperammonaemia
- ↑ blood urea (renal)
🔑KEY POINTS TO REMEMBER- Urea cycle: ammonia → urea (liver).
- 2 nitrogens (ammonia + aspartate); rate enzyme CPS-I.
- Defects → hyperammonaemia.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Ammonia is a toxic product of amino acid metabolism, detoxified mainly by conversion to urea and glutamine.
Sources
- Amino acid deamination
- Gut bacteria (protein breakdown)
- Purine / pyrimidine breakdown
Detoxification / Transport
- Liver → urea (urea cycle)
- Glutamine (transport form)
- Alanine (muscle → liver)
Toxicity
- Ammonia is neurotoxic
- Depletes α-ketoglutarate (impairs TCA in brain)
Ammonia is carried safely as glutamine/alanine to the liver for urea formation. Transport form Tissue Glutamine Most tissues Alanine Muscle Applied
- Hyperammonaemia (liver failure)
- Hepatic encephalopathy
🔑KEY POINTS TO REMEMBER- Ammonia is toxic; detoxified to urea & glutamine.
- Transported as glutamine / alanine.
- Excess → hepatic encephalopathy.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Inborn errors of amino acid metabolism are inherited enzyme deficiencies causing accumulation of amino acids or their metabolites.
Common Disorders
- Phenylketonuria — phenylalanine hydroxylase
- Alkaptonuria — homogentisate oxidase
- Maple syrup urine disease — branched-chain keto acid DH
- Albinism — tyrosinase
- Homocystinuria
A missing enzyme lets an amino acid or its metabolite build up and harm the body. Disease Enzyme PKU Phenylalanine hydroxylase Alkaptonuria Homogentisate oxidase Albinism Tyrosinase Applied
- Newborn screening (PKU)
- Dietary management
🔑KEY POINTS TO REMEMBER- Inborn errors: PKU, alkaptonuria, MSUD, albinism, homocystinuria.
- Each = a specific enzyme deficiency.
- Screened for and diet-managed.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
One-carbon metabolism transfers single-carbon units using carriers like folate and SAM; transmethylation is transfer of methyl groups.
Carriers
- Tetrahydrofolate (THF) — various one-carbon units
- S-adenosylmethionine (SAM) — methyl donor
- Vitamin B12 links folate & methionine
Uses
- Purine / pyrimidine synthesis
- Methylation (DNA, neurotransmitters)
- Creatine, choline synthesis
Folate and SAM shuttle one-carbon units for synthesis and methylation. Carrier Role THF One-carbon units SAM Methyl donor Applied
- Folate/B12 deficiency → megaloblastic anaemia
- Methotrexate blocks folate
🔑KEY POINTS TO REMEMBER- One-carbon carriers: THF & SAM (methyl donor).
- Used for nucleotides, methylation, creatine.
- Folate/B12 deficiency → megaloblastic anaemia.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Nitrogen balance is the difference between nitrogen intake (protein) and nitrogen loss (urea, etc.).
Types
- Positive — intake > loss (growth, pregnancy)
- Negative — loss > intake (starvation, trauma)
- Equilibrium — healthy adult
Comparing nitrogen in and out shows whether the body is building or losing protein. Balance Situation Positive Growth, pregnancy Negative Starvation, illness Zero Healthy adult Applied
- Assess protein nutrition
- Catabolic states
🔑KEY POINTS TO REMEMBER- Nitrogen balance = intake – loss.
- Positive (growth), negative (illness), zero (healthy adult).
- Assesses protein nutrition.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Creatine is synthesised from amino acids and stored as creatine phosphate in muscle; creatinine is its waste product.
Creatine
- From arginine, glycine, methionine
- Stored as creatine phosphate (energy)
- Mainly in muscle
Creatinine
- Non-enzymatic breakdown of creatine phosphate
- Constant daily production
- Excreted by the kidney (marker of GFR)
Creatine stores muscle energy; its breakdown gives creatinine, a renal marker. Substance Role Creatine phosphate Energy store Creatinine Renal marker Applied
- Serum creatinine = renal function
- Creatinine clearance
🔑KEY POINTS TO REMEMBER- Creatine from arginine, glycine, methionine.
- Stored as creatine phosphate; breaks down to creatinine.
- Creatinine = renal-function marker.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Several neurotransmitters are synthesised from amino acids by decarboxylation or hydroxylation.
Examples
- GABA — from glutamate
- Serotonin — from tryptophan
- Dopamine / noradrenaline / adrenaline — from tyrosine
- Histamine — from histidine
Amino acids are converted into signalling molecules of the nervous system. Precursor Neurotransmitter Glutamate GABA Tryptophan Serotonin Tyrosine Dopamine Applied
- Parkinson’s (dopamine)
- Depression (serotonin)
🔑KEY POINTS TO REMEMBER- GABA (glutamate), serotonin (tryptophan), dopamine (tyrosine), histamine (histidine).
- Made by decarboxylation / hydroxylation.
- Relevant to Parkinson’s, depression.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Decarboxylation is the removal of the carboxyl group (as CO₂) from amino acids, forming biologically active amines.
Products
- Histidine → histamine
- Glutamate → GABA
- Tyrosine → tyramine
- 5-HTP → serotonin
Coenzyme
- Pyridoxal phosphate (vitamin B6)
Removing the carboxyl group yields active amines, needing vitamin B6. Amino acid Amine Histidine Histamine Glutamate GABA Applied
- Vitamin B6 needed
- Amines: neurotransmitters, mediators
🔑KEY POINTS TO REMEMBER- Decarboxylation removes CO₂ → active amines.
- Needs pyridoxal phosphate (B6).
- Histamine, GABA, serotonin.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Glutathione (GSH) is a tripeptide (glutamate-cysteine-glycine) that is the major intracellular antioxidant.
Functions
- Antioxidant (reduces peroxides)
- Maintains protein SH groups
- Detoxification (conjugation)
- Regenerated by NADPH
GSH neutralises peroxides and is recycled using NADPH. Form State GSH Reduced (active) GSSG Oxidised Applied
- G6PD deficiency → ↓ GSH
- Paracetamol overdose (depletes GSH)
🔑KEY POINTS TO REMEMBER- Glutathione = tripeptide, major antioxidant.
- Reduces peroxides; regenerated by NADPH.
- Depleted in paracetamol overdose.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Phenylketonuria (PKU) is an inborn error due to deficiency of phenylalanine hydroxylase, causing accumulation of phenylalanine.
Mechanism
- Phenylalanine cannot convert to tyrosine
- ↑ Phenylalanine + phenylketones
- Toxic to the developing brain
Features
- Intellectual disability (untreated)
- Musty odour, fair skin (↓ melanin)
- Seizures
Without the enzyme, phenylalanine and its ketones accumulate and harm the brain. Feature Detail Enzyme Phenylalanine hydroxylase Diet Low phenylalanine Applied
- Newborn screening (Guthrie test)
- Phenylalanine-free diet
🔑KEY POINTS TO REMEMBER- PKU = phenylalanine hydroxylase deficiency.
- ↑ phenylalanine → brain damage if untreated.
- Screened at birth; low-phenylalanine diet.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Glutamine is the most abundant amino acid in blood, serving as a non-toxic transport form of ammonia and a key nitrogen donor.
Functions
- Transports ammonia (non-toxic)
- Nitrogen donor (nucleotide synthesis)
- Fuel for enterocytes & immune cells
- Renal ammoniagenesis (acid-base)
Glutamine safely carries ammonia and donates nitrogen for synthesis. Role Detail Ammonia transport Non-toxic Nitrogen donor Nucleotides Applied
- Renal ammoniagenesis in acidosis
- Gut / immune-cell fuel
🔑KEY POINTS TO REMEMBER- Glutamine = most abundant blood amino acid.
- Non-toxic ammonia transport + nitrogen donor.
- Renal ammoniagenesis in acidosis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).