Biochemistry
MBBS Biochemistry question bank: biomolecules, enzymes, metabolism, molecular biology, vitamins, nutrition and clinical biochemistry.
12chapters144questions75High-Yield
Definition
Enzymes are biological catalysts (mostly proteins) that speed up reactions by lowering activation energy, without being consumed.
IUB Classification (6 classes)
- Oxidoreductases — redox reactions
- Transferases — group transfer
- Hydrolases — hydrolysis
- Lyases — add/remove groups (no water)
- Isomerases — rearrangement
- Ligases — join molecules (ATP)
Mechanism of Action
- Bind substrate at the active site
- Lower activation energy
- Lock-and-key / induced-fit models
- Form an enzyme-substrate complex
Enzymes bind substrate and lower activation energy to speed the reaction. Class Reaction Oxidoreductase Redox Transferase Group transfer Hydrolase Hydrolysis Ligase Joining (ATP) Applied
- Enzyme-deficiency diseases
- Enzymes as drug targets
🔑KEY POINTS TO REMEMBER- 6 IUB classes: oxidoreductase, transferase, hydrolase, lyase, isomerase, ligase.
- Lower activation energy via active site.
- Lock-and-key / induced fit.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Enzyme kinetics studies reaction rates; the Michaelis–Menten equation relates velocity to substrate concentration, defining Km.
Michaelis–Menten
- V = Vmax[S] / (Km + [S])
- Gives a hyperbolic curve
- Low [S] — first order; high [S] — zero order (Vmax)
Km (Michaelis constant)
- [S] at which velocity = ½ Vmax
- A measure of enzyme affinity
- Low Km = high affinity
Lineweaver-Burk Plot
- Double-reciprocal plot (straight line)
- Used to find Km and Vmax
Velocity rises with substrate to a maximum (Vmax); Km marks half-maximal rate. Parameter Meaning Vmax Maximum velocity Km [S] at ½ Vmax Low Km High affinity Applied
- Km differs between isoenzymes
- Drug-enzyme kinetics
🔑KEY POINTS TO REMEMBER- V = Vmax[S]/(Km+[S]); hyperbolic curve.
- Km = [S] at ½ Vmax; low Km = high affinity.
- Lineweaver-Burk linearises the plot.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Enzyme inhibition is the decrease in enzyme activity by inhibitors; it is reversible or irreversible.
Reversible Inhibition
- Competitive — binds active site (↑ Km, same Vmax)
- Non-competitive — binds elsewhere (same Km, ↓ Vmax)
- Uncompetitive
Irreversible Inhibition
- Covalent, permanent binding
- E.g. organophosphates, aspirin
Inhibitors reduce activity reversibly or by permanent covalent binding. Type Km Vmax Competitive ↑ Same Non-competitive Same ↓ Applied
- Methanol poisoning (ethanol competes)
- Statins, allopurinol
🔑KEY POINTS TO REMEMBER- Competitive: ↑ Km, same Vmax (active site).
- Non-competitive: same Km, ↓ Vmax.
- Irreversible = covalent (organophosphates).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Enzyme activity depends on factors (temperature, pH, substrate) and often requires coenzymes / cofactors.
Factors Affecting Activity
- Temperature — optimum ~37°C
- pH — an optimum pH
- Substrate concentration
- Enzyme concentration, activators/inhibitors
Coenzymes & Cofactors
- Coenzyme — organic (often vitamin-derived: NAD, FAD)
- Cofactor — inorganic (metal ions: Mg²⁺, Zn²⁺)
- Prosthetic group — tightly bound
Physical factors and helper molecules together set enzyme activity. Factor Effect ↑ Temp (to optimum) ↑ rate Beyond optimum Denatures pH Optimum needed Applied
- Vitamin-derived coenzymes
- Enzyme assays run at 37°C
🔑KEY POINTS TO REMEMBER- Factors: temperature, pH, [S], enzyme concentration.
- Coenzymes (organic, NAD/FAD) vs cofactors (metal ions).
- Vitamin deficiency → ↓ coenzyme.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Clinical enzymology uses serum enzyme levels for diagnosis; isoenzymes are different forms of an enzyme catalysing the same reaction.
Diagnostic Enzymes
- Troponin, CK-MB — myocardial infarction
- ALT, AST — liver disease
- Amylase, lipase — pancreatitis
- ALP — bone / liver
Isoenzymes
- Same reaction, different structure
- LDH (5 isoenzymes), CK (3)
- Tissue-specific → localise damage
Damaged tissue releases enzymes whose pattern points to the organ involved. Enzyme Indicates Troponin/CK-MB Myocardial infarction ALT/AST Liver disease Amylase/lipase Pancreatitis Applied
- Cardiac markers in MI
- Liver function tests
🔑KEY POINTS TO REMEMBER- Serum enzymes localise organ damage.
- Troponin/CK-MB (heart), ALT/AST (liver), amylase/lipase (pancreas).
- Isoenzymes are tissue-specific.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Isoenzymes are physically distinct forms of the same enzyme that catalyse the same reaction but differ in structure and tissue distribution.
Features
- Same reaction, different properties
- Differ in Km and electrophoretic mobility
- Tissue-specific
Examples
- LDH — 5 isoenzymes (LDH1 heart, LDH5 liver)
- CK — CK-MM (muscle), CK-MB (heart), CK-BB (brain)
Isoenzymes catalyse the same reaction but mark different tissues. Isoenzyme Tissue LDH1 Heart LDH5 Liver CK-MB Heart Applied
- CK-MB in myocardial infarction
- LDH flip in MI
🔑KEY POINTS TO REMEMBER- Isoenzymes: same reaction, different structure/tissue.
- LDH (5), CK (3) isoenzymes.
- CK-MB for MI.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Coenzymes and cofactors are non-protein components required by many enzymes for activity.
Coenzymes
- Organic molecules
- Often vitamin-derived
- E.g. NAD (niacin), FAD (riboflavin), coenzyme A, TPP
Cofactors
- Inorganic ions (metals)
- E.g. Mg²⁺, Zn²⁺, Fe²⁺
- Prosthetic group tightly bound (haem)
The protein apoenzyme plus its helper forms the active holoenzyme. Type Example Coenzyme NAD, FAD, CoA Cofactor Mg²⁺, Zn²⁺ Applied
- Vitamin deficiency → ↓ enzyme activity
🔑KEY POINTS TO REMEMBER- Coenzyme (organic, vitamin-derived) vs cofactor (metal ion).
- Apoenzyme + coenzyme = holoenzyme.
- Vitamin deficiency impairs enzymes.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Allosteric enzymes have a regulatory (allosteric) site besides the active site, allowing modulation of activity.
Features
- Sigmoid kinetics (not hyperbolic)
- Regulated by effectors (activators/inhibitors)
- Usually multi-subunit
- Often catalyse rate-limiting steps
Regulation
- Positive effectors → ↑ activity
- Negative effectors (feedback) → ↓ activity
Effectors binding away from the active site switch the enzyme up or down. Effector Effect Positive ↑ activity Negative ↓ activity Applied
- PFK-1 (glycolysis regulation)
- Feedback inhibition
🔑KEY POINTS TO REMEMBER- Allosteric enzymes: regulatory site, sigmoid kinetics.
- Effectors modulate activity.
- Often rate-limiting (PFK-1).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Feedback inhibition is regulation of a pathway in which the end product inhibits an earlier enzyme (usually the rate-limiting step).
Mechanism
- End product accumulates
- Inhibits the first / committed enzyme (allosteric)
- Prevents overproduction
The end product switches off an early step, preventing its own excess. Feature Detail Target First / committed enzyme Type Usually allosteric Applied
- Cholesterol inhibits HMG-CoA reductase
- Metabolic economy
🔑KEY POINTS TO REMEMBER- End product inhibits an early (committed) enzyme.
- Usually allosteric.
- E.g. cholesterol → HMG-CoA reductase.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Enzyme specificity is the ability of an enzyme to select a particular substrate or reaction.
Types
- Absolute — one substrate (urease)
- Group / relative — a class of substrates
- Stereospecificity — one stereoisomer (L or D)
- Bond specificity
The active site’s shape restricts which substrate the enzyme accepts. Type Example Absolute Urease Stereo L-amino acid oxidase Applied
- Basis of metabolic control
🔑KEY POINTS TO REMEMBER- Specificity: absolute, group, stereo, bond.
- Determined by active-site shape.
- Basis of metabolic control.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
Zymogens (proenzymes) are inactive enzyme precursors that become active after specific cleavage.
Features
- Synthesised in inactive form
- Activated by partial proteolysis
- Prevents self-digestion of tissue
Examples
- Pepsinogen → pepsin
- Trypsinogen → trypsin
- Prothrombin → thrombin
A precise cut converts the inactive zymogen into the active enzyme. Zymogen Active form Pepsinogen Pepsin Trypsinogen Trypsin Prothrombin Thrombin Applied
- Acute pancreatitis (premature activation)
- Clotting cascade
🔑KEY POINTS TO REMEMBER- Zymogen = inactive precursor, activated by cleavage.
- Pepsinogen, trypsinogen, prothrombin.
- Premature activation → pancreatitis.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).Definition
The Michaelis constant (Km) is the substrate concentration at which the reaction velocity is half of Vmax.
Significance
- Km = [S] at ½ Vmax
- A measure of enzyme-substrate affinity
- Low Km = high affinity
- A characteristic constant for each enzyme
Km is the substrate level giving half-maximal velocity — an affinity measure. Km Affinity Low High High Low Applied
- Compare isoenzyme affinity
- Glucokinase (high Km) vs hexokinase (low Km)
🔑KEY POINTS TO REMEMBER- Km = [S] at ½ Vmax.
- Low Km = high affinity.
- Glucokinase (high Km) vs hexokinase (low Km).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Textbook of Medical Biochemistry (M.N. Chatterjea).