Unit 4: Enzymes
Biochemical Metabolism notes · PTU syllabus (BMLS202-18)
On this page
Unit summary
Enzymes make the chemistry of life fast enough to sustain it, and many are measured in the laboratory. This unit covers the importance and classification of enzymes, their properties, the mechanism of enzyme action, factors affecting activity, enzyme kinetics and inhibitors.
After this unit you can
- Classify enzymes into the six IUBMB classes
- Explain the mechanism of enzyme action
- Describe factors affecting enzyme activity and kinetics
- Distinguish types of enzyme inhibition
PTU syllabus topics
- Introduction
- importance and classification of enzymes
- properties
- mechanism of enzyme action
- factors affecting enzyme action
- enzyme kinetics and inhibitors
Competitive
The active site
Km rises, Vmax unchanged
Non-competitive
Another site
Km unchanged, Vmax falls
Uncompetitive
The enzyme-substrate complex
Both Km and Vmax fall
Topic 1
Importance and classification of enzymes
- Enzymes: biological catalysts, mostly proteins, that speed reactions without being consumed. Clinically, serum enzymes diagnose disease (ALT for liver, CK-MB and troponin for heart muscle, amylase and lipase for pancreas, ALP for bone and biliary disease).
1 Oxidoreductases
Oxidation–reduction
Lactate dehydrogenase
2 Transferases
Transfer of groups
ALT, AST, hexokinase
3 Hydrolases
Hydrolysis
Amylase, lipase, trypsin
4 Lyases
Removal of groups forming double bonds
Aldolase, fumarase
5 Isomerases
Rearrangement
Phosphoglucose isomerase
6 Ligases
Joining with ATP
DNA ligase, pyruvate carboxylase
- A seventh class, translocases (moving ions across membranes), was added in 2018.
Topic 2
Properties of enzymes
- Specificity
- Absolute, group, stereo-specificity
- Catalytic efficiency
- Speed reactions millions of times
- Active site
- Region binding the substrate
- Cofactors
- Metal ions (Mg²⁺, Zn²⁺) or coenzymes (vitamin-derived — NAD, FAD)
- Holoenzyme
- Apoenzyme + cofactor
- Isoenzymes
- Different forms catalysing the same reaction — LDH1–5, CK-MM, CK-MB, CK-BB
- Zymogens
- Inactive precursors — pepsinogen, trypsinogen
Topic 3
Mechanism of enzyme action
- Enzymes lower the activation energy by forming an enzyme–substrate complex: E + S ⇌ ES → E + P.
Idea
Rigid active site exactly fits the substrate
Active site changes shape to fit the substrate
Explains
Specificity
Specificity and flexibility; better supported by evidence
Topic 4
Factors affecting enzyme action
- Substrate concentration
- Rate rises then plateaus at Vmax
- Enzyme concentration
- Rate proportional to enzyme amount
- Temperature
- Rises to an optimum (about 37 °C) then falls as the enzyme denatures
- pH
- Optimum pH — pepsin 2, trypsin 8, most about 7.4
- Activators and inhibitors
- Increase or decrease activity
- Products
- Accumulation may slow the reaction
- Clinical relevance: samples for enzyme assays must be fresh or properly stored; haemolysis raises LDH and AST falsely.
Topic 5
Enzyme kinetics
Michaelis–Menten equation
v = Vmax [S] ÷ (Km + [S])
Km
Substrate concentration giving half Vmax; low Km = high affinity
Lineweaver–Burk plot
1 ÷ v = (Km ÷ Vmax)(1 ÷ [S]) + 1 ÷ Vmax; intercepts −1 ÷ Km and 1 ÷ Vmax
Example
If Vmax = 100 units and Km = 2 mM, at [S] = 2 mM, v = 100 × 2 ÷ 4 = 50 units (half Vmax).
- Enzyme units: 1 IU = amount converting 1 µmol substrate per minute; 1 katal = 1 mol per second.
Topic 6
Enzyme inhibitors
Competitive
Inhibitor resembles the substrate; binds the active site; overcome by excess substrate
Km increases; Vmax unchanged — e.g., methotrexate, statins, sulphonamides
Non-competitive
Binds elsewhere; changes enzyme shape
Km unchanged; Vmax decreases — e.g., heavy metals
Uncompetitive
Binds only the ES complex
Both Km and Vmax decrease
Irreversible
Covalent binding destroys activity
Aspirin (cyclooxygenase), organophosphates (acetylcholinesterase)
- Allosteric regulation and feedback inhibition control pathways — e.g., ATP inhibits phosphofructokinase-1.
Key terms
- Enzyme
- Biological catalyst
- Active site
- Region where the substrate binds
- Isoenzymes
- Different forms of an enzyme catalysing the same reaction
- Km
- Substrate concentration at half maximum velocity
- Competitive inhibitor
- Inhibitor competing with substrate for the active site
Quick revision
- Six IUBMB classes plus translocases; diagnostic enzymes.
- Specificity, active site, cofactors, coenzymes, isoenzymes, zymogens.
- Activation energy; lock and key vs induced fit.
- Substrate, enzyme, temperature, pH effects.
- Michaelis–Menten, Km, Vmax, Lineweaver–Burk; competitive, non-competitive, uncompetitive, irreversible inhibition; allosteric control.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Name the six classes of enzymes.
- Q2.What is a coenzyme?
- Q3.Give an example of isoenzymes.
- Q4.Define Km.
- Q5.How does competitive inhibition affect Km and Vmax?
- Q6.Name an irreversible inhibitor.
Long-answer questions
- Q1.Classify enzymes with examples.
- Q2.Explain the mechanism of enzyme action and the factors affecting it.
- Q3.Explain Michaelis–Menten kinetics and the Lineweaver–Burk plot.
- Q4.Describe the types of enzyme inhibition.
Stuck on this unit?
Message SBS on WhatsApp for help with Biochemical Metabolism, or to ask about studying B.Sc MLS at Synetic.