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Biology · Unit 6

Enzymes: MDCAT Notes and MCQs

Characteristics

  • Enzymes are (mostly) globular proteins acting as biological catalysts; they lower activation energy and are not consumed.
  • Highly specific; work in small amounts; reactions are usually reversible; sensitive to temperature, pH and inhibitors.
  • Active site: small region (a few amino acids) where substrate binds. Has binding site + catalytic site.
  • Cofactors: non-protein parts. Activators = inorganic ions (Mg2+, Zn2+, Cl− for amylase). Prosthetic group = tightly (covalently) bound organic cofactor (e.g. haem, FAD). Coenzyme = loosely bound organic cofactor (NAD+, vitamins). Protein part alone = apoenzyme; apoenzyme + cofactor = holoenzyme.

Mechanism

  • E + S ⇌ ES complex → E + P.
  • Lock and key model — Emil Fischer (1894): rigid active site exactly fits the substrate.
  • Induced fit model — Koshland (1959): active site is flexible and changes shape to fit the substrate.

Factors affecting rate

  • Temperature: rate rises with temperature up to the optimum (≈ 37 °C for human enzymes); above it, enzyme is denatured. Low temperature only inactivates (reversible).
  • pH: each enzyme has an optimum pH — pepsin ≈ 2, salivary amylase ≈ 6.8, trypsin ≈ 8, arginase ≈ 9.7.
  • Enzyme concentration: rate ∝ [E] when substrate is in excess.
  • Substrate concentration: rate rises then levels off at Vmax when all active sites are saturated.

Inhibitors

  • Competitive (reversible): resembles substrate, binds the active site; overcome by adding more substrate (e.g. malonate inhibits succinic dehydrogenase; sulpha drugs).
  • Non-competitive: binds at another (allosteric) site, changes active site shape; not reversed by excess substrate.
  • Irreversible: bind permanently (covalently) — e.g. cyanide (cytochrome oxidase), heavy metals (Hg, Pb), nerve gases, some insecticides.
PMDC learning outcomes for this unit
  • Enzymes: 6.1 Describe the distinguishing characteristics of enzymes
  • Mode of Enzyme Action: 6.2 Explain mechanism of action of enzymes
  • Factors that Affect the Rate of Enzyme Reactions: 6.3 Describe effects of factors on enzyme action (temperature, pH and concentration)
  • Inhibitors: 6.4 Describe enzyme inhibitors

Enzymes MCQs with answers

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Q1
Enzymes increase the rate of reaction by:
AIncreasing activation energy
BLowering activation energy
CChanging the equilibrium
DIncreasing temperature
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Answer: B (Lowering activation energy). Catalysts provide an alternative pathway of lower activation energy.
Q2
The induced fit model of enzyme action was proposed by:
AWatson
BKoshland
CEmil Fischer
DSanger
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Answer: B (Koshland). Koshland (1959) proposed that the active site changes shape on substrate binding; Fischer gave lock & key (1894).
Q3
A non-protein organic cofactor tightly bound to the enzyme is a:
AProsthetic group
BCoenzyme
CActivator
DApoenzyme
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Answer: A (Prosthetic group). Prosthetic groups are covalently/tightly bound organic cofactors; coenzymes are loosely attached.
Q4
The optimum pH of pepsin is about:
A2
B9.7
C8
D7
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Answer: A (2). Pepsin works in the acidic stomach (pH ≈ 2); trypsin ≈ 8.
Q5
At high temperature (e.g. 70 °C) most human enzymes:
AAre denatured
BWork faster
CBecome more specific
DAre temporarily inactivated
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Answer: A (Are denatured). Heat breaks bonds holding the 3-D shape, permanently denaturing the enzyme.
Q6
A competitive inhibitor:
ACannot be overcome by excess substrate
BResembles the substrate and binds the active site
CAlways binds covalently
DBinds to an allosteric site
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Answer: B (Resembles the substrate and binds the active site). It competes with substrate for the active site; adding more substrate reduces its effect.
Q7
Cyanide is a poison because it irreversibly inhibits:
AAmylase
BHexokinase
CPepsin
DCytochrome oxidase
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Answer: D (Cytochrome oxidase). Cyanide blocks cytochrome oxidase, stopping the electron transport chain.
Q8
When all active sites are occupied, increasing substrate concentration:
ADecreases rate sharply
BDenatures enzyme
CDoes not increase the rate
DIncreases rate further
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Answer: C (Does not increase the rate). Enzyme is saturated; the reaction proceeds at Vmax.
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