Enzymes are generally
A carbohydrates
B proteins
C lipids
D salts
Most enzymes are globular proteins (some RNA also acts as enzyme—ribozymes, but basic concept: proteins).
The region where substrate binds to enzyme is called
A coenzyme site
B active site
C allosteric site
D prosthetic site only
Active site has specific shape and functional groups for binding and catalysis.
Enzymes are highly specific because of
A random collisions
B complementary shape of active site
C high molecular mass
D high melting point
Structure of active site fits substrate (lock-key / induced fit).
Lock-and-key model suggests enzyme active site is
A rigid and exactly complementary to substrate
B flexible and changes shape only after binding
C destroyed after reaction
D same for all substrates
Classic model: rigid active site + perfect fit.
Induced-fit model suggests
A enzyme is rigid always
B enzyme changes shape to fit substrate on binding
C substrate changes to fit enzyme only
D enzyme works only at 100°C
Active site adjusts to bind substrate more strongly.
Enzymes increase rate by
A increasing ΔG°
B lowering activation energy by stabilizing transition state
C increasing equilibrium constant
D converting reactants into catalysts
Enzyme forms ES complex and stabilizes transition state → lower Ea.
Enzymes do NOT change
A reaction rate
B activation energy
C equilibrium constant
D pathway
Enzymes change kinetics only, not thermodynamics.
Optimum pH means
A pH where enzyme is fully denatured
B pH where enzyme activity is maximum
C pH where reaction stops
D pH where substrate precipitates always
Enzyme structure and ionization state are best at optimum pH.
If pH becomes too acidic or too basic, enzyme activity falls mainly due to
A increase in pressure
B denaturation/alteration of active site charges
C increase in molecularity
D increase in equilibrium constant
Extreme pH disrupts ionic bonds and structure.
Temperature effect on enzyme activity shows maximum at
A 0 K
B very high temperature always
C optimum temperature
D independent of temperature
Activity rises with T initially, then falls due to denaturation.
The enzyme–substrate complex is represented as
A E + P ⇌ ES
B E + S ⇌ ES → E + P
C ES + P → E + S
D E → S + P
Standard catalytic cycle: binding then conversion then release.
Cofactors are
A always proteins
B non-protein components required for enzyme activity
C always carbohydrates
D always inhibitors
Some enzymes need metal ions or coenzymes to function.
A coenzyme is usually a
A metal ion
B organic molecule (often vitamin-derived)
C protein chain
D salt crystal
Coenzymes like NAD⁺, FAD are derived from vitamins.
Competitive inhibition occurs when inhibitor
A binds to active site competing with substrate
B binds permanently destroying enzyme
C binds to enzyme-substrate complex only
D binds to product only
Inhibitor resembles substrate and blocks active site.
Competitive inhibition can be reduced by increasing
A inhibitor concentration
B substrate concentration
C temperature to 200°C
D pH to extreme values
More substrate outcompetes inhibitor at active site.
In non-competitive inhibition, inhibitor binds
A only at active site
B at a site other than active site (allosteric site)
C only with substrate present
D only after denaturation
It reduces catalytic activity without directly blocking substrate binding.
In non-competitive inhibition, increasing substrate concentration
A fully removes inhibition
B partially removes inhibition
C does not remove inhibition significantly
D doubles inhibition always
Because inhibitor affects enzyme function, not substrate binding competition.
Irreversible inhibition involves
A weak temporary binding
B strong permanent binding/covalent modification
C binding only to substrate
D increasing enzyme concentration instantly
Enzyme gets permanently inactivated.
The term “enzyme specificity” means enzyme acts on
A any substrate
B only a particular substrate (or closely related)
C only at 0°C
D only at pH 7 always
Specificity arises from structure and active site chemistry.
The maximum rate of enzyme-catalysed reaction is achieved when
A enzyme concentration is zero
B all enzyme active sites are saturated with substrate
C pH is extreme
D temperature is infinite
At saturation, rate reaches Vmax.
Michaelis constant (Km) is the substrate concentration at which
A rate is zero
B rate is maximum
C rate is half of maximum (Vmax/2)
D enzyme is denatured
Km indicates affinity; lower Km → higher affinity.
Lower Km indicates
A lower enzyme-substrate affinity
B higher enzyme-substrate affinity
C no binding
D irreversible inhibition
Less substrate needed to reach half-max rate.
In competitive inhibition, Km
A increases
B decreases
C remains unchanged
D becomes zero
More substrate is needed to reach Vmax/2 due to competition.
In competitive inhibition, Vmax generally
A increases
B decreases
C remains unchanged
D becomes zero
At very high substrate concentration, enzyme can still reach same Vmax.
In non-competitive inhibition, Vmax generally
A increases
B decreases
C unchanged
D becomes infinite
Effective enzyme concentration reduced; maximum possible rate falls.
In non-competitive inhibition, Km generally
A increases
B decreases
C remains unchanged (basic level)
D becomes infinite
Substrate binding affinity is not directly affected; catalytic turnover reduced.
Enzymes are most effective under mild conditions because
A they require high pressure
B biological systems operate near neutral pH and moderate temperature
C they work only at 300°C
D they always need UV light
Enzymes evolved to work under physiological conditions.
Denaturation of enzyme means
A increase in enzyme activity
B loss of 3D structure and activity
C conversion into coenzyme
D increase in Km always
Denaturation disrupts active site shape and function.
If enzyme concentration is doubled (substrate in excess), the rate
A halves
B doubles
C becomes zero
D remains unchanged
More enzyme active sites → more ES formation per unit time (when substrate not limiting).
If substrate concentration is increased at fixed enzyme concentration, rate initially
A decreases
B increases and then becomes constant
C remains constant from start
D becomes negative
Rate rises then reaches Vmax when enzyme saturates.
Which is TRUE about enzyme catalysis
A Enzyme is consumed in reaction
B Enzyme remains unchanged after reaction
C Enzyme increases ΔG°
D Enzyme changes equilibrium constant
Enzyme acts as catalyst and is regenerated.
The primary reason enzymes catalyse reactions fast is
A they provide heat
B they stabilize transition state
C they increase product energy
D they lower reactant concentration
Stabilizing transition state reduces activation energy.
If an enzyme works best at pH 7, then at pH 2 it will likely
A work faster
B work slower due to structural change
C work unchanged
D increase Vmax
Extreme pH disrupts ionic interactions and active site charge.
Enzyme inhibition is important in medicine because
A it always increases digestion
B many drugs work by inhibiting enzymes
C it increases equilibrium constant
D it produces catalysts
Enzyme inhibitors can block harmful biochemical pathways.
The enzyme + cofactor together is called
A apoenzyme
B holoenzyme
C zymogen
D substrate
Apoenzyme (protein part) + cofactor = holoenzyme (active form).
Protein part of enzyme alone (inactive without cofactor) is called
A holoenzyme
B apoenzyme
C prosthetic group
D inhibitor
Apoenzyme requires cofactor to become active holoenzyme.
Prosthetic group is a cofactor that is
A loosely bound
B tightly bound to enzyme
C always inorganic
D always a gas
Prosthetic groups remain attached (e.g., heme in some enzymes).
Which condition best represents enzyme saturation
A [S] ≪ Km
B [S] ≫ Km
C [E] ≫ [S]
D Ea becomes zero
At very high substrate, almost all enzyme active sites are occupied.
If an inhibitor resembles the substrate structure, it most likely causes
A non-competitive inhibition
B competitive inhibition
C irreversible inhibition only
D no inhibition
Substrate analogs compete at active site.
Enzyme activity vs temperature graph typically shows
A straight line rise only
B straight line fall only
C rise to maximum then sharp decline
D constant line
Higher T increases collisions up to optimum; then denaturation lowers activity.
Which is NOT a factor affecting enzyme activity
A pH
B temperature
C substrate concentration
D standard enthalpy change (ΔH°)
ΔH° is not the direct control factor for enzyme activity.
If enzyme is denatured, it mainly loses
A primary structure always first
B active site shape (secondary/tertiary structure)
C atomic number
D molecularity
Denaturation disrupts higher-order structure, destroying active site.
An allosteric inhibitor binds to
A active site
B a different site causing conformational change
C substrate only
D product only
Binding at allosteric site changes active site geometry.
The reaction rate in enzyme catalysis is maximum when
A ES complex formation is minimum
B ES complex formation is maximum and turnover is fastest
C enzyme is absent
D product inhibits enzyme completely
High ES formation + efficient conversion gives high rate (Vmax region).
If substrate concentration is very low, the enzyme reaction rate is approximately
A independent of substrate
B directly proportional to substrate concentration
C inversely proportional to substrate
D equal to Vmax always
At low [S], enzyme not saturated; rate increases nearly linearly with [S].
Which is correct about enzyme specificity
A enzyme acts equally on all substrates
B enzyme acts mainly on one specific substrate or similar substrates
C enzyme activity depends only on pressure
D enzyme has no active site
Active site structure determines specificity.
A strong acid can inactivate enzyme mainly by
A increasing collision frequency
B breaking ionic/hydrogen bonds in enzyme structure
C decreasing substrate concentration
D increasing equilibrium constant
Extreme pH disrupts bonding responsible for tertiary structure.
Enzymes are more efficient than ordinary catalysts because they
A work at very high temperatures
B show high specificity and high catalytic power
C increase ΔH
D are consumed permanently
Enzymes bind substrates precisely and stabilize transition state strongly.
In enzyme catalysis, the energy barrier decreases mainly due to
A higher product stability
B formation of ES complex and transition state stabilization
C increased ΔG°
D increased Kc
ES formation provides alternate pathway with lower Ea.
Which statement is most correct
A Enzymes increase equilibrium constant
B Enzymes reduce activation energy and increase rate
C Enzymes change ΔH of reaction
D Enzymes make endothermic reactions exothermic
Core role of enzymes is kinetic acceleration via lower Ea; thermodynamics remain same.