Deck 8: Enzymes: Biological Catalysts

Question 1

The lock and key model of substrate binding and enzymatic catalysis explains:

A) the release of product.
B) substrate specificity.
C) formation of a transition state.
D) the catalytic mechanism.
E) structural changes that occur on substrate binding.

Answer 1

B

Question 2

Pyruvate carboxylase is an example of the ligase class of enzymes.

Answer 2

True

Question 3

In a favorable reaction the free energy of the products is ________ than the free energy of the reactants.

Answer 3

less

Question 4

Electrostatic catalysis proceeds via covalent bonding interactions.

Question 5

The formation of an enzyme-substrate complex tends to be thermodynamically favorable due to ________ interactions between the substrate and enzyme.

Question 6

A Lineweaver-Burk plot for a first order enzyme-catalyzed reaction gives values of 1/KM = 2.5 × 104 (M)-1 and 1/Vmax of 1.25 × 10-2 (µmolL-1 sec-1)-1. Calculate the rate constant k.

Question 7

The equilibrium state of a biochemical reaction is approached ________ in the presence of a catalyst.

Question 8

The cofactor NAD+ is:

A) a reductant.
B) an oxidant.
C) oxidized to NADH/H+ in dehydrogenase reactions.
D) able to accept 2 electrons and 2 protons.
E) covalently linked to enzymes in whose catalytic activity it assists.

Question 9

Enzymes can accelerate reactions by:

A) binding a substrate or substrates.
B) promoting the removal or addition of protons.
C) correctly positioning a metal ion for catalysis.
D) lowering the energy for activation.
E) all of the above.

Question 10

Catalysts affect the thermodynamic f of a chemical reaction.

Question 11

Which of the following statements about the proposed mechanisms of action for hen egg white lysozyme does NOT support either model?

A) Glycosidic bond cleavage occurs by general acid/base catalysis.
B) A covalent intermediate is formed between an active site aspartate and C1 of the substrate.
C) A water molecule is deprotonated, which then attacks C1 of the substrate.
D) The active site aspartic acid changes between being protonated and deprotonated.
E) The active site glutamic acid changes between being protonated and deprotonated.

Question 12

Which of the following statement is FALSE?

A) The free energy barrier in a chemical reaction must be overcome in order for products to form.
B) An increase in temperature can result in an increased reaction rate.
C) Lowering the free energy of the transition state can increase a reaction rate.
D) At a given temperature and time all molecules in a solution or a sample will have the same energy.
E) An enzyme can increase a reaction rate by lowering the activation energy.

Question 13

In an enzyme-catalyzed reaction, the lifetime of the transition state is similar to the vibrational frequencies of covalent bonds.

Question 14

Non-catalyzed biochemical reactions always occur at physiological useful timescales.

Question 15

A common feature of serine proteases is the catalytic triad consisting of a nucleophile, a general base and an acid. In chymotrypsin these amino acid residues are ________, ________, and ________ respectively.

Question 16

A second-order reaction:

A) occurs when one substrate is converted into one product.
B) is characterized by two molecules coming together to form a product.
C) is the rate-limiting step of a reaction.
D) has a rate constant with units of (time)-1.
E) only occurs in multistep processes.

Question 17

Serine proteases make use of covalent catalysis as well as electrostatic stability of the transition state to achieve rate enhancement.

Question 18

The equilibrium constant for a first-order ________ reaction is equivalent to the ratio of the rate constant for the forward and reverse reactions.

Question 19

Which of the following amino acid residues are often involved in proton transfers in enzyme-catalyzed reactions?

A) Histidine, aspartate, serine, and cysteine
B) Serine, tyrosine, arginine, and cysteine
C) Glutamine, asparagine, lysine, and tyrosine
D) Histidine, aspartate, lysine, and serine
E) Histidine, aspartate, glutamate, arginine, and lysine

Question 20

The ________ hypothesis suggests that an enzyme can induce distortion of the substrate or the substrate can induce conformational changes in the enzyme that stabilize the transition state.

Question 21

A mutation causing an amino acid change in an enzyme that affects the turnover number kcat will always affect the KM as well.

Question 22

Proteolytic cleavage is an example of an ________ post-translational modification.

Question 23

A graph of initial velocity vs substrate concentration will be ________ for an enzyme that obeys Michaelis-Menton kinetics.

Question 24

A reversible ________ inhibitor is one that binds to the active site of an enzyme but cannot undergo the chemical conversion step of the reaction.

Question 25

Feedback regulation of a metabolic pathway can either be activation or inhibition.

Question 26

Random substrate binding, ordered substrate binding and the ping-pong mechanism are all features of ________ enzyme-catalyzed reactions.

Question 27

Which of the following statements about inhibitors of enzyme-catalyzed reactions is TRUE?

A) An uncompetitive inhibitor typically affects KM but not kcat.
B) A competitive inhibitor does not affect Vmax.
C) An uncompetitive inhibitor will always bind at the active site.
D) A competitive inhibitor binds irreversibly to the enzyme at the active site.
E) Reversible inhibitors bind to either free enzyme or the enzyme-substrate complex but not both.

Question 28

Which of the following statements applies to metalloenzymes?

A) Some metal ions assist in ATP binding.
B) Amino acid residues in the enzyme are never covalently linked to the metal ion.
C) The metal does not bind at the catalytic site.
D) Many are oxido-reductases.
E) A and D

Question 29

Catalytically inactive enzyme precursors are called ________.

Question 30

A Lineweaver-Burk plot can be used to determine KM using initial-rate data for an enzyme-catalyzed reaction.

Question 31

A stopped-flow apparatus is used to measure rates of pre-steady state slow enzymatic reactions.

Question 32

Which of the following is a feature of allosteric regulation of enzyme activity?

A) Ligand binding causes a conformation change in the enzyme.
B) Allosteric enzymes often have multiple active sites.
C) Cooperativity in substrate binding.
D) There is often a range of different effectors for a single enzyme.
E) All of the above

Question 33

Metal ions are often required for catalytic efficiency but they may not remain permanently bound to the protein or take part in the catalytic process.

Question 34

Which of the following is NOT a feature of substrate-level enzyme regulation?

A) A high substrate concentration will speed up the rate of reaction.
B) A high product concentration will slow the rate of reaction.
C) The product can be a competitor.
D) It is sufficient for regulation of most enzyme-catalyzed reactions.
E) Sometimes products can be competitive or uncompetitive.

Question 35

A ribonucleic acid that can act as a biological catalyst is called a ________.

Question 36

Covalent modification can either activate or inhibit enzymes.

Question 37

Almost all irreversible enzyme inhibitors bind noncovalently to the enzyme.

Question 38

Which of the following statements BEST describes the Michaelis-Menton constant KM?

A) It is numerically equal to the affinity between the enzyme and its substrate.
B) It is numerically equal to the substrate concentration required to reach half maximal velocity for an enzyme-catalyzed reaction.
C) It is a measure of the rate of a catalytic process.
D) It is a measure of enzyme efficiency.
E) It has units of concentration.

Question 39

Protein kinases add a phosphoryl group to the -OH group of a ________, ________, or ________ residue of the target protein.

Question 40

Coenzymes or cofactors are irreversibly changed during catalysis.