3. Enzymes Kinetics

Question 1 What is an enzyme?

A protein catalyst, except for ribozymes, that increases the rate of a reaction without being consumed in the overall process.

Question 2 What is enzyme kinetics?

The quantitative measurement of the rates of enzyme-catalyzed reactions and the study of factors that affect those rates.

Question 3 What general reaction scheme is used to represent enzyme catalysis?

E + S

Question 4 In the enzyme reaction scheme, what does E represent?

The enzyme.

Question 5 In the enzyme reaction scheme, what does S represent?

The substrate.

Question 6 In the enzyme reaction scheme, what does ES represent?

The enzyme-substrate complex.

Question 7 In the enzyme reaction scheme, what does P represent?

The product.

Question 8 How can enzymes influence metabolic pathways?

They regulate metabolic rates by accelerating selected biochemical reactions.

Question 9 Why can enzyme deficiency produce disease?

Because loss or reduction of an enzyme can impair the metabolic reaction or pathway that depends on it.

Question 10 Why are enzymes important therapeutic targets?

Some drugs act by inhibiting or otherwise modifying specific enzyme activities.

Question 11 What can elevated enzyme activity in plasma indicate?

Tissue damage.

Question 12 Which serum enzyme is associated with myocardial infarction in the lecture but is described as nonspecific?

Aspartate aminotransferase (AST/SGOT).

Question 13 Which serum enzyme is associated with viral hepatitis in the lecture?

Alanine aminotransferase (ALT/SGPT).

Question 14 Which serum enzyme is associated with acute pancreatitis?

Amylase.

Question 15 Which serum enzyme is associated with muscle disorders and myocardial infarction?

Creatine kinase (CK).

Question 16 Which serum enzyme is associated with various liver diseases?

Gamma-glutamyl transferase (GGT).

Question 17 Which lactate dehydrogenase isozyme is associated with liver disease in the lecture?

Lactate dehydrogenase isozyme 5 (LDH-5).

Question 18 Deficiency of which enzyme is associated with Gaucher disease in the lecture table?

Beta-glucocerebrosidase.

Question 19 Which serum enzyme is associated with various bone disorders and obstructive liver disease?

Alkaline phosphatase.

Question 20 What does the term rate mean in enzyme kinetics?

Speed or velocity; a measure of change over a period of time.

Question 21 What is reaction rate?

The formation of product over a period of time.

Question 22 If product concentration increases rapidly over a short time, what does this imply about reaction rate?

The reaction rate is high.

Question 23 What two major perspectives are used to explain the mechanism of enzyme action?

Energy changes during the reaction and chemical catalysis at the active site.

Question 24 What is the active site?

The region of an enzyme where substrate binding and catalytic events occur.

Question 25 Why is the active site important to enzyme kinetics?

Its binding and catalytic properties determine formation of the ES complex and conversion of substrate to product.

Question 26 What happens to the enzyme after product formation in the basic kinetic model?

The free enzyme is regenerated and can participate in another catalytic cycle.

Question 27 What is the practical purpose of studying enzyme kinetics?

To quantify enzyme reaction rates and determine how variables such as substrate, pH, temperature, inhibitors, and regulation affect those rates.

Question 28 A patient has elevated plasma enzyme activity after tissue injury. What general principle explains this finding?

Damaged cells can release intracellular enzymes into the blood.

Question 29 A drug decreases the velocity of an enzyme-catalyzed reaction. What general term describes the drug's action on the enzyme?

Enzyme inhibition.

Question 30 What four major themes are emphasized in Enzymes 2?

Reaction rates and their determinants, Michaelis-Menten and Lineweaver-Burk analysis, enzyme inhibition, and enzyme regulation with clinical correlations.

Question 31 What is activation energy (Ea)?

The energy barrier separating reactants from products.

Question 32 What is the transition state?

A high-energy intermediate state formed as reactants are converted toward product.

Question 33 Why must reactant molecules have sufficient energy before a reaction can proceed?

They must overcome the activation-energy barrier and reach the transition state.

Question 34 What is the relationship between activation energy and reaction rate?

Lower activation energy produces a faster reaction rate.

Question 35 How does an enzyme accelerate a chemical reaction?

It provides an alternate reaction pathway with a lower activation energy.

Question 36 Does an enzyme change the free-energy difference between reactants and products?

No. It lowers activation energy but does not change the overall free-energy difference.

Question 37 Does an enzyme change the equilibrium position of a reaction?

No. It accelerates the approach to equilibrium without changing the equilibrium itself.

Question 38 What is transition-state stabilization?

Stabilization of the high-energy intermediate by the enzyme, making formation of the transition state more likely.

Question 39 How does transition-state stabilization increase catalytic rate?

It increases the effective concentration of the reactive intermediate and lowers the energetic barrier to product formation.

Question 40 What can the active site provide to enhance formation of the transition state?

Catalytic groups.

Question 41 What four catalytic strategies are listed in the Enzymes 2 lecture?

General acid-base catalysis, transient covalent catalysis, metal catalysis, and catalysis by altered proximity and orientation.

Question 42 What is general acid-base catalysis?

Catalysis in which groups in the active site donate or accept protons to facilitate the reaction.

Question 43 What is transient covalent catalysis?

Catalysis involving temporary covalent bonding between the enzyme and substrate during the reaction.

Question 44 What is metal catalysis?

Catalysis in which a metal ion helps stabilize charges or otherwise facilitates the chemical reaction.

Question 45 What is catalysis by alteration in proximity and orientation?

The enzyme positions reacting groups close together and in a favorable orientation for reaction.

Question 46 What three major factors affecting reaction velocity are emphasized in the lecture?

pH, temperature, and substrate concentration.

Question 47 How can pH alter enzyme activity?

By changing the ionization state of groups in the active site and substrate that are needed for binding or catalysis.

Question 48 Why may an enzyme require a specific ionization state at its active site?

Correct protonation or deprotonation of catalytic groups may be necessary for substrate binding and chemical catalysis.

Question 49 What can extremes of pH do to an enzyme?

Cause denaturation and loss of catalytic activity.

Question 50 Do all enzymes have the same optimum pH?

No. Different enzymes have different optimum pH values.

Question 51 Which enzyme is shown as functioning best in a strongly acidic environment?

Pepsin.

Question 52 Which enzyme is shown as having an optimum near neutral pH?

Human amylase.

Question 53 Which enzyme is shown as functioning best in a more alkaline environment?

Trypsin.

Question 54 Why does pepsin function well at an acidic pH?

Its catalytic groups and structure are suited to the acidic environment in which it normally acts.

Question 55 What happens to reaction velocity as temperature initially rises?

Reaction velocity increases up to an optimum temperature.

Question 56 Why does moderate warming increase enzyme reaction rate?

More molecules have sufficient kinetic energy to reach the transition state.

Question 57 Why does reaction velocity fall at very high temperature?

The enzyme becomes denatured and loses functional structure.

Question 58 According to the Topnotch handout, what temperature range begins to raise concern for protein denaturation in the human body?

Above about 42 degrees Celsius.

Question 59 How does increasing substrate concentration affect an enzyme-catalyzed reaction at low substrate levels?

It increases reaction velocity because more enzyme active sites encounter substrate.

Question 60 What happens to reaction velocity as substrate concentration becomes very high?

Velocity approaches a maximum because enzyme active sites become saturated.

Question 61 What is enzyme saturation?

The condition in which essentially all available enzyme active sites are occupied by substrate.

Question 62 Why does adding more substrate after saturation fail to markedly increase velocity?

There are no additional free active sites available to process the extra substrate.

Question 63 What variable must be increased to raise Vmax once all enzyme molecules are saturated?

Enzyme concentration.

Question 64 A reaction rate rises with added substrate and then plateaus. What phenomenon explains the plateau?

Saturation of enzyme active sites.

Question 65 A patient's enzyme works poorly at both very low and very high pH but best at an intermediate pH. What concept does this illustrate?

An optimum pH for enzyme activity.

Question 66 A reaction becomes faster as temperature rises from 20 to 37 degrees Celsius but then slows at much higher temperature. What causes the decline?

Heat-induced enzyme denaturation.

Question 67 Professor Question: If an enzyme lowers Ea but leaves delta G unchanged, what aspect of the reaction has changed?

The reaction rate has increased, but the thermodynamic free-energy difference and equilibrium position have not changed.

Question 68 Professor Question: If the transition state is stabilized, what happens to the probability of product formation?

It increases.

Question 69 Professor Question: A mutation disrupts active-site residues that orient the substrate correctly. Which catalytic mechanism is impaired?

Catalysis by altered proximity and orientation.

Question 70 Professor Question: A catalytic residue can no longer donate a proton. Which catalytic strategy is most directly impaired?

General acid-base catalysis.

Question 71 What does the Michaelis-Menten equation describe?

How initial reaction velocity varies with substrate concentration.

Question 72 Write the Michaelis-Menten equation.

V0 = Vmax[S] / (Km + [S]).

Question 73 What does V0 represent in the Michaelis-Menten equation?

Initial reaction velocity.

Question 74 What does Vmax represent?

Maximal reaction velocity.

Question 75 What does Km represent?

The Michaelis constant.

Question 76 What does [S] represent?

Substrate concentration.

Question 77 What is the shape of a typical Michaelis-Menten plot of V0 versus [S]?

Hyperbolic.

Question 78 Why is initial velocity used in Michaelis-Menten analysis?

At the start of the reaction, product concentration is negligible, so the reverse reaction from product to substrate can be ignored.

Question 79 What assumption does Michaelis-Menten kinetics make about substrate concentration relative to enzyme concentration?

[S] is much greater than [E].

Question 80 Why must [S] be much greater than [E] in the Michaelis-Menten model?

So only a small fraction of the total substrate is bound to enzyme at any one time.

Question 81 What is the steady-state assumption for the ES complex?

The concentration of ES remains approximately constant because its rate of formation equals its rate of breakdown.

Question 82 At what substrate concentration is reaction velocity equal to one-half Vmax?

When [S] = Km.

Question 83 What does a low Km indicate about enzyme-substrate affinity?

High affinity.

Question 84 What does a high Km indicate about enzyme-substrate affinity?

Low affinity.

Question 85 Why does a low Km reflect high affinity?

Only a low substrate concentration is needed to achieve one-half Vmax.

Question 86 Why does a high Km reflect low affinity?

A higher substrate concentration is needed to achieve one-half Vmax.

Question 87 Does Km change simply because enzyme concentration is increased?

No. Km is characteristic of the enzyme-substrate relationship and does not vary with enzyme concentration.

Question 88 How is Vmax related to enzyme concentration?

Vmax is directly proportional to enzyme concentration.

Question 89 If enzyme concentration doubles while other conditions are unchanged, what happens to Vmax?

Vmax doubles.

Question 90 If enzyme concentration is halved, what happens to Vmax?

Vmax is halved.

Question 91 At low [S] compared with Km, what is the reaction order with respect to substrate?

First order.

Question 92 What does first-order kinetics mean in this context?

Reaction velocity depends on substrate concentration and increases as [S] increases.

Question 93 At high [S] compared with Km, what is the reaction order with respect to substrate?

Zero order.

Question 94 What does zero-order kinetics mean in this context?

Reaction velocity is essentially independent of substrate concentration because the enzyme is saturated.

Question 95 What velocity is approached during zero-order kinetics for an enzyme-catalyzed reaction?

Vmax.

Question 96 Why is the reaction first order when [S] is much less than Km?

Many active sites are unoccupied, so adding substrate increases the frequency of ES formation and increases velocity.

Question 97 Why is the reaction zero order when [S] is much greater than Km?

The enzyme is saturated, so additional substrate cannot increase the catalytic capacity.

Question 98 What region of the Michaelis-Menten curve corresponds mainly to first-order kinetics?

The low-substrate, rising portion of the curve.

Question 99 What region of the Michaelis-Menten curve corresponds mainly to zero-order kinetics?

The high-substrate plateau near Vmax.

Question 100 How is Vmax defined in terms of product formation?

The maximal number of substrate molecules converted to product per unit time at a given enzyme concentration.