Enzyme Kinetics and Inhibition - Biochemistry

To answer this question, we need to understand what competitive inhibition is. When a competitive inhibitor is present, an enzyme's active site will be able to bind either substrate or the inhibitor. Thus, for a given substrate concentration, the reaction will be slower in the presence of the inhibitor because sometimes the inhibitor will interfere with the binding of the substrate to the enzyme's active site. However, we're not looking for reaction rate at a given substrate concentration. Instead, we are looking at the maximum possible reaction rate given any amount of substrate concentration. If we keep adding more and more substrate in the presence of the inhibitor, eventually we will get to a point where there is so much substrate present that having the inhibitor around doesn't make a difference on the reaction rate. Therefore, the addition of a competitive inhibitor has no effect on the of an enzyme catalyzed reaction.

Competitive inhibitors bind the active site of enzymes, and compete with the substrate for this binding site. Thus, the does not change since if enough substrate is added, regardless of the differential affinities between the substrate and inhibitor for the active site, the substrate will outcompete the inhibitor. However, increases upon the addition of a competitive inhibitor.

Competitive inhibition is characterized by an increase in the Michaelis-Menten constant, . Note that this constant represents the substrate concentration at which half the enzymes are occupied with substrate. If increases, then it suggests that a higher concentration of substrate is needed to occupy half the enzymes. is also a measure of the affinity between substrate and enzyme. As increases, the affinity decreases and more substrate is required to bind 50% of the enzyme. Competitive inhibitors bind to the active site of the enzyme and compete with the substrate for the binding site on the enzyme, thereby decreasing the affinity and increasing .

Competitive inhibitors do not alter the maximum velocity of an enzyme-substrate reaction. Recall that enzymes speed up reactions; therefore, the velocity of a reaction is a direct measure of its efficacy. This means that competitive inhibitors do not alter the efficacy of the enzyme.

Competitive inhibitors bind to the active site of the enzyme and prevent substrates from binding to enzyme. This prevents the enzyme-substrate reaction from happening, thereby decreasing the activity of enzymes; however, competitive inhibitors can be overcome by increasing the concentration of substrates. Increase in the amount of substrates will displace the inhibitors from the active site and allow for substrates to bind. This will bring the efficacy of the enzyme back up to normal levels.

Increasing and decreasing the volume of the solution will concentrate or dilute all species in the solution, respectively. This will not decrease the effects of competitive inhibitors on the enzyme.

There are two types of sites on the enzyme where molecules can bind. Active sites are the main location for substrate-enzyme binding. These sites usually involve weak, reversible bonds (such as hydrogen bonds) between substrate and enzyme. Allosteric site, on the other hand, are found at a different location on the enzyme and bind certain types of inhibitors and modulators of the enzyme. These are usually more permanent bonds (covalent bonds) and are irreversible.

Competitive inhibitors bind to active sites and form weak, reversible bonds. This is why we can dissociate competitive inhibitors from the active site by increasing the concentration of substrates. Substrates will compete for the active site and displace bound competitive inhibitors.

Competitive inhibitors bind to the substrate binding site of an enzyme and have the following effect: Increase , No change in .

Noncompetitive inhibitors bind to a site other than the substrate binding site and have the following effect: No change in , Decrease in .

Increasing the , lowers the affinity since the is the substrate concentration at which the reaction proceeds as one-half of .

Because carbon monoxide binds at the same site as oxygen, this is a form of competitive inhibition. In order to overcome this type of inhibition, the concentration of substrate (oxygen) needs to be increased.

The type of inhibition being described here is competitive. The carbon monoxide binds to the same site that oxygen does. Therefore, by increasing the amount of substrate available, the inhibitor can be outcompeted. This is why Vmax for competitive inhibition is unchanged. Km on the other hand, is decreased for competitive inhibition.

A molecule can only competitively inhibit another molecule if it fits into the same active site in the enzyme. In the reaction, goes into the active site of the enzyme, and so only a molecule with its similar structure can competitively inhibit it - .

The hydrolytic enzymes found in lysosomes are pH-sensitive and function best in acidic environments. More specifically, when the pH of lysosomes change to an acidic pH of 4.8, the enzymes will become more active. These enzymes will not function well, or potentially at all, in alkaline (basic) environments. Since these enzymes have digestive capabilities, their pH activation helps to restrict their activity to the lumen of the lysosome.

is the maximal velocity of an enzyme-catalyzed reaction. This occurs when all active sites of enzymes are occupied with substrate, and new openings will be filled immediately from the excess substrate. The enzyme is saturated with excessive substrate, so the reaction velocity no longer depends on substrate concentration.

Apoenzymes are holoenzymes without a coenzyme. There are no prosthetic enzymes, only prosthetic groups. Phosphorylases generally remove phosphate groups from substrates, and kinases generally add phosphate groups to substrates.

From the answer choices, we see that there are a variety of constants that we're presented with. However, it is the Michaelis constant that signifies the amount of substrate at which an enzymatic reaction will be at half of its maximum value. The other constants, though related in some way to enzymatic reactions, do not answer the question.

The rate constant signifies the reaction rate at any given point in time, while the equilibrium constant is a thermodynamic value that tells us the spontaneity of the reaction.

The size of an enzyme is typically not indicative of the rate of the reaction that it catalyzes. All other parameters are taken into account when considering reaction velocity.

A first-order reaction is one in which the reaction rate varies directly with the concentration of the reactant. A zero-order reaction is one in which the reaction rate is independent of the concentration of the reactant. A second-order reaction is one in which the reaction rate varies directly with the square of the concentration of one reactant, or one in which the reaction rate varies directly with the concentration of two reactants.

Both myoglobin (Mb) and hemoglobin (Hb) use heme groups to bind to oxygen. However, Hb contains four heme groups, whereas Mb contains only one. Single-ligand binding appears as a hyperbolic curve in ligand binding graphs, whereas sigmoidal curves indicate cooperative binding. As one ligand (oxygen) binds to hemoglobin, this makes it easier and more favorable for the second oxygen to bind, and so on for the third and finally the fourth oxygen; each oxygen binding allows the one following it to bind more easily. This behavior is responsible for creating the sigmoidal curve - the slope of the curve increases with pressure, indicating better binding capability, up to the point where the Hb starts to become totally saturated with oxygen molecules.

Allosteric enzymes have multiple active sites, which affect each other. More often than not, allosteric enzymes will have sigmoidal plots when reaction velocity is plotted against enzyme concentration, and thereby display cooperativity. Cooperativity means that when one active site is bound by substrate, the other sites become easier to bind for substrate. Hemoglobin is a notable example of a protein that exhibits this type of enzyme kinetics.

Covalent modification - e.g phosphorylating a molecule to activate it.

Proteolytic cleavage - e.g zymogen becoming activated when cleaved.

Association with other polypeptides - e.g enzyme may have both catalytic and regulatory subunits - regulatory controls activity of the catalytic.

Allosteric regulation - e.g allosteric site on an enzyme that can become bound by a molecule, altering the protein's function.

Enzymes are substrate specific molecules that lower the activation energy and increase the reaction rate of product formation. Enzymes do not change the equilibrium of product formation; this characteristic stays the same under the influence of enzymes.