Enzyme Kinetics and Models - Biochemistry

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.

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.

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.

A catalyst works to speed up a reaction by lowering the Ea (activation energy), which is related to chemical kinetics. A catalyst has nothing to do with thermodynamics, and can neither change the of a reaction, nor make a non-spontaneous one occur.

Additionally, by definition a catalyst is not consumed in a reaction, and it is regenerated after the reaction occurs. This allows for the possibility of a catalyst having an enormous impact on the rate of a reaction, as it is can once again exert its effects after being regenerated.

When substrate concentration is high, all of the active sites can be constantly occupied by enzyme. Further, the active sites can be saturated by enzyme when concentration is very high. But, when the concentration of substrate is low, the reaction velocity is considered to be linear.

The question describes the "induced fit model", in which the binding of a substrate induces an actual change in the shape of the enzyme in order for the substrate to fit properly. This models differs from "the lock and key hypothesis" and "the active site model" (which are actually the same thing, and describes an enzyme and its substrate as fitting perfectly together). Cooperative binding is a concept that refers to binding becoming easier for subsequent molecules in comparison to previous molecules (the sixth molecule binds more readily than the fifth, which binds more readily than the fourth, etc.). Finally, affinity dependent binding is not a real term used in biochemistry (all binding is related to affinity anyway).

The most important concept about chronic alcoholism is based off the fact that the increased ratio of from alcohol metabolism interferes with gluconeogenesis; 2) the conversion of pyruvate to lactate is favored, decreasing gluconeogenesis and increasing blood lactate. Pyruvate dehydrogenase, the enzyme that catalyzes the conversion of pyruvate to lactate. The conversion of pyruvate to acetyl-CoA, catalyzed by alanine aminotransferase, does not require and is not directly affected by this ratio. An increase in pyruvate to oxaloacetate (catalyzed by pyruvate carboxylase), is a reaction used in gluconeogenesis. If increased, it would favor this process or stimulate the citric acid cycle.

When the body is in a well fed state, insulin stimulates glycolysis (glucose uptake) and the first step to convert glucose to glucose-6-phosphatase. When you have an increase of insulin, it stimulates protein phosphatase (PP1), which stimulates synthase while inhibiting phosphorylase. This leads to a decreased conversion of glycogen to glucose. Gluconeogeneisis occurs in the starvation state as well as fatty acid degradation.

A first order reaction is one in which the amount of product is dependent on only one reactant. For instance:

The above reaction is first order because the amount of product produced depends solely on the concentration of A. Therefore, a first order reaction is one in which the reaction is directly proportional to the reactant concentration.

Note: bimolecular reaction and second order reaction are synonymous.

Zero order reactions are reactions in which the enzyme is saturated with substrate (in first order, the substrate concentration is below Km). The velocity in a zero-order reaction is independent of substrate concentration, therefore the two variables are not related. In a first order reaction, velocity is proportional to substrate concentration.

All of the above statements are true concerning enzymes. They work to lower the activation energy of reactions and do not change the concentrations of substrates or products. They are highly specific and do not get changed by the reaction they are catalyzing.

A molecule that binds to an allosteric site on an enzyme does not necessarily have a similar structure to the actual substrate. That would only be the case for a molecule that binds to the enzyme's active site. Moreover, the molecule binding to the allosteric site will almost always cause a conformational change in the enzyme's structure, but it can either enhance or inhibit the catalytic activity by doing so.

The fractional saturation of an enzyme is defined as the amount of enzyme that is bound to substrate divided by the total amount of enzyme. To calculate the fractional saturation, we'll need to use the Michaelis-Menton equation:

In addition, we'll need to define the rate and maximum rate in terms of enzyme concentrations:

From the above equations, we can calculate the fractional saturation of the enzyme: