Types of reversible enzyme inhibition:

Two types reversible enzyme inhibition, representing the opposite ends of a spectrum, will be discussed here; there are other types of inhibition, as well as mixtures of types.

Competitive inhibition:

In the case of competitive inhibition, the inhibitor is similar, electronically, to substrate but does not convert to product; it competes for binding at the active site with substrate.

Competitive inhibition reduces the amount of ES formed by having some of the enzyme exist as the EI complex. The binding of I can be overcome at high enough substrate concentrations (Le Chatelier's principle). The scheme shown below demonstrates this:

E  +  S   <==>    ES  -->  E  +  P

   +I

EI

The modified Lineweaver-Burk equation for this type of inhibition is

1/V = 1/V_max + [K_m/ V_max ] x {1/[S]}{1 + [I]/K_i}

Non-Competitive inhibition:

In the case of non-competitive inhibition, the binding site does not overlap with the active site; the inhibitor can bind to the ES complex. This type of inhibition does not affect substrate binding; in other words, it affects Vmax not Km.

E  <==>   ES  -->  E  +  P

+I              +I

EI  <==>  ESI

Increasing the substrate concentration does not overcome this type of inhibition.

The kinetic equation for this type of inhibition is V_Imax = V_max /{1 + [I]/K_i}.

Double reciprocal (Lineweaver-Burk) plots permit these two types of inhibition to be distinguished very easily one from the other. In competitive inhibition the straight lines all cross the vertical axis (1/v) at the same point (v_max values are the same). Non-competitive inhibition gives a series of lines that all converge to the same point on the horizontal axis (1/[S]) indicating that the K_m is the same under all conditions.

Uncompetitive inhibition:

In the case of uncompetitive inhibition, the inhibitor binds to ES complex and both the K_m and k_cat are affected.

Irreversible inhibition of enzymes

Chemicals capable of reacting covalently with amino acid residues of the enzyme, usually at the active site, produce irreversible inhibition. A well-studied example of this is the reaction of diisopropylfluorophosphate with the enzyme acetylcholine esterase, which is essential for normal nerve conduction. This organophosphorus compound acts as a nerve gas.

Irreversible inhibition of enzymes can also occur if (non-covalent) binding is so strong that it is irreversible.

Coenzymes:

Some of the chemical reactions catalyzed by enzymes involve only the participation of amino acid side chain residues.

Many of the more complex and more interesting chemical reactions in living systems involve a series of relatively small organic molecules called coenzymes or prosthetic groups. Some of these coenzymes are vitamins or are derived from vitamins.

Coenzymes combine with a purely protein component, called an apoprotein, to form a complete, active enzyme called a holoenzyme.

We will illustrate the chemical contributions of coenzymes with some examples of redox cofactors.

Pyridine Coenzymes (NAD⁺ and NADP⁺):

NADH is the biochemist's shorthand for nicotinamide adenine dinucleotide (structure); as NADPH is for nicotinamide adenine dinucleotide phosphate (structure).

As a rule of thumb, NADH participates in catabolic reactions, where it is usually formed, and NADPH participates in biosynthetic (anabolic) reactions, where it is usually consumed.

Addition of hydrogen to NAD⁺ to create NADH occurs stereospecifically. The para (to N) position in the nicotinamide ring is what is called a prochiral center; some enzymes add the hydrogen to the HA (the pro-R position) face, others add to the HB side or pro-S position. They never cross over!

Alcohol dehydrogenase, which catalyzes the reaction shown below, was one of the first enzymes for which a capability to choose between two chemically equivalent atoms at a prochiral center was demonstrated. It attaches (and removes) hydrogen only to the pro-R side of ring (HA side).

CH₃-CH₂-OH + NAD⁺ ----> CH₃CHO + NADH

Flavin Coenzymes (FAD and FMN):

There are a very large number of different flavoproteins.

For example, in the TCA cycle succinate dehydrogenase is a flavoprotein; it catalyzes the following reaction:

-OOC-CH2-CH2-COO- --->    -OOC-CH=CH-COO-
  succinate fumarate (trans)                 

The flavin coenzymes are usually tightly bound to an enzyme (i.e. they are not found in free solution to any significant degree); they are also stronger oxidizing agents than the nicotinamide-based coenzymes (NAD⁺ and NADP⁺).