Original Question (E-03)
Answer: ( c ) Relative specificity
One of the main characteristics of enzymes is their high specificity.
Enzymes are specific for:
a) the substrate
b) the reaction
It means that they catalyze the transformation of just one substrate or a family of substrates that are structurally related, catalyzing only one of the possible reactions of the substrate(s).
When the enzyme only can act on one substrate, it is said that the enzyme shows absolute specificity for the substrate. It is the case of succinate dehydrogenase, that is specific for succinate, or the L-glutamate dehydrogenase, specific for glutamate.
If the enzyme binds to some structurally related substrates, the enzyme shows relative specificity over substrate. L- amino acid Oxidase, for example can catalyze the oxidation of different amino acid of the L series, but not the oxidation of D- amino acids.
This characteristic of some enzymes is used with advantages in some clinical situations. For example, en patients intoxicated with methanol, ethanol is used in the treatment. The enzyme can bind to any of both alcohols (relative specificity), but has 10 to 20 more affinity for ethanol, so ethanol is metabolized instead of methanol. Avoiding the oxidation of methanol for favoring its elimination without transformation, is very important, since the metabolic oxidation of methanol produces in the body the very dangerous formaldehide and formic acid. (For an interesting review of Methanol intoxication, click here: Korabathina, K: Methanol)
The specificity of action is related to the fact that the enzyme only catalyzes one of the possible transformations of a substrate.
In the case of glutamate, for example, it can experiment different transformations, but each of them requires a different enzyme, for example:
Glutamine (Fixation of ammonia) : Glutamine synthase
GABA (Decarboxylation): Glutamate Decarboxyase
Alfaketoglutarate: Gutamate Dehydrogenase
The specificity of enzymes depends on the characteristics of the active site. It is the region of the enzyme where it binds to the substrate before the substrate transformations happens.
The binding of the enzyme to specific substrate(s) depends of different groups (usually lateral chains of amino acids) related to the active site:
a) groups that form the backbone, that gives the appropriate conformation for the binding;
b) the orientation groups, that “oblige” the substrate(s) to adopt the appropriate orientation for the reaction,
c) the environmental groups, that give the hydrophobic, polar or negative o positive environment necessary for warranting an appropriate affinity between the enzyme and the specific substrate, and
d) the catalytic groups, responsible of creating the necessary “tensions” for breaking old linkages and forming new ones among the metabolites implied in the reaction: substrate(s), coenzymes or other prosthetic groups.