Enzyme Activity and Regulation: A Comprehensive Look
Enzymes: Biological Catalysts
Enzymes are usually proteins that specifically catalyze certain biochemical reactions by binding to the molecule or metabolite that is going to transform, the substrate. There are also ribonucleoprotein enzymes called ribozymes. The region of the enzyme where the substrate fits is the active site. The bond between enzyme and substrate involves a steric recognition, i.e., related to the shape and size of the substrate itself, which binds specifically. Therefore, the variety of enzymes is incalculable, as they are specific to each substrate and for each biochemical reaction.
How Enzymes Function
Enzymes act like any other catalyst in that:
- They reduce the activation energy in the process involved, i.e., they accelerate biochemical reactions.
- They do not change the sign or the amount of free energy change, they only increase the speed. They do that processes are thermodynamically more favorable.
- They do not change the balance of a reaction, but they accelerate the arrival there.
- After the reaction, they are free and without changing, like any other catalyst, and may work other times.
Influence of pH and Temperature on Enzyme Activity
Temperature variations induce conformational changes in the tertiary or quaternary structure of enzymes, altering their active sites and therefore their biological activity. Each enzyme has an optimum temperature and pH to act. Most enzymes act at the temperature of the living being. Variations in the pH of the medium cause a change in the surface electrical charges of the enzymes, altering their tertiary structure and therefore their activity. Each enzyme acts at an optimum pH.
Enzymatic Cofactors
Some enzymes are not exclusively protein, but are associated with other molecules, both protein and non-protein in nature, on which their activity depends. These associations are called conjugated enzymes or holoenzymes, the molecules with which they are associated, cofactors, and the protein of the enzyme, apoenzyme. Cofactors have different natures, and can include:
- Metallic cations, which bind to the apoenzyme or regulate its activation.
- Complex organic molecules: They are called coenzymes when weakly bound to the apoenzyme. When they bind tightly to the apoenzyme by covalent bonds, they are known as prosthetic groups.
Classification of Enzymes
The suffix -ase is added to the root of the name of the substrate on which they operate: for example, amylase acts on amylose and amylopectin starch. In other cases, they are named according to the reaction they catalyze, such as hydrolases, which are involved in hydrolysis reactions, or isomerases, which catalyze isomerization reactions of different substrates.
- Hydrolases: Catalyze hydrolysis reactions involving diverse substrates and water.
- Lyases: Catalyze the release of various functional groups.
- Transferases: Catalyze the transfer of radicals or functional groups to other molecules. When the transfer is a phosphate group, they are called kinases.
- Isomerases: Catalyze isomerization reactions, i.e., the transformation of molecules into their isomers.
- Oxidoreductases: Catalyze oxidation-reduction reactions of substrates with the transfer of hydrogen, oxygen, or electrons.
- Synthetases: Catalyze the synthesis of molecules with ATP hydrolysis.
Enzyme-Substrate Complex Formation
An enzyme-catalyzed biochemical reaction always passes through substrate binding to the enzyme, forming the enzyme-substrate complex, essential for the chemical reaction to take place.
Specificity of Enzyme Action
One of the most important characteristics of enzyme activity is their specificity in the reaction they catalyze. This property is because the three-dimensional conformation of the enzyme’s active site is such that it complements the substrate molecule to which it binds. The complementarity of the binding of enzyme to substrate has been likened to that between a key and a lock, so it is said that the union is modeled as a lock-and-key model. However, this union cannot be rigid, i.e., the binding of the substrate itself induces a conformational change in the enzyme’s active site, which finally provokes a perfect and definitive link between this and the substrate. This model is called the induced-fit model.
Regulation of Enzyme Activity
Enzymes remain active when needed to form a product at some point in the cell. A regulatory mechanism is the inhibition of enzymes in the active site by inhibitors (cellular components).
Types of Inhibition
- Irreversible: The inhibitor binds covalently to the enzyme permanently, altering its structure.
- Reversible: The inhibitor binds non-covalently (by bonds easier to break) to the enzyme temporarily. There are two types:
- Competitive: The inhibitor binds to the active site, preventing substrate binding.
- Noncompetitive: The inhibitor binds to another area other than the active site, modifying its structure and preventing the coupling of the substrate.
Allosteric Regulation of Enzymes
There are several molecules that bind specifically to the enzyme, causing a conformational change in it. This change originates the transformation between the inactive form of the enzyme and the functionally active form thereof, or vice versa. Both conformations of the enzyme are different and stable. These ligands bind to the enzyme in so-called regulatory centers, which are different from the active site. There are ligand activators and inhibitors: In general, the substrates of enzymes often behave as ligand activators, so that the binding of one substrate molecule to the enzyme facilitates the binding of more molecules of substrate; the reaction products, however, often behave as inhibitor ligands, preventing the binding of substrate molecules to the enzyme and therefore the enzymatic reaction. These enzymes, which are regulated by the substrate and product of the reaction, are called allosteric enzymes. Allostery is a major regulatory mechanism in enzymatic reactions.
Kinetics of Enzymatic Reactions
In enzymatic reactions, there is a limit to the amount of substrate that the enzyme is able to transform over time. The reaction rate increases linearly to its peak when the enzyme is saturated. At that time, the velocity only depends on the speed with which the enzyme is able to process the substrate. Another parameter widely used in enzyme kinetics is the Michaelis constant, whose value refers to the affinity of the enzyme for its substrate. Therefore, the Michaelis constant is directly related to the speed at which the enzymatic reaction takes place. This is known as catalytic efficiency. A small value of the Michaelis constant could indicate a very close union between the enzyme and substrate, since half of the maximum speed is reached when the substrate concentrations are low.