Many biochemical reactions in living cells can go both ways. For instance, cells of mammals both synthesize and catabolize glucose. The rates of occurrence of these reactions must be regulated to prevent wastage of energy through the futile cycle. This cycle carries out opposing reactions at very high rates without a net substrate flow in any direction. According to the second law of thermodynamics, entropy increases in favored reactions, entropy is an energy that is wasted and that cannot be used to do work.
Enzymes are important for every physical and chemical change in cells. Therefore, the regulation of catalytic activity contributes to understanding in-born errors and preserving homeostasis.
Regulation of the actions of enzymes can be accomplished through:
- Compartmentalization. Different enzymes having different tasks can be localized in particular compartments. This guarantees metabolic efficiency as well as the simplification of regulation. For instance, chloroplasts have photosynthetic enzymes, lysosomes have hydrolytic enzymes, and mitochondria has enzymes for energy metabolism, oxidative phosphorylation and TCA cycle.
- Covalent modification. This is also known as enzymatic interconversion. The majority of enzymes are regulated through the addition of a phosphate (phosphorylation), removal of phosphate (dephosphorylation), the addition of AMP (adenylylation) or other covalent modifications. Covalent modification cause changes in the tertiary enzyme structure that change its catalytic activity.
- Partial proteolysis. This refers to an irreversible covalent modification where zymogens or inactive proenzymes are activated through hydrolysis of one or many peptide bonds. For instance, the activation of proteases (protein-digesting enzymes) only in the digestive area avoids proteolysis of cellular constituents. In the same way, blood clotting factors are only activated at the sites of a cut to prevent internal clots.
- Control of enzyme concentration. The concentration of a certain enzyme in a cell depends on the rate of its degradation and synthesis. The rate of synthesis of enzymes is regulated through induction as well as repression of the gene. Apart from a few exceptions, enzymatic reaction rates increase with an increase in the concentration of enzymes.
- The concentration of the substrate. The speed of an enzymatic reaction normally increases with an increase in the substrate concentration up to a particular maximum.
- The concentration of the end product. When the end products of a reaction accumulate, the rate of the reaction decreases. In some cases, the end product combines with the enzyme, therefore, reducing the rate further.
- Temperature. The rate of enzymatic reactions is greatly influenced by temperature. Generally, the initial rate of enzymatic reaction increases with an increase in temperature until a particular optimum is achieved. Above the optimum temperature, the destruction of enzyme starts thus reducing the rate of the enzymatic reaction.
- pH of the medium. The concentration of hydrogen ions in the medium affects the activity of enzymes. Enzyme activity is maximum at a particular pH and decreases rapidly on either side of this value.
- Hydration. The effect of increased hydration on the activity of enzymes of the tissues of plants is mostly demonstrated during seed germination. As water imbibition takes place during germination, enzyme activity increases.
- Activators. Activators refer to specific compounds that accelerate the rate of the enzymatic reaction. Some activators increase the activity of almost all the enzymatic reactions like salts of alkaline earth metals such as chlorine ions, cobalt, nickel, manganese, and magnesium.
