Enzymes and Vitamins: Essential Roles in Metabolism
Enzymes: Biological Catalysts
Enzymes are biological catalysts crucial for metabolic reactions and sustaining life in living creatures. They are usually proteins, although ribonucleoprotein enzymes, known as ribozymes, also exist. The region of the enzyme where the substrate fits is called the active site. The bond between the enzyme and substrate involves steric recognition. Enzymes are highly specific for each substrate and each biochemical reaction.
Key Features of Enzymes:
- Decrease the activation energy of the process.
- Do not change the sign or the amount of free energy variation.
- Do not change the balance of a reaction but accelerate its progress.
- Remain free and unchanged at the end of the reaction, like any other catalyst, and can be reused.
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, consequently, their biological activity. Each enzyme has an optimum temperature and pH for activity. Variations in the pH of the medium cause changes in the surface electrical charges of enzymes, altering their tertiary structure and activity. Each enzyme functions at an optimum pH.
Enzyme Cofactors
Some enzymes are not exclusively proteins but are associated with other non-protein molecules necessary for their activity (holoenzymes). The molecules associated with the enzyme are called cofactors, and the protein part of the enzyme is called the apoenzyme. There are two main types of cofactors:
- Metal cations: These bind to or regulate the activation of the apoenzyme.
- Complex organic molecules: When weakly bound to the apoenzyme, they are called coenzymes. When tightly bound through covalent bonds, they are known as prosthetic groups.
The Enzymatic Reaction
The binding of an enzyme to its substrate to form an enzyme-substrate complex is essential for the chemical reaction to occur.
Specificity
This property arises because the three-dimensional conformation of the enzyme’s active site is complementary to the substrate molecule. This interaction can be described by the lock-and-key model or the induced-fit model, where the binding of the substrate induces a conformational change in the enzyme’s active site, leading to a perfect and definitive link between the enzyme and substrate.
Inhibition of Enzyme Activity
- Reversible inhibitors: These bind temporarily to the enzyme. Competitive inhibitors have a similar spatial conformation to the substrate and compete for binding to the enzyme’s active site.
- Irreversible inhibitors (poisons): These bind irreversibly to the enzyme’s active site, completely suppressing its activity.
Allostery
Certain ligands or effectors (non-competitive inhibitors) can specifically bind to the enzyme, causing a conformational change. These ligands bind to regulatory sites on the enzyme. The substrate can act as an activating ligand, where the binding of one substrate molecule facilitates the binding of more. Reaction products often act as inhibitory ligands, preventing further substrate binding and thus regulating the enzymatic reaction. Enzymes regulated by substrate and product are called allosteric enzymes. Allostery is a major regulatory mechanism in enzymatic reactions.
Kinetics of the Enzymatic Reaction
The reaction rate increases linearly until it reaches a peak, indicating enzyme saturation. At this point, the velocity depends solely on the speed at which the enzyme processes the substrate. The catalytic constant expresses the maximum number of substrate molecules an enzyme can transform per unit of time and is equivalent to the enzyme’s activity at maximum reaction rate. The Michaelis constant reflects the enzyme’s affinity for its substrate, indicating catalytic efficiency. A small Michaelis constant suggests a strong bond between the enzyme and substrate, as half the maximum speed is reached at low substrate concentrations.
Vitamins and Metabolism
Vitamins are biomolecules of varied complexity, essential in the diet as they generally cannot be synthesized by the animal organism, with some exceptions like vitamin B5. Metabolic disorders related to vitamins include:
- Vitamin deficiency: Absence of one or more vitamins.
- Hypovitaminosis: Insufficient presence of a particular vitamin in the diet.
- Hypervitaminosis: Excess of vitamins due to accumulation, as the body cannot dispose of them through normal methods.
Many vitamins are coenzymes or precursors of active molecules in metabolism. They are classified as:
Water-Soluble Vitamins
- Vitamin C: Source: Milk, fruits, and vegetables; Deficiency Disease: Scurvy, susceptibility to infections; Function: Antioxidant, hydroxylation cofactor, coenzyme in collagen synthesis.
- Vitamin B1: Source: Cereals, legumes, yeast, and bacteria; Deficiency Disease: Beriberi; Function: Transfers aldehyde groups.
Fat-Soluble Vitamins
- Vitamin A: Source: Green and yellow vegetables, liver, cod liver oil, eggs; Deficiency Disease: Night blindness, xerophthalmia, epithelial growth arrest; Function: Visual cycle, growth, protection, and maintenance of epithelial tissue.
- Vitamin E: Source: Vegetable oils, cereal seeds; Deficiency Disease: Cellular aging, impaired growth; Function: Inhibits the oxidation of unsaturated fatty acids.