Proteins and Enzymes: Structure, Function, and Types

Posted on Feb 22, 2025 in Biology

Proteins

Proteins are CHON (Carbon, Hydrogen, Oxygen, Nitrogen) unions formed by amino acids, which contain one amino group and one carboxyl group. They are the most abundant biomolecules, making up more than 50% of a cell’s dry weight. Ribosomes create proteins from genetic information. Amino acids forming proteins bind through peptide bonds, which can be broken by hydrolysis. The resulting structures are classified by size:

• Peptide: A few amino acids
• Oligopeptide: Fewer than 10 amino acids
• Polypeptide: More than 10 amino acids
• Protein: More than 50 amino acids

Proteins have a three-dimensional structure, determined by different levels of folding:

• Primary structure: The amino acid sequence (number, type, and order).
• Secondary structure: Folding patterns created by peptide bonds, such as alpha-helices or beta-sheets.
• Tertiary structure: The overall three-dimensional shape of a protein. This can be filamentous (insoluble structural proteins like keratin) or globular (dynamic proteins like enzymes).
• Quaternary structure: Multiple protein subunits combined (e.g., hemoglobin).

Protein inactivation, or denaturation, can occur due to changes in pH, temperature, or agitation. The basic process of protein synthesis is: DNA -> RNA -> protein.

Protein Types

Proteins can be classified as simple or complex:

• Simple proteins (Holoproteins): Composed only of amino acids.
  • Globular:
    • Protamines (associated with nucleic acids)
    • Histones (component of chromosomes)
    • Albumins (transport)
    • Globulins (defense)
  • Fibrillary (structural):
    • Insoluble: Keratin (hair, nails), Collagen (tissue), Elastin (ligaments, tendons, artery walls)
    • Soluble: Fibrinogen (coagulation), Myosin, Actin, Tropomyosin (muscle contraction)
• Complex proteins (Heteroproteins): Composed of amino acids and a non-protein component. These are generally globular.
  • Phosphoproteins (e.g., casein in milk)
  • Glycoproteins (e.g., enzymes, hormones, antibodies)
  • Lipoproteins (e.g., cholesterol)
  • Chromoproteins (pigmented proteins, e.g., hemoglobin, rhodopsin, chlorophyll)
  • Nucleoproteins (e.g., chromatin fiber)

Enzymes

Enzymes are globular proteins that act as biological catalysts (biocatalysts). They accelerate reaction rates by lowering activation energy. Key characteristics of enzymes include:

• They are not altered during the reaction.
• They exhibit specificity.
• They significantly increase reaction rates.

Enzymes act on substrates, which bind to the enzyme’s active site, to form products. Reaction rate depends on:

• Temperature: Affects enzyme effectiveness.
• pH: Affects amino and carboxyl groups; can lead to inactivation.
• Substrate concentration: Beyond a certain point, increasing substrate concentration does not change the reaction rate.
• Inhibitors and cofactors.

Metabolic processes include catabolism (degradation) and anabolism (cell formation). Enzyme deficiencies can cause diseases like Tay-Sachs and Sandhoff disease. Substances like cyanide and mercury can act as enzyme inhibitors. Proteins have many enzymatic functions, and some proteins regulate hormone levels in physiological processes (nervous system, cellular reproduction, sexuality, sugar metabolism). Examples of protein hormones include insulin, glucagon, STH, and ACTH.

Heteroprotein enzymes (Holoenzymes) have a non-protein component (coenzyme) that binds to the protein part. Coenzymes often derive from vitamins, and neither the protein part nor the coenzyme can function independently in important processes. Examples include Acetyl coenzyme A (CoA), Nicotinamide adenine dinucleotide (NAD), and Flavin adenine dinucleotide (FAD). Metal ions, such as copper (Cu²⁺), can also act as cofactors.

Enzyme names often derive from the substrate they act on and end in “-ase” or describe their function.

Vitamins

Vitamins are organic substances needed in small quantities that the body cannot produce itself; they must be obtained through diet. Funk coined the term “vitamins” (“vital amines”). They are classified as:

• Liposoluble (A, D, E, K)
• Hydrosoluble (B complex, C)

Vitamins are named by letters and have similar properties. Plants, especially generating plants, are a primary source of vitamins, often storing them as pro-vitamins. Animals, such as fish, store vitamins (A, D) in their liver. The liver can produce vitamin A from carotene. Vitamins A, D, and B₁₂ are found in animal sources. Some vitamins combine with protein molecules to form enzymes. Vitamin B₁₂ is involved in methyl synthesis; a deficiency can lead to decreased red blood cell production and pernicious anemia. Vitamin D is involved in calcium and phosphorus metabolism; a deficiency can lead to poor bone formation and rickets.