Proteins: Structure, Functions, and Properties

Posted on Jan 6, 2025 in Human Nutrition and Dietetics

Proteins: Essential Biomolecules in Cells

Structure and Composition

Proteins are organic molecules and the most abundant in cells, making up 50% of their dry weight. They are distributed throughout the cell and are fundamental structurally and functionally. Chemically, they are formed by Carbon (C), Oxygen (O), Hydrogen (H), and Nitrogen (N). Proteins are polymers formed by the union of amino acids through peptide bonds.

Classification of Proteins

Proteins can be classified into two main categories:

  • Holoproteins: Composed exclusively of amino acids.
    • Fibrous: Simple structure, insoluble in water, important structural or protective roles. Examples include:
      • Collagen (skin, bones, tendons)
      • Myosin (muscle contractions)
      • Keratin (nails)
    • Globular: Folded polypeptide chains integrated into a spherical structure, soluble in water, responsible for the biological activity of the cell. Examples include:
      • Actin (forms filaments, responsible for muscle contractions)
      • Albumin (amino acid reserve functions)
      • Casein
      • Globulin (immunoglobulins, antibodies)
      • Histones (associate with DNA)
  • Heteroproteins: Contain a protein part (amino acids) and a non-protein part called the prosthetic group. Examples include:
    • Chromoproteins (pigments)
    • Nucleoproteins (chromosomes)
    • Glycoproteins (carbohydrates, immunoglobulins)
    • Phosphoproteins (casein)
    • Lipoproteins (lipids, LDL, HDL cholesterol transport)

Functions of Proteins

Proteins have diverse functions, including:

  • Reserve: Casein
  • Transport: Lipoproteins, hemoglobin
  • Contractile: Actin, myosin
  • Defense: Thrombin, fibrinogen
  • Hormonal: Insulin, glucagon
  • Structural: Collagen
  • Homeostatic: Equilibrium of the internal environment, pH regulation
  • Enzymatic: Biological catalysts

Properties of Proteins

Protein properties depend on the nature of their R-groups (side chains) of the amino acids.

  • Solubility: Fibrous proteins are generally not soluble, while globular proteins are.
  • Denaturation: Due to pH or temperature changes. Denaturing agents or substances (urea) can cause proteins to lose their structure and, consequently, their biological function. They may be able to renature.
  • Specificity of Function: Determined by the position of amino acids in the linear sequence. A single variation can cause a loss of functionality. Proteins with the same function in different species have similar structures (homologous proteins, e.g., insulin).
  • Buffering Capacity: Proteins can act as buffers, helping to regulate the pH of their environment.

Amino Acids: Building Blocks of Proteins

Amino acids are simple compounds of low molecular weight. Chemically, they are composed of C, H, O, and N. They have a carboxyl group (-COOH), an amino group (-NH2), and an H atom covalently bonded to an R group. All these are joined to a central carbon atom (C?).

Classification of Amino Acids

Amino acids are classified based on their R group. There are 20 basic amino acids that make up proteins.

  • Hydrophobic: Form the hydrophobic core of soluble proteins, hidden from the aqueous medium (e.g., alanine, valine).
  • Polar: Hydrophilic, form stable hydrogen bonds with water, favoring solubility (e.g., serine, glycine).
  • Basic: Positively charged radicals (e.g., lysine, arginine).
  • Acidic: Negatively charged radicals (e.g., aspartic acid, glutamic acid).

Properties of Amino Acids in Aqueous Solution

Amino acids form dipolar amphoteric ions in aqueous solutions. The pH at which an amino acid forms a dipolar ion is called the isoelectric point. The dipolar ion can act as an acid or a base, depending on the pH of the solution, allowing for pH regulation in the organism.

Stereoisomers and Optical Activity

The central carbon (C?) can have two spatial configurations: D and L (amino group on the right or left). Proteins are made of L-series amino acids because cellular enzymes involved in protein synthesis are stereospecific, similar to monosaccharides.

Essential Amino Acids

Essential amino acids are those that heterotrophic organisms cannot synthesize and must be ingested in the diet.