Proteins: The Workhorses of Life


  • Proteins are complex biomolecules composed of amino acids. They are one of the four major biomolecule classes and play essential roles in living organisms.


  • The monomers of proteins are amino acids. There are 20 different amino acids commonly found in proteins. The only difference between them is the specific side chain (R group) attached to the central carbon.

Structure of Proteins:

Amino Acids:

  • Proteins are composed of long chains of amino acids.
  • Each amino acid consists of an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R group).

Classification of Amino Acids Based on Function :

  • Essential Amino Acids: These amino acids cannot be synthesized by the body and must be obtained through the diet. There are nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

  • Non-essential Amino Acids: These amino acids can be synthesized by the body, so they do not need to be obtained from the diet. There are 11 non-essential amino acids.

Peptide Bonds:

  • Amino acids are linked together by peptide bonds.
  • A peptide bond forms between the amino group of one amino acid and the carboxyl group of another, releasing a water molecule in a condensation reaction.


Structural Level of Proteins:

Proteins have a hierarchical structural organization that can be categorized into four primary levels: primary, secondary, tertiary, and quaternary structures. Each level of structure is essential for the protein's function. Here's an overview of these structural levels:

  1. Primary Structure:

    • The primary structure of a protein is its linear sequence of amino acids. The order and number of amino acids in the polypeptide chain are determined by the gene encoding the protein.
    • The primary structure is crucial because it dictates the unique identity and function of the protein.

  1. Secondary Structure:

    • Secondary structure refers to the regular, repeating spatial arrangements of the polypeptide chain. The most common secondary structures are alpha-helices and beta-sheets.
    • Alpha-helices: These are right-handed coils where the polypeptide chain winds around a central axis. This structure is stabilized by hydrogen bonds between amino acids.
    • Beta-sheets: These are formed when adjacent segments of the polypeptide chain run alongside each other, forming hydrogen bonds between the amino acids. Beta-sheets can be either parallel or antiparallel.
    • Secondary structures contribute to the protein's overall shape and stability.



  1. Tertiary Structure:

    • Tertiary structure represents the three-dimensional folding of the entire polypeptide chain. It is driven by interactions between the side chains (R groups) of amino acids.
    • The interactions responsible for tertiary structure include hydrogen bonds, ionic bonds, disulfide bridges (covalent bonds between cysteine side chains), and hydrophobic interactions.
    • Tertiary structure is critical for the protein's function, as it determines the precise arrangement of amino acids and the functional domains of the protein.

  1. Quaternary Structure:

    • Quaternary structure is relevant only for proteins composed of multiple polypeptide chains. It describes the spatial arrangement and interactions between these subunits.
    • Quaternary structure is critical for proteins with multiple subunits, such as hemoglobin, which consists of four subunits.
    • Interactions between subunits can include hydrogen bonds, ionic bonds, and hydrophobic interactions.


These levels of protein structure collectively define the protein's overall shape and function. The primary structure provides the foundation, while the secondary, tertiary, and quaternary structures add complexity and versatility to protein function. Protein structure is intimately linked to its function, as the unique three-dimensional arrangement of amino acids determines how a protein interacts with other molecules in biological processes.

- Collagen is the most abundant protein in the animal world.

- Ribulose bisphosphate Carboxylase-Oxygenase(RuBisCO) is the most abundant protein in the whole of the biosphere.

Functions of Proteins:

  1. Enzymes:

    • Act as biological catalysts, speeding up chemical reactions in cells.
  2. Structural Proteins:

    • Provide support and shape to cells and tissues. Examples include collagen in connective tissues and keratin in hair and nails.
  3. Transport Proteins:

    • Facilitate the movement of substances (e.g., oxygen, ions) across cell membranes and throughout the body.
  4. Hormones:

    • Act as signaling molecules that regulate various physiological processes.
  5. Antibodies:

    • Part of the immune system, these proteins defend against foreign invaders like bacteria and viruses.
  6. Contractile Proteins:

    • Enable muscle contraction and movement.
  7. Storage Proteins:

    • Store essential molecules, such as iron in ferritin or oxygen in myoglobin.