Proteins: Structure, Function, and Biochemistry in Anatomy & Physiology

Organic and Inorganic Chemistry in the Cell

Introduction to Cellular Chemistry

Cells are composed of both organic and inorganic molecules, which together account for the mass and functions of living organisms. Organic molecules, such as proteins, carbohydrates, lipids, and nucleic acids, are essential for cellular structure and function.

• Organic molecules contain carbon and are typically large, complex compounds.

• Inorganic molecules include water, salts, acids, and bases, and are generally simpler.

• Proteins are a major class of organic molecules, making up a significant portion of cell mass.

Amino Acids and Peptide Bonds

Structure and Types of Amino Acids

Proteins are polymers made from 20 different types of amino acids, which are joined together by peptide bonds. Each amino acid contains an amine group, an acid group, and a unique side chain (R group).

• Amino acid: Organic molecule with a central carbon (α-carbon), an amino group (–NH2), a carboxyl group (–COOH), and a variable R group.

• Peptide bond: Covalent bond formed between the carboxyl group of one amino acid and the amino group of another.

• Essential amino acids: Must be obtained from the diet (e.g., leucine, lysine, methionine, phenylalanine, tryptophan, valine).

• Histidine is essential in infants.

Example: The backbone of all amino acids is N–C–C, with the side chain (R group) determining the specific properties of each amino acid.

Structural Levels of Proteins

Hierarchy of Protein Structure

The function and shape of a protein are determined by its structure, which is organized into four levels:

• Primary structure: Linear sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns stabilized by hydrogen bonds, including:

  • Alpha (α) helix: Coiled structure resembling a spring.

  • Beta (β) pleated sheet: Folded structure resembling ribbons.

• Tertiary structure: Overall 3-D shape formed by interactions among secondary structures.

• Quaternary structure: Association of two or more polypeptide chains.

Example: Hemoglobin is a protein with quaternary structure, consisting of multiple polypeptide subunits.

Fibrous and Globular Proteins

Classification and Functions

Proteins are classified as either fibrous (structural) or globular (functional) based on their shape and role in the body.

• Fibrous proteins:

  • Long, strand-like, water-insoluble, and stable.

  • Provide mechanical support and tensile strength.

  • Examples: Keratin, elastin, collagen (most abundant protein in the body), and contractile fibers.

• Globular proteins:

  • Spherical, water-soluble, and sensitive to environmental changes.

  • Have functional regions called active sites.

  • Examples: Antibodies, hormones, molecular chaperones, and enzymes.

Protein Denaturation

Loss of Structure and Function

Denaturation occurs when proteins lose their functional 3-D shape due to environmental changes, such as alterations in pH or temperature.

• Denaturation: Unfolding of globular proteins, resulting in loss of biological activity.

• Fibrous proteins are generally more stable than globular proteins.

• Enzymes become deactivated when denatured.

• Denaturation is often reversible if conditions are restored, unless changes are extreme (e.g., cooking an egg).

Enzymes

Biological Catalysts

Enzymes are globular proteins that act as biological catalysts, speeding up chemical reactions without being consumed.

• Catalyst: Substance that increases the rate of a chemical reaction without undergoing permanent change.

• Enzymes lower the activation energy required for reactions.

• Highly specific for their substrate.

• Enzyme names often end in –ase (e.g., oxidases) or –in.

• Can catalyze thousands of reactions per minute.

Example: Lactase is an enzyme that catalyzes the breakdown of lactose into glucose and galactose.

Equation for enzyme-catalyzed reaction:

$E + S \rightarrow ES \rightarrow E + P$

Where E is enzyme, S is substrate, ES is enzyme-substrate complex, and P is product.