Proteins

Overview – Proteins

Proteins are complex biological polymers made from amino acid monomers and are central to nearly every physiological function in the body. They serve structural, enzymatic, transport, hormonal, immune, and regulatory roles. Importantly, all genetic information stored in DNA exists to encode the synthesis of proteins, making them the functional end-products of gene expression. This guide covers protein structure, synthesis, function, and enzymatic action in a concise, clinically relevant format.

Definition

Proteins are biological polymers made of linked amino acids via peptide bonds. They are the most functionally diverse macromolecules in living organisms.

Functions of Proteins

Proteins have a wide range of essential roles in human physiology:

• Enzymes (e.g. digestive enzymes, metabolic catalysts)
• Hormones (e.g. insulin, growth hormone)
• Carrier proteins (e.g. albumin)
• Membrane transporters (e.g. Na⁺/K⁺-ATPase)
• Receptor proteins (e.g. cell signaling)
• Structural proteins (e.g. collagen, keratin)
• Contractile proteins (e.g. actin, myosin in muscle)
• Storage proteins
• Defensive proteins (e.g. antibodies)
• Sensory and regulatory proteins

Key Concept: The primary purpose of DNA is to encode instructions for protein synthesis via transcription and translation.

Amino Acids

Each amino acid contains:

• A central carbon atom
• An amino group (–NH₂)
• A carboxyl group (–COOH)
• A variable side chain (R group)
• There are 20 biologically relevant amino acids:
  • Essential amino acids: Must be consumed in the diet
  • Non-essential amino acids: Can be synthesized by the body
  • Some are considered conditionally essential under certain physiological states

Protein Structure

Primary Structure

• Linear sequence of amino acids
• Joined via peptide bonds through dehydration synthesis
• Directionality: written from amino (N) → carboxy (C) end

Secondary Structure

• Local folding via hydrogen bonds
• Forms:
  • Alpha-helix
  • Beta-pleated sheet

Tertiary Structure

• 3D folding due to side chain interactions
• Creates a functionally active shape for many proteins

Quaternary Structure

• Combination of multiple polypeptide subunits
• Example: Hemoglobin (4 subunits, oxygen transport)

Note: Protein shape is sensitive to environmental changes — denatured by extremes in temperature or pH

Proteins as Enzymes

What Are Enzymes?

• Proteins that catalyze biological reactions
• Function by lowering activation energy
• Often require cofactors (e.g. metal ions or vitamins)

Naming of Enzymes

• Usually end in “-ase”
  • E.g. Sucrase: breaks down sucrose
  • E.g. Lipase: breaks down lipids
• Can also reflect the type of reaction:
  • Oxidase: catalyzes oxidation
  • Hydrolase: catalyzes hydrolysis
• Exceptions: Pepsin, trypsin

Enzyme Mechanisms

Lock and Key Model

• Enzyme active site fits substrate precisely
• High specificity

Induced Fit Model

• Active site conforms to fit the substrate
• More versatile, fits a broader range of substrates

Steps in Enzymatic Reaction

1. Substrate binds to active site
2. Enzyme-substrate complex forms
3. Reaction occurs
4. Product is released
5. Enzyme returns to original shape

Factors Affecting Enzyme Activity

Temperature

• Low temp = slow activity
• Optimal temp = peak performance (~37°C in humans)
• High temp = denaturation

Substrate Concentration

• Increased substrate → increased activity (up to saturation point)

pH

• Each enzyme has an optimal pH range
• Outside this range: activity drops due to structural changes

Summary – Proteins

Proteins are essential, multifunctional biological polymers made from amino acids. They form the molecular machinery of life, from enzymes and hormones to transporters and structural components. Protein function is dictated by their complex structure, and their activity is sensitive to environmental factors. For a broader context, see our Cell Biology & Biochemistry Overview page.