Proteins and enzymes

Proteins are polymers built from amino acid monomers joined by peptide bonds in condensation reactions.

The primary structure (the sequence of amino acids) determines all higher levels of structure.

Secondary structure forms when hydrogen bonds between groups along the polypeptide backbone create α-helices or β-pleated sheets.

Tertiary structure is the overall 3D shape of one polypeptide chain, stabilised by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.

Quaternary structure occurs when two or more polypeptide chains assemble together, sometimes with prosthetic groups.

Globular proteins

• Are roughly spherical and soluble in water (hydrophilic R-groups face outwards)
• Suited to transport and metabolic roles, e.g. haemoglobin

Fibrous proteins

• Are long, thin and insoluble in water
• Suited to structural roles, e.g. collagen

Key Definition Enzymes are globular proteins that act as biological catalysts, lowering activation energy and increasing reaction rates.

Each enzyme has an active site with a shape complementary to its substrate, which explains its specificity.

The induced fit model shows how the active site changes shape slightly on substrate binding, straining bonds in the substrate and lowering the activation energy.

Intracellular enzymes such as catalase work inside cells, while extracellular enzymes such as amylase and trypsin are secreted to work outside cells.

The rate of an enzyme-catalysed reaction depends on several factors:

• Temperature — rate increases with temperature up to the optimum, then falls as the enzyme denatures
• pH — each enzyme has an optimum pH; moving away from it alters the active site shape
• Enzyme concentration — rate increases with enzyme concentration provided substrate is in excess
• Substrate concentration — rate increases with substrate concentration until the active sites become saturated

Outside the optimum temperature or pH, the enzyme denatures: the active site changes shape and is lost, so the substrate can no longer bind.

The initial rate of reaction is measured because at the start substrate is in excess and the reaction proceeds at its maximum (fastest) rate, giving a fair basis for comparing conditions.