Protein Structure & Enzymes

A protein's job is set by its three-dimensional shape, and that shape is built up in four levels — each determined ultimately by the sequence of amino acids (the primary structure).

• Primary — the sequence of amino acids, joined by peptide bonds.
• Secondary — local folding into α-helices and β-sheets, held by hydrogen bonds.
• Tertiary — the overall 3-D fold, held by bonds between R-groups including disulfide bridges.
• Quaternary — two or more polypeptide chains together (e.g. haemoglobin, DNA polymerase).

Because shape is everything, if a protein is denatured (unfolded) it stops working. Proteins with specific shapes include enzymes, some hormones, receptor proteins and antibodies.

Enzymes

Enzymes are biological catalysts: they speed up reactions by lowering activation energy, and each is specific for its substrate. In the induced-fit model, the substrate binds the active site and the enzyme changes shape slightly to grip it — like a handshake, not a rigid lock and key.

What affects enzyme activity

• Temperature — rate rises to an optimum, then falls sharply as the enzyme denatures.
• pH — each enzyme has an optimum pH; far from it, the active site distorts.
• Inhibitors — molecules that block or slow the enzyme (the basis of many drugs and pesticides).
• Concentration of substrate and of enzyme both change the reaction rate.

Enzyme rate climbs to an optimum temperature, then collapses as the protein denatures.