Explain how the specificity of enzymes arises from their molecular structure, and state one consequence of this specificity for metabolic pathways.

Enzymes are globular proteins whose tertiary structure gives them a specific three‑dimensional shape.
The active site is a pocket or cleft whose shape and chemical properties match those of a particular substrate, so that only that substrate can fit.
The binding is stabilised by interactions such as hydrogen bonds and ionic bonds between the R‑groups of the amino acids that line the active site and the substrate.
Because each enzyme recognises only one substrate (or a very small group of substrates), each step in a metabolic pathway is carried out by a different, specific enzyme, which allows the pathway to be tightly regulated and controlled.