Which one of the following statements is not true about enzymes?

Enzymes are biological catalysts that are highly specific in their action — a key property known as enzyme specificity. Each enzyme catalyses only one particular type of reaction and acts on a specific substrate. This specificity arises from the unique three-dimensional structure of the active site of the enzyme (the "lock and key" model and induced fit model).

The statement that "Enzymes are non-specific for a reaction and substrate" is therefore NOT true. Enzymes are in fact highly specific both for the type of reaction they catalyse and for the substrate they act upon.

The other statements are all correct: almost all enzymes are proteins (some are RNA-based ribozymes, but the vast majority are proteins); enzymes lower the activation energy of biochemical reactions, acting as catalysts; and enzyme activity is highly dependent on temperature and pH, with optimal conditions for each enzyme.