Which of the following is not a characteristic of enzymes?

Enzymes are biological catalysts that speed up chemical reactions in living organisms by lowering the activation energy required for the reaction to proceed. The correct choice indicates that enzymes are not always active on more than one kind of substrate. In fact, many enzymes are highly specific, often catalyzing reactions for a single substrate or a very limited range of substrates. This specificity is determined by the unique shape of the enzyme's active site, which only allows certain substrate molecules to bind effectively, facilitating the reaction.

The other characteristics mentioned highlight important functions of enzymes. The ability to be reused after a reaction emphasizes their efficiency, as they do not get consumed in the reaction process. Additionally, the ability to lower activation energy is a fundamental characteristic of enzymes, making reactions occur more readily. Therefore, stating that enzymes are always active on more than one kind of substrate contrasts with the well-established fact about enzyme specificity. This understanding is crucial for studying enzyme kinetics and their role in metabolic pathways.