Which macromolecule acts as a catalyst in biochemical reactions?

The correct answer is that enzymes act as catalysts in biochemical reactions. Enzymes are a specific type of protein that accelerate chemical reactions by lowering the activation energy required for the reaction to proceed. This means that enzymes enable reactions to occur more quickly and efficiently, often facilitating reactions that would be too slow to maintain life without their presence.

While proteins, in general, can serve a variety of functions including structural roles and transport, enzymes are specialized proteins specifically designed to catalyze reactions. This distinction is crucial to understanding the role of enzymes in biological systems.

Carbohydrates and lipids, on the other hand, primarily function as energy sources or structural components rather than as catalysts. Therefore, while proteins encompass enzymes, it is the enzymes specifically that are responsible for catalyzing biochemical reactions. This is why "D. Enzyme" is the most accurate choice for this question.