In biochemistry, which single statement about enzymes is incorrect? Read each carefully: consider catalysis, whether all enzymes are proteins, reusability across cycles, and regulation by the cell.

Introduction / Context:
Enzymes are biological catalysts that accelerate chemical reactions in living systems. A solid understanding of what enzymes do—and just as importantly, what they do not do—is foundational in biochemistry, biotechnology, and medicine. This item tests your grasp of catalytic principles, regulation, and the molecular nature of enzymes.

Given Data / Assumptions:

• We are evaluating general statements about enzyme catalysis.
• Consider both protein enzymes and catalytic RNAs (ribozymes).
• Assume standard aqueous cellular conditions.

Concept / Approach:
Enzymes accelerate the approach to equilibrium by lowering the activation energy (Ea). They do not supply or “provide” activation energy; rather, they stabilize the transition state so that less energy is required to reach it. Many enzymes are proteins, but not all—ribozymes are RNA molecules with catalytic activity. Enzymes are not consumed by the reactions they catalyze and can be regulated by multiple mechanisms, including allosteric effectors, covalent modification, and changes in expression or localization.

Step-by-Step Solution:

Verification / Alternative check:
Energy profiles of catalyzed vs. uncatalyzed reactions show a decreased Ea for the catalyzed pathway. No energy is donated by the enzyme; instead, binding interactions and precise orientation lower Ea.

Why Other Options Are Wrong:

• Most enzymes are proteins: accurate; proteins dominate enzyme classes.
• Regulation occurs via allostery, covalent modification, and gene expression: accurate.
• Reusability: enzymes emerge unchanged overall and can catalyze many cycles: accurate.
• Ribozymes exist (e.g., peptidyl transferase center, self-splicing introns): accurate.

Common Pitfalls:
Confusing “lowering activation energy” with “providing energy.” Enzymes do not shift equilibrium or supply energy; only reaction kinetics change.

Final Answer:
Enzymes provide activation energy to reactants.