Polyphenol oxidase (phenolase/tyrosinase), the browning enzyme in fruits and vegetables, contains which metal as its essential prosthetic group at the active site?

Introduction:
Enzymatic browning in cut fruits and vegetables is chiefly driven by polyphenol oxidase, also called phenolase or tyrosinase. Understanding its metal cofactor helps in designing strategies to control browning and preserve colour and nutrition in minimally processed produce.

Given Data / Assumptions:

• Enzyme class: polyphenol oxidase (PPO).
• Observable effect: browning via o-quinone formation and subsequent polymerization.
• Question asks for the metal prosthetic group.

Concept / Approach:
PPO is a copper-containing oxidoreductase. The dicopper centre cycles between oxidation states to catalyse hydroxylation of monophenols and oxidation of o-diphenols to o-quinones. The resulting quinones polymerize, yielding brown melanins observed on cut surfaces.

Step-by-Step Solution:

Verification / Alternative check:
Use of chelating agents, modified atmospheres, or ascorbic acid reduces browning, consistent with copper-dependent catalysis and quinone reduction chemistry.

Why Other Options Are Wrong:

• Mg: Common in chlorophyll and ATP-related enzymes, not PPO.
• Ca: Structural roles in many enzymes but not the PPO active site.
• Fe: Characteristic of peroxidases and cytochromes, not tyrosinase.

Common Pitfalls:
Confusing enzymatic browning (copper dependent) with non-enzymatic Maillard reactions. Assuming refrigeration alone stops PPO; low temperatures slow but do not eliminate activity without additional hurdles.

Final Answer:
Cu