Question

Arrange the sequence of steps involved in the reaction of enzymatic browning 
[A.] Formation of DOPA 
[B.] Action of PPO enzyme on cut fruits 
[C.] Polymerization of ortho-quinones to form melanins 
[D.] Formation of ortho-quinones 
Choose the most appropriate answer from the options given below:

• A. C, B, A, D
• B. B, D, C, A
• C. B, A, D, C
• D. B, A, C, D

Hint:

The enzymatic browning cascade is: Enzyme (PPO) acts \(\rightarrow\) makes DOPA \(\rightarrow\) makes Quinones \(\rightarrow\) Quinones link together (polymerize) to make brown Melanins.

Answer 1

The Correct Option is C

Step 1: Understand the trigger and key players in enzymatic browning. The reaction is initiated when tissue damage (cutting) exposes the enzyme polyphenol oxidase (PPO) and phenolic substrates to oxygen.

Step 2: Order the biochemical steps.
1. (B) Initial Action: The process begins with the action of the PPO enzyme on its substrates, which are present in the cut fruit tissue.
2. (A) Hydroxylation: PPO first catalyzes the hydroxylation of monophenols (like tyrosine) to form o-diphenols (like DOPA).
3. (D) Oxidation: PPO then catalyzes the oxidation of these o-diphenols into highly reactive ortho-quinones.
4. (C) Polymerization: These ortho-quinones are unstable and undergo further non-enzymatic reactions, including polymerization, to form the complex brown, black, or red pigments known as melanins.
The correct sequence is B \(\rightarrow\) A \(\rightarrow\) D \(\rightarrow\) C.