Question

1. describe and explain the effect of ascorbic acid on PPO activity

2. describe and explain the effect of cysteine on PPO activity.

Answer 1

Polyphenol oxidase (PPO, monophenol monooxygenase or polyphenol oxidase i, chloroplastic) is a tetramer that contains four atoms of copper per molecule, and binding sites for two aromatic compounds and oxygen.^[1] The enzyme catalyses the o-hydroxylation of monophenol molecules in which the benzene ring contains a single hydroxyl substituent to o-diphenols (phenol molecules containing two hydroxyl substituents). It can also further catalyse the oxidation of o-diphenols to produce o-quinones.

PPO causes the rapid polymerization of o-quinones to produce black, brown or red pigments (polyphenols) that cause fruit browning. The amino acid tyrosine contains a single phenolic ring that may be oxidised by the action of PPOs to form o-quinone. Hence, PPOs may also be referred to as tyrosinases. Common foods producing the enzyme include mushrooms, apples and lettuce.

PPO inhibitors :

There are two types of inhibitor of PPO, those competitive to oxygen in the copper site of the enzyme and those competitive to phenolics.

Ascorbic acid and cysteine are the inhibitors of PPO enzyme.

Ascorbic acid reduces the formed quinone instantly to the original substrate (catechol) at high concentration while at lower concentrations acts as competitive inhibitor.

Cysteine, at higher concentrations reacts with the resulted quinone to give a colorless products while at the low concentrations, cysteine acts as competitive inhibitor.

Ascorbic acid reduces o-quinones to diphenols and prevents the formation of browning

Pigments.( Sometimes ascorbic acid is oxidized completely and browning may occur due to

formation of melanins)

Cysteine reacts with O- quinones and forms a

colourless compound and thereby reduces PPO activity by delaying discolorations.

The combination of both ascorbic acid and Cysteine gives good results of Antibrowning effect.