Food Biochemistry and Food Processing (2 edition)

BLBS102-c04 BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come

58 Part 1: Principles/Food Analysis

OH OH

OH

O

O

R

Polymer

R R

Monohydroxyphenol o-Dihydroxyphenol

Cresolase reactions

Protein-Cu 2 + + O 2

Protein-Cu 2 – O 2 + monophenol + 2H+

Protein-Cu 2 +2 + 2 e–

Protein-(Cu 2 +2)n Protein-(Cu+)n + O 2 Protein-(Cu+)n – O 2

Protein-(Cu+)nO + OH–

+ 2e–

+ ne–

o-Diphenol

o-Diphenol

Monophenol

n = 2 in high cresolase preparations

n = 1 in high catecholase preparations

o-Quinone

o-Diphenol

o-Quinone

+ 2e

Protein-Cu 2 +

Overall oxidation reactions

activation

Protein-Cu 2 +2 + o-diphenol + H 2 O

Protein-Cu 2 – O 2

o-Quinone

Monophenolase

(cresolase)

Diphenoloxidase

(catecholase)

Figure 4.2.Simplified mechanism for the hydroxylation and oxidation of diphenol by phenoloxidase.

Catechol oxidase and laccase are distinguishable both on the

basis of their phenolic substrates (Figure 4.1), and their inhibitor

specificities (Marshall et al. 2000).

Catecholase activity is more important than cresolase action

in food because most of the phenolic substrates in food are

dihydroxyphenols (Mathew and Parpia 1971).

This bifunctional enzyme, PPO, containing copper in its

structure, has been described as an oxygen and four electron-

transferring phenol oxidase (Jolley et al. 1974). Figure 4.2 shows

a simplified mechanism for the hydroxylation and oxidation of

phenols by PPO. Both mechanisms involve the two copper moi-

eties on the PPO.

PPO, active between pH 5 and 7, does not have a very sharp pH

optimum. At lower pH values of approximately 3, the enzyme

is irreversibly inactivated. Reagents that complex or remove

copper from the prosthetic group of the enzyme inactivate the

enzyme (Fennema 1976).

Substrates

Although tyrosine is the major substrate for certain phenolases,

other phenolic compounds such as caffeic acid and chlorogenic

acid also serve as substrate (Fennema 1976). Structurally, they

contain an aromatic ring bearing one or more hydroxyl groups,

together with a number of other substituents (Marshall et al.

2000). Structures of common phenolic compounds present in

foods are shown in Figure 4.3. Phenolic subtrates of PPO in

fruits, vegetables, and seafood are listed in Table 4.1. The sub-

strate specificity of PPO varies in accordance with the source of

the enzyme. Phenolic compounds and PPO are, in general, di-

rectly responsible for enzymatic browning reactions in damaged

fruits during postharvest handling and processing. The relation-

ship of the rate of browning to phenolic content and PPO activity

has been reported for various fruits. In addition to serving as

PPO substrates, phenolic compounds act as inhibitors of PPOs

(Marshall et al. 2000).