2009_Book_FoodChemistry

106 2 Enzymes
Table 2.5. Redox potentials of Fe 3+ /Fe 2+ complex
compounds at pH 7 (25 ◦ C) as affected by the ligand
Redox-System E ′ 0
(Volt)
[Fe III (o-phen a ) 3 ] 3⊕ /[Fe II (o-phen) 3 ] 2⊕ +1.10
[Fe III (OH 2 ) 6 ] 3⊕ /[Fe II (OH 2 ) 6 ] 2⊕ +0.77
[Fe III (CN) 6 ] 3⊖ /[Fe II (CN) 6 ] 4⊖ +0.36
Cytochrome a(Fe 3⊕ )/Cytochrome a (Fe 2⊕ ) +0.29
Cytochrome c (Fe 3⊕ )/Cytochrome c (Fe 2⊕ ) +0.26
Hemoglobin (Fe 3⊕ )/Hemoglobin (Fe 2⊕ ) +0.17
Cytochrome b (Fe 3⊕ )/Cytochrome b (Fe 2⊕ ) +0.04
Myoglobin (Fe 3⊕ )/Myoglobin (Fe 2⊕ ) 0.00
(Fe III EDTA) 1⊖ /(Fe II EDTA) 2⊖ −0.12
(Fe III (oxin b ) 3 )/(Fe II (oxin) 3 ) 1⊖ −0.20
Ferredoxin (Fe 3⊕ )/Ferredoxin (Fe 2⊕ ) −0.40
a o-phen: o-Phenanthroline.
b oxin: 8-Hydroxyquinoline.
Polyphenol oxidase catalyzes two reactions: first
the hydroxylation of a monophenol to o-diphenol
(EC 1.14.18.1, monophenol monooxygenase)
followed by an oxidation to o-quinone (EC
1.10.3.1, o-diphenol: oxygen oxidoreductase).
Both activities are also known as cresolase
and catecholase activity. At its active site,
polyphenol oxidase contains two Cu 1⊕ ions with
two histidine residues each in the ligand field.
In an “ordered mechanism” (cf. 2.5.1.2.1) the
enzyme first binds oxygen and later monophenol
with participation of the intermediates
shown in Fig. 2.8. The Cu ions change their
valency (Cu 1⊕ → Cu 2⊕ ). The newly formed
complex ([] in Fig. 2.8) has a strongly polarized
O−O=bonding, resulting in a hydroxylation to
o-diphenol. The cycle closes with the oxidation
of o-diphenol to o-quinone.
2.4 Theory of Enzyme Catalysis
It has been illustrated with several examples (Table
2.1) that enzymes are substantially better catalysts
than are protons or other ionic species used
in nonenzymatic reactions. Enzymes invariably
surpass all chemical catalysts in relation to substrate
and reaction specificities.
Theories have been developed to explain the
exceptional efficiency of enzyme activity. They
are based on findings which provide only indirect
insight into enzyme catalysis. Examples are the
identification of an enzyme’s functional groups
involved in catalysis, elucidation of their arrangement
within the tertiary structure of the enzyme,
and the detection of conformational changes
induced by substrate binding. Complementary
studies involve low molecular weight model
substrates, the reactions of which shed light on
the active sites or groups of the enzyme and
their coordinated interaction with other factors
affecting enzymatic catalysis.
2.4.1 Active Site
An enzyme molecule is, when compared to
its substrate, often larger in size by a factor
Fig. 2.8. Mechanism of polyphenol oxidase activity