2009_Book_FoodChemistry

104 2 Enzymes
(2.14)
of the amino acid with formation of an
α-keto acid. The reaction may also proceed
through a decarboxylation (Fig. 2.7) and yield an
amine. Which of these two pathway options will
prevail is decided by the structure of the protein
moiety of the enzyme.
2.3.3 Metal Ions
Metal ions are indispensable cofactors and stabilizers
of the conformation of many enzymes.
They are especially effective as cofactors with
enzymes converting small molecules. They influence
the substrate binding and participate in catalytic
reactions in the form of a Lewis acid or play
the role of an electron carrier. Only the most important
ions will be discussed.
2.3.3.1 Magnesium, Calcium and Zinc
Mg 2⊕ ions activate some enzymes which
hydrolyze phosphoric acid ester bonds (e. g.
phosphatases; cf. Table 2.4) or transfer phosphate
residues from ATP to a suitable acceptor (e. g.
kinases; cf. Table 2.4). In both cases, Mg 2⊕ ions
act as an electrophilic Lewis acid, polarize the
P−O-linkage of the phosphate residue of the
substrate or cosubstrate and, thus, facilitate
a nucleophilic attack (water with hydrolases;
ROH in the case of kinases). An example is the
hexokinase enzyme (cf. Table 2.16) which, in
glycolysis, is involved in catalyzing the phosphorylation
of glucose to glucose-6-phosphate with
ATP as cosubstrate. The effect of a Mg 2⊕ ion
within the enzyme-substrate complex is obvious
from the following formulation:
Fig. 2.7. The role of pyridoxal phosphate in transamination
and decarboxylation of amino acids
(2.15)