Agriculture Reference
In-Depth Information
Enzyme reactions in flavour generation
When fresh tissue is damaged, flavour precursors react under the control of
alliinase (S-alk(en)yl-L-cysteine sulphoxide lyase) to release the highly reactive
sulfenic acids, plus ammonia and pyruvate, as illustrated in Fig. 8.2. The
enzyme is confined to the cell vacuole, whereas the flavour precursors are
confined to the cytoplasm, probably within small vesicles associated with their
presence in the cell. Hence, the enzyme has access to the precursors only when
cells are disrupted. This explains why onion bulbs boiled whole, without
previously damaging the tissue, lack flavour, since the enzyme will be destroyed
before having access to the flavour precursors. In garlic bulbs the alliinase is
confined to the bundle sheath cells that surround the vascular bundles within
the cloves.
The enzyme alliinase
Some details of the alliinase reaction are shown in Fig. 8.2. Piridoxal-5
-
phosphate is an essential cofactor that is tightly bound to the enzyme protein in
a 1:1 ratio. Alliinase is a glycoprotein with approximately 4.6% of its weight
comprising mannose-rich carbohydrate. Alliinase comprises about 6% of the
soluble protein of onion bulb tissue and exists in the cell vacuole both as single
units (monomers) or linked as several enzyme units in multimers, in
association with mannose-specific lectins. The amino acid sequence of several
alliinases has been established from the DNA sequence of the corresponding
Fig. 8.2.
Catalysis of the conversion of flavour precursors to highly reactive sulfenic
acids by the alliinase enzyme. The group R depends on species (see Table 8.1).
Piridoxal phosphate acts on the precursor so that it forms an enzyme-substrate
complex involving electrostatic attraction of the substrate for a metal ion (M
+
). A
basic group on the enzyme then removes a hydrogen ion (H
+
) from the substrate,
triggering its collapse to release a sulfenic acid, ammonia and pyruvate (redrawn
from Block, 1985).
