Agriculture Reference
In-Depth Information
genes. The enzyme contains a chain of 445 amino acids, with the pyridoxal
phosphate binding with a lysine unit at position 285. A tryptophan unit at
position 182 is essential for activity. Several forms of the enzyme may exist that
may differ in affinity for the varying ACSOs.
Volatile flavour compounds
The highly reactive sulfenic acids, once released, proceed to undergo rearrange-
ment and inter-reactions to produce a wide range of volatile, strongly smelling
products. The 1-propenyl sulfenic acid (1-PECSO) produced in onions rapidly
enters a reaction catalysed by the recently discovered enzyme lachrymatory
factor synthase (Imai
et al.
, 2002) to form the tear-inducing thiopropanal S-
oxide (see Fig. 8.3). In garlic alliinase releases 2-PECSO, two of which
spontaneously combine to form the thiosulfinate allicin, which gives fresh garlic
its characteristic smell.
O
O
↑
alliinase
↑
2 CH
2
= CH-CH
2
-S-CH
2
(NH
2
)COOH
→
CH
2
= CH-CH
2
-S-S-CH
2
-CH = CH
2
Alliin
Allicin
ammonia
pyruvate
Fig. 8.3.
The involvement of the enzyme 'lachrymatory factor synthase' in the
production of the tear-inducing volatile propanthial S-oxide (the lachrymatory
factor (LF)) when onion bulbs are sliced. Previously, LF was believed to be formed
spontaneously following the action of alliinase, but the enzyme LF synthase,
alliinase and PECSO must all be present for LF to be produced (redrawn from Imai
et al
., 2002).
