Wine Chemistry and Biochemistry

1 Biochemistry of Alcoholic Fermentation 15

P l a s m a M e m b r a n e

Grape juice

Cytoplasme

Acetyl-coA

OO

HOHO

HOHO

Acetoacetyl-coA

Squalene

Hydroxymethyl

glutaryl-coA

Mevalonicacid

Lanosterol

Squalene

momooxygenase

NAPD+

NADPH + H+

H 2 O O 2

Squalene

2,3-epoxyde

Squaleneepoxide

lanosterol

cyclase

Membrane

insertion

Ergosterol

Mevalonicacid

pathway

P l a s m a M e m b r a n e

P l a s m a M e m b r a n e

Cytoplasme

Acetyl-coA

O

HOHO

HOHO

Acetoacetyl-coA

Squalene

Hydroxymethyl

glutaryl-coA

Mevalonic acid

Lanosterol

Squalene

momooxygenase

NAPD+

NADPH + H+

H 2 O O 2

Squalene

2,3-epoxyde

Squalene epoxide

lanosterol

cyclase

Membrane

insertion

Ergosterol

Mevalonicacid

pathway

Fig. 1.7Synthesis of ergosterol in yeasts

lanosterol. Afterwards, lanosterol is used to obtain ergosterol, which is the main

sterol inSaccharomyces cerevisiae(Nes et al. 1993).

Therefore, in the absence of oxygen, this pathway is completely blocked and its

final yeast product, ergosterol, cannot be produced.

Figure 1.8 shows the synthesis of fatty acids. This complex process is catalysed

by the multienzymatic complex, fatty acid synthetase. This enzyme uses as sub-

strates acetyl-coA and malonyl-coA to produce palmitic acid. Afterwards, palmitic

acid, a saturated fatty acid of 16 carbon atoms, can be used to produce other fatty

acids (Ratledge and Evans 1989). Fatty acids with more carbon units, such as

estearic acid, are obtained by elongation of palmitic acid.