Dairy Chemistry And Biochemistry

82 DAIRY CHEMISTRY AND BIOCHEMISTRY

0

&-OH

0 I

I

CH,&-S-C~A + co2 + ATP Mn2+ b CH2 t ADP + Pi

Acetyl CoA Acetyl CoA C-S-CoA
carboxylase d

Malonyl CoA

Reduced bicarbonate supply (source of CO,) depresses fatty acid synthesis.
Some P-hydroxybutyrate is reduced to butyrate and incorporated directly
into milk fat; hence, the high level of this acid in ruminant milk fat.
In non-ruminants, the malonyl CoA is combined with an ‘acyl carrier
protein’ (ACP) which is part of a six-enzyme complex (molecular weight
c. 500 kDa) located in the cytoplasm. All subsequent steps in fatty acid
synthesis occur attached to this complex; through a series of steps and
repeated cycles, the fatty acid is elongated by two carbon units per cycle
(Figure 3.8, see also Lehninger, Nelson and Cox, 1993).
The net equation for the synthesis of a fatty acid is:
n Acetyl CoA + 2(n - 1)NADPH + 2(n - 1)H’ + (n - 1)ATP
0
il

• (n - 1)C02 -+ CH,CH,(CH~CH,),-~CH,C-COA + (n - 1)CoA


• (n - 1)ADP + (n - 1)Pi + 2(n - 1)NADP + (n - 1)COl
  The large supply of NADPH required for the above reactions is ob-
  tained through the metabolism of glucose-6-phosphate via the pentose
  pathway.
  In ruminants, P-hydroxybutyrate is the preferred chain initiator (labelled
  P-hydroxybutyrate appears as the terminal four carbons of short- to
  medium-chain acids), i.e. the first cycle in fatty acid synthesis commences at
  P-hydroxybut yryl-S- ACP.
  Synthesis of fatty acids via the malonyl CoA pathway does not proceed
  beyond palmitic acid (C,,,,) and mammary tissue contains an enzyme,
  thioacylase, capable of releasing the acyl fatty acid from the carrier protein
  at any stage between C, and c16. Probable interspecies differences in the
  activity of thioacylase may account for some of the interspecies differences in
  milk fatty acid profiles.
  The malonyl CoA pathway appears to account for 100% of the C,,, C,,
  and C14, and c. 50% of the C,,:, acids in ruminant milk fat, as indicated by
  labelling experiments (Figure 3.9). However, C,, c6 and C, are synthesized