O. Pines et al., THE CYTOSOLIC PATHWAY OF L-MALIC ACID SYNTHESIS IN SACCHAROMYCES-CEREVISIAE - THE ROLE OF FUMARASE, Applied microbiology and biotechnology, 46(4), 1996, pp. 393-399
Saccharomyces cerevisiae accumulates L-malic acid but only minute amou
nts of fumaric acid. A C-13-nuclear magnetic resonance study following
from glucose to L-malic acid indicates L-malic acid is synthesized fr
om pyruvic oxaloacetic acid. previously published studies, a cytosolic
reductive pathway leading from pyruvic acid via oxaloacetic acid to L
-malic acid is responsible for the L-malic acid production in yeast. T
he nonproduction of fumaric acid can be explained by the conclusion th
at, in the cell, cytosolic fumarase catalyzes the conversion of fumari
c acid to L-malic acid but not the reverse. This conclusion is based o
n the following findings. (a) The cytosolic enzyme exhibits a 17-fold
higher affinity towards fumaric acid than towards L-malic acid; the K-
m for L-malic acid is very high indicating that L-malic acid is not an
in vivo substrate of the enzyme. (b) Overexpression of cytosolic fuma
rase does not cause accumulation of fumaric acid (but rather more L-ma
lic acid). (c) According to C-13 NMR studies there is no interconversi
on of cytosolic L-malic and fumaric acids.