Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 527

Cell wall containing organisms, such as plants, fungi, and bacteria, require very large
amounts of carbohydrates during growth for the biosynthesis of complex structural polysac-
charides, such as cellulose, glucans, and chitin. In these organisms, in the absence of available
carbohydrates (for example, in certain microbial environments or during seed germination in
plants), the glyoxylate cycle permits the synthesis of glucose from lipids via acetate generated
in fatty acid b -oxidation.
The glyoxylate cycle bypasses the steps in the TCA cycle where carbon is lost in the form of
CO 2 . The two initial steps of the glyoxylate cycle are identical to those in the TCA cycle:
acetate

isocitrate. In the next step, catalyzed by the first glyoxylate cycle enzyme,
isocitrate lyase, isocitrate undergoes cleavage into succinate and glyoxylate (the latter gives
the cycle its name). Glyoxylate condenses with acetyl-CoA (a step catalyzed by malate syn-
thase), yielding malate. Both malate and oxaloacetate can be converted into PEP, which is the
substrate of PEP carboxykinase, the first enzyme in gluconeogenesis. The net result of the
glyoxylate cycle is therefore the production of glucose from fatty acids. Succinate generated
in the first step can enter into the citric acid cycle to eventually form oxaloacetate.

/ citrate /

10.7.3. Metabolism of Common Plant Biomass Derived Monosaccharides
Major monomeric sugars obtainable from plant biomass can be categorized as six carbon
sugars: glucose, fructose, mannose and galactose, and five carbon sugars: xylose and arabi-
nose. By far, glucose is the preferred substrate to microorganisms. There are few known
organisms that can utilize these nonglucose sugars. Table 10.5 shows various natural micro-
organisms and their natural ability to metabolize these monosaccharides.
Figure 10.26 shows the uptake of six carbon sugars, which replaces the front end of glucose
metabolism shown in the previous section. One can observe that the metabolism of fructose
and mannose does not go through Glucose-6P intermediate before reaching Fructose-6P.
Galactose needs two additional steps to get to the intermediate Glucose-6P.
Figure 10.27 shows the pentose metabolisms. Xylose and arabinose are metabolized into
Xylulose-5P, from which enters into the HMP or PP pathways.

TABLE 10.5 Various Natural Microorganisms and Their Ability to Metabolize Monosaccharides

Natural sugar utilization pathways Major products

Organism Glucose Mannose Galactose Xylose Arabinose Ethanol Other

Anaerobic
bacteria

Yes Yes Yes Yes Yes Yes Yes

E. coli Yes Yes Yes Yes Yes No Yes

Z. mobilis Yes No No No No Yes No

S. cerevisiae Yes Yes Yes No No Yes No

P. stipitis Yes Yes Yes Yes Yes Yes No

Filamentous
fungi

Yes Yes Yes Yes Yes Yes No