Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 528

Mannose Fructose

Galactose Glucose

ATP
ADP

ATP

ATP ATP

Galactose-1P

ADP

UDP-GLU
UDP-GAL

ADP ADP

Glucose-6P

Glucose-1P

Glucose-6P
isomerase

Mannose-6P Fructose-6P

Mannose-6P
isomerase

FIGURE 10.26 Six carbon sugar uptake pathways.

10.7.4. Fermentative Pathways
When the oxidative phosphorylation is inactive (due to the absence of oxygen or lack of
some of the necessary proteins), pyruvate is not oxidized in the TCA cycle since that would
lead to an accumulation of NADH inside the cells. In this situation, NADH is oxidized with
simultaneous reduction of pyruvate to acetate, lactic acid, or ethanol. These processes are
collectively called fermentative metabolism. Fermentative metabolism is not the same in all
microorganisms, but there are many similarities.

NAD(P)H

NADH

L-xylulose

NAD(P) +

NAD +

L-Xyliulose
reductase

Arabitol
dehydrogenase

NAD(P) +

L-arabitol

NAD(P)H

Xylitol

NAD(P) +

Xylose

Arabionse reductase

NAD(P)H NAD +

Xylitol dehydrogenase

NADH

L-arabinose

D-xylulose

Arabinose isomerase

ATP

Xylulokinase

ADP

L-ribulose

D-xylulose-5P

Ribulokinase

ATP

Ribulose 5-phosphate
epimerase

ADP

L-ribulose-5P

FIGURE 10.27 Pentose metabolism.