Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 182

7.2 FERMENTATIVE PATHWAYS

Mono- or disaccharide carbohydrates are the substrate for the majority of industrial
fermentations. Carbohydrates are reduced in a series of reactions, known as glycolysis, that
result in the production of two moles of pyruvate (Figure 7.1). In glycolysis sugars are
degraded to produce ATP, NADH, and pyruvate (Pyr) by the overall reaction:

+ − +

Glucose 2 ADP + + 2 NAD + 2 PO →+ 2 Pyr 2 ATP 2 (NADH + + H )

3

The overall glycolysis reaction is accomplished by a series of ten enzymatic reactions
(Figure 7.1). Glycolysis is also known as the Embden-Meyerhoff-Parnas pathway. A few
organisms use an alternate pathway to produce pyruvate from glucose known as the Entner-
Doudoroff pathway (Shuler and Kargi, 2002). This pathway produces only one mole of ATP
per mole of glucose instead of two moles of ATP. The production of only one mole of
ATP leaves the cell with less energy to produce cell mass and results in a greater yield of
fermentation products. Zymomonas mobilis , a bacterium that produces ethanol from glucose
and fructose, uses the Entner-Doudoroff pathway.
If oxygen is present, pyruvate is converted to acetyl-CoA, which enters the citric acid
cycle. The citric acid cycle generates more ATP, reducing compounds, and chemical

Glucose, Fructose,
Galactose, Mannose

2 ATP

Xylose

2 NAD +

2 NADH

2 ADP

Pentose phosphate
pathway

2 Glycerol 2 G3P

2 ATP 2 ADP

2 ADP+ 2 Pi

2 NAD +

2 NADH 2 ATP

2 CO 2

2 NAD +

2 NADH

2 Acetaldehyde 2 Lactate

2 Pyruvate

4 NADH

2 CO 2

4 NAD +

2 Ethanol 2 Acetyl-CoA

Figure 7.1 Reduction of carbohydrates by glycolysis.