S. cerevisiae is not the only microorganism capable of producing ethanol (Table 6.6). Other yeasts, bacteria and fungi can also ferment sugars to produce ethanol. Ethanol — producing bacteria have been of commercial interest because they have a faster growth rate and can be easily genetically engineered. Escherichia coli uses a different pathway to produce ethanol from pyruvate (Fig. 6.8) with an enzyme pyruvate for­mate lyase forming acetyl-CoA, which is then reduced by two steps with alcohol dehydrogenase to ethanol.

Other ethanol-producing bacteria metabolize glucose via the Entner-Doudoroff pathway which consumes less ATP than glycolysis (Fig. 6.9). One of the best examples

Table 6.6. The substrates that can be utilized by ethanol-producing microorganisms. Organism Substrate utilized

Glucose, fructose, galactose, maltose, maltotriose, xylulose Glucose, fructose, galactose, maltose, maltotriose, xylulose Glucose, galactose, lactose Glucose, xylose, xylulose

Glucose, fructose, sucrose, engineered to use xylose Glucose, cellobiose, cellulose Xylose, no ethanol tolerance Xylose, cellobiose, glucose

Xylose to acetone and butanol, ethanol in small quantities Uses cellobiose faster than glucose Lactose, useful for whey utilization Uses cellobiose if nutrients supplied, glucose, xylose and arabinos

is the bacterium Zymomonas mobilis which has a higher growth rate, is more ethanol- tolerant than yeasts but still only metabolizes glucose, fructose and sucrose.