Discussion
An E.coli mutant defective for an enzyme is unable to grow on acetate but grows on glycerol as the sole carbon source. Which ONE of the following enzymes is likely to be defective in this mutant?
Answer: D
The defective enzyme is likely Isocitrate lyase.
To grow on acetate, a two-carbon compound, a bacterium must make four-carbon compounds for gluconeogenesis without losing both carbons as CO2 in the citric acid cycle. The glyoxylate shunt does this: isocitrate lyase splits isocitrate into succinate and glyoxylate, and malate synthase joins glyoxylate to acetyl CoA. Without isocitrate lyase there is no net synthesis from acetate. Glycerol enters glycolysis, so growth on it does not need the shunt.
Glyceraldehyde 3-phosphate dehydrogenase and isocitrate dehydrogenase are needed on glycerol too, so their loss would block growth on both sources. Pyruvate dehydrogenase is not needed on acetate, which enters as acetyl CoA directly.
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