Question

Draw a diagram that describes how E. coli obtains oxaloacetate via the glyoxylate cycle when acetate, not glucose, is provided as a food source.

Answer 1

The Glyoxylate cycle is an anabolic metabolic pathway of plants and microorganisms, it is resorted to by organisms for eg. E.coli when complex sources of carbohydrate like glucose are unavailable.

It occurs in the cytoplasm of E.coli. It starts with the condensation of Acetyl Coenzyme A and oxaloacetate to form citrate. With the attack of water molecule the CoA is released from the complex. Then citracte is converted to isocitrate by the enzyme aconitase.

Isocitrate is cleaved by lyase into succinate and Glyoxylate. (Analogous to aldolase reaction of the Glycolysis)

Acetyl CoA condenses with glyoxylate to form malate. Finally malate is oxidized to oxaloacetate as in Citric acid cycle or CAA.

Glyoxylate cycle results in consumption of two Acetyl CoA molecules per cycle and produces 4C units as in oxaloacetate.

E.coli uses this path way when it is exposed to media that provide acetate as the only carbon source. Each turn of cycle uses 2 two-carbon fragments and results in the formation of a 4C molecule. The succinate generated is transported to mitochondrion where it is converted to OAA or oxaloacetate which subsequently is used for carbohydrate synthesis via gluconeogenesis.