Practice Quiz 4

Question 1

Citric acid cycle intermediates can be used for gluconeogenesis. Why is there no net synthesis of glucose from fatty acids with an even number of carbons?

Answer 1

For every acetate that enters the CAC two molecules of CO2 are generated. Since there is a net of 0 new carbons from acetyl-CoA entering the CAC, there can be no net glucose synthesis from acetyl-CoA entering the CAC.

Question 2

Why can fatty acids with an odd number of carbons be used to synthesize a net gain of glucose?

Answer 2

Odd chain fatty acids are degraded to propionyl-CoA which is subsequently carboxylated to generate succinyl-CoA. Succinyl-CoA can enter directly into the CAC downstream of isocitrate dehydrogenase and a-ketoglutarate dehydrogenase. Therefore, none of the carbons from propionyl-CoA generate CO2 and can be used for transport out of the mitochondria as malate and used for gluconeogenesis.

Question 3

Why can the degradation of triacylglycerides contribute to a small net synthesis of glucose?

Answer 3

The glycerol generated from lipolysis can be used to generate glyceraldehyde 3-phosphate and enter gluconeogenesis.