Like glycolysis, much of the energy consumed is used in the irreversible steps of the process.
Six high-energy phosphate bonds are consumed: two from GTP and four from ATP. Furthermore, two molecules of NADH are required for the reduction of two molecules of 1,3-bisphosphoglycerate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase. The oxidation of NADH causes the lack of production of 5 molecules of ATP that are synthesized when the electrons of the reduced coenzyme are used in oxidative phosphorylation.
Also these energetic considerations show that gluconeogenesis is not simply glycolysis in reverse, in which case it would require the consumption of two molecules of ATP, as shown by the overall glycolytic equation.
Propionyl-CoA carboxylase catalyses the carboxylation reaction of propionyl CoA in the mitochondrial matrix . The enzyme is biotin -dependent. The product of the reaction is (S)- methylmalonyl CoA . Propionyl CoA is the end product of metabolism of odd-chain fatty acids, and is also a metabolite of most methyl-branched fatty acids . It is also the main metabolite of valine, and together with acetyl-CoA , is a metabolite of isoleucine, as well as a methionine metabolite. Propionyl-CoA is thus of great importance as a glucose precursor. (S)-Methylmalonyl-CoA is not directly utilizable by animals; it is acted on by a racemase to give (R)-methylmalonyl-CoA. The latter is converted by methylmalonyl-CoA mutase (one of a very few Vitamin B 12 -dependent enzymes) to give succinyl-CoA . The latter is converted to oxaloacetate and then malate in the Krebs cycle . Export of malate into the cytosol leads to formation of oxaloacetate , phosphoenol pyruvate , and other gluconeogenic intermediates.