文档介绍：Chapter 16 The Citric Acid Cycle
mon pathway leading plete oxidation of carbohydrates, fatty acids, and amino acids to CO2.
A pathway providing many precursors for biosynthesis
1. The cellular respiration (complete oxidation of fuels) can be divided into three stages
Stage I All the fuel molecules are oxidized to generate mon two-carbon unit, acetyl-CoA.
Stage II The acetyl-CoA pletely oxidized into CO2, with electrons collected by NAD and FAD via a cyclic pathway (named as the citric acid cycle, Krebs cycle, or tricarboxylic acid cycle).
Stage III Electrons of NADH and FADH2 are transferred to O2 via a series carriers, producing H2O and a H+ gradient, which will promote ATP formation.
Mitochondria is the major site for
fuel oxidation to generate ATP.
2. Pyruvate is oxidized to acetyl-CoA by the catalysis of pyruvate plex
Pyruvate is first transported into mitochondria via a specific transporter on the inner membrane.
Pyruvate is converted to acetyl-CoA and CO2 by oxidative decarboxylation.
The pyruvate plex is a huge multimeric assembly of three kinds of enzymes, having 60 subunits in bacteria and more in mammals.
Pyruvate is first decarboxylated after binding to the prosthetic group (TPP) of pyruvate dehydrogenase (E1), forming hydroxyethyl-TPP.
The hydroxyethyl group attached to TPP is oxidized and transferred: First two electrons, then the acetyl group formed are all transferred to the lipoyllysyl (硫辛酰赖氨酰)group of dihydrolipoyl transacetylase (E2).
The lipoyllysyl group serves as both electron and acetyl carriers.
The acetyl group is then transferred (still catalyzed by E2) from acetyllipoamide to CoA-SH,forming acetyl-CoA.
The oxidized lipoamide group is then regenerated by the action of dihydrolipoyl dehydrogenase (E3), with electrons collected by FAD and then by NAD+.
Substrates of the five reactions catalyzed by pyruvate plex are efficiently channeled : The lipoamide group attached to E2 swings between E1 (accepting the electrons and acetyl group) a