Saturday, August 20, 2011

Krebs cycle (general ideas) - part 1

The Krebs cycle is also called the citric acid cycle or the tricarboxylic acids cycle. It is a catabolic process that occurs in mitochondria, specifically in the mitochondrial matrix (as I will highlight in a next post, there is just a reaction that occurs in association with the inner membrane of mitochondria). In the cycle, cells oxidize acetyl-CoA molecules to CO2, and the energy released is conserved in the form of NADH and FADH2. The Krebs cycle is exclusively aerobic, because although O2 does not participate directly in the cycle, the NAD + and FAD can only be regenerated in the mitochondria by transferring electrons to O2 (in the post on the regulation of the Krebs cycle, which will place soon, it will be possible to see that if NADH accumulates, that is what happens in the absence of O2, the Krebs cycle is inhibited ...).
We oxidize in the Krebs cycle many moles of acetyl-CoA per day. The oxidants, NAD+ and FAD, are reduced to NADH and FADH2. In the cell there are only a few micromoles of NAD + and FAD and within the mitochondria (where the cycle occurs) the regeneration of NAD+ and FAD depends on the respiratory chain, so the Krebs cycle does not occur under anaerobic conditions. The Krebs cycle is like a "mill" where the "grain" (the substrate) is the acetyl group of acetyl-CoA and the "flour" (products) are CO2 and electrons (NADH and FADH2), the "millstone" are the enzymes and intermediate compounds.
Main bibliographic sources:
- Quintas A, Freire AP, Halpern MJ, Bioquímica - Organização Molecular da Vida, Lidel
- Nelson DL, Cox MM, Lehninger - Principles of Biochemistry, WH Freeman Publishers

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