why cant pyruvic acid directly starts in krebs cycle ? .what is there need for conversion of pyruvic acid to citric acid ?.
Answers
Answer:Pyruvate, Lactate, and Energy Metabolism Pyruvic and lactic acid metabolism . The oxidation of glucose and, to a much smaller degree, the deamination of alanine, generates pyruvate, which has four metabolic fates: (1) enter mitochondria and be oxidized to acetyl-CoA via pyruvate dehydrogenase. (2) Enter mitochondria and be carboxylated to form oxaloacetate via pyruvate carboxylase. (3) Remain in cytosol and be aminated to alanine. (4) Remain in cytosol and be reduced to lactic acid.
In most cells, the major metabolic pathway for pyruvate is mitochondrial oxidation to acetyl-CoA. Generally, the rate of mitochondrial pyruvate uptake and oxidation matches the pyruvate generation rate and the cytosol pyruvate concentration is stabilized. Should mitochondrial uptake fail to increase in response to accelerated generation, the pyruvate concentration increases. Decreased mitochondrial uptake, during periods of rapid generation, will increase the pyruvate concentration sharply.
The metabolic–hormonal set of the cell determines how pyruvate, acetyl-CoA, and oxaloacetate are used. In the fed state, energy-yielding substrate is abundant. Exogenous carbohydrates are partially oxidized and partially used to synthesize glycogen and fat. In the fasted state, adipose tissue releases fatty acids, which are oxidized; oxaloacetate and other protein-derived substrates are used to synthesize glucose.
Krebs cycle oxidation of acetyl-CoA transfers electrons and protons, representing chemical potential energy, from this compound to NAD+, forming NADH+H+. The energy is subsequently released in small steps as the electrons flow down the mitochondrial respiratory (electron transport) chain from the NADH+H+, eventually reduce to oxygen, and form H2O