<P> However, it is also possible for pyruvate to be carboxylated by pyruvate carboxylase to form oxaloacetate . This latter reaction "fills up" the amount of oxaloacetate in the citric acid cycle, and is therefore an anaplerotic reaction, increasing the cycle's capacity to metabolize acetyl - CoA when the tissue's energy needs (e.g. in muscle) are suddenly increased by activity . </P> <P> In the citric acid cycle all the intermediates (e.g. citrate, iso - citrate, alpha - ketoglutarate, succinate, fumarate, malate and oxaloacetate) are regenerated during each turn of the cycle . Adding more of any of these intermediates to the mitochondrion therefore means that that additional amount is retained within the cycle, increasing all the other intermediates as one is converted into the other . Hence the addition of any one of them to the cycle has an anaplerotic effect, and its removal has a cataplerotic effect . These anaplerotic and cataplerotic reactions will, during the course of the cycle, increase or decrease the amount of oxaloacetate available to combine with acetyl - CoA to form citric acid . This in turn increases or decreases the rate of ATP production by the mitochondrion, and thus the availability of ATP to the cell . </P> <P> Acetyl - CoA, on the other hand, derived from pyruvate oxidation, or from the beta - oxidation of fatty acids, is the only fuel to enter the citric acid cycle . With each turn of the cycle one molecule of acetyl - CoA is consumed for every molecule of oxaloacetate present in the mitochondrial matrix, and is never regenerated . It is the oxidation of the acetate portion of acetyl - CoA that produces CO and water, with the energy thus released captured in the form of ATP . The three steps of beta - oxidation resemble the steps that occur in the production of oxaloacetate from succinate in the TCA cycle . Acyl - CoA is oxidized to trans - Enoyl - CoA while FAD is reduced to FADH2, which is similar to the oxidation of succinate to fumarate . Following, trans - Enoyl - CoA is hydrated across the double bond to beta - hydroxyacyl - CoA, just like fumarate is hydrated to malate . Lastly, beta - hydroxyacyl - CoA is oxidized to beta - ketoacyl - CoA while NAD+ is reduced to NADH, which follows the same process as the oxidation of malate to oxaloacetate . </P> <P> In the liver, the carboxylation of cytosolic pyruvate into intra-mitochondrial oxaloacetate is an early step in the gluconeogenic pathway which converts lactate and de-aminated alanine into glucose, under the influence of high levels of glucagon and / or epinephrine in the blood . Here the addition of oxaloacetate to the mitochondrion does not have a net anaplerotic effect, as another citric acid cycle intermediate (malate) is immediately removed from the mitochondrion to be converted into cytosolic oxaloacetate, which is ultimately converted into glucose, in a process that is almost the reverse of glycolysis . </P>

How many atp molecules are produced in krebs cycle