一个cetyl-CoA is not part of the cycle but is oxidized by it. There are two decarboxylations in the cycle, from isocitrate to alpha-ketoglutarate, and from alpha-ketoglutarate to succinyl-CoA. In total, three equivalents ofare produced in the cycle. Isocitrate is a compound in the cycle, produced from citrate.

Even though anis generated when malate is dehydrogenated to oxaloacetate, this oxidation is very unfavorable because of the addition of a reactive ketone in place of an alcohol on the 2nd carbon. In fact, the only way this reaction can proceed is if oxaloacetate concentration is very low. All of the other reactions have large negativevalues.

The formation of alpha-ketoglutarate from isocitrate using the enzyme alpha-ketoglutarate dehydrogenase is an irreversible reaction due to its largely negativevalue.

This question is providing us with a scenario in whichions enter a cell. We're further told that it will take a single molecule of ATP to move three of these ions out of the cell. Finally, we are being asked to determine the total number of acetyl-CoA molecules that must pass through the Krebs cycle in order to provide the energy necessary for the export of theseions.

First, we'll need to determine the total number of ATP molecules generated from the passage of a single molecule of acetyl-CoA through the Krebs cycle. It's important to remember that the passage of acetyl-CoA through the Krebs cycle generates one molecule of ATP directly by substrate-level phosphorylation, but it also produces other intermediate energy carriers in the form ofand.

对于每一个乙酰辅酶a穿过周期,一个摩尔ecule ofand three molecules ofare produced. Furthermore, each molecule ofwill go on to donate its electrons to the electron transport chain to generatemolecules of ATP per molecule ofoxidized. Likewise, eachwill also produce ATP via oxidative phosphorylation, but at a rate ofmolecules of ATP per molecule ofoxidized.

添加these up, we obtain:

一个TP via substrate-level phosphorylation

添加these values up, we have a total ofmolecules of ATP produced for every molecule of acetyl-CoA oxidized. Now that we know how much ATP is produced from one acetyl-CoA, we can calculate the number needed to move theions out of the cell.

The only step of the citric acid cycle (also known as the Krebs cycle, or the TCA cycle) thatdirectlyproduces ATP or GTP is the conversion of succinyl-CoA to succinate.

在this reaction, succinyl-CoA is converted to succinate with the assistance of the enzyme, succinyl-CoA synthetase. During this reaction, ADP + Pi (or GDP + Pi) is also converted to ATP (or GTP) using the energy from the breaking of the bond between CoA and succinate. Thus, the overall reaction appears as:

While side products of some of the other reactions listed produce intermediaries that may be used to produce ATP in the future, these reactions do notdirectlyproduce ATP.

The only citric acid cycle (also known as the Krebs cycle or TCA cycle) step listed that does not result in the production ofas a side product is the conversion of fumarate to malate.

在the conversion of fumarate to malate, fumarate is chemically combined with water in the presence of the enzyme fumarase to produce malate. In this conversion, there is no concomitant production of.

在each of the other reactions listed,is converted toandas side products.

正确的答案是没有柠檬酸cycle steps listed result in the production of. The only step of the citric acid cycle that results in the production ofis the conversion of succinate to fumarate (catalyzed by succinate dehydrogenase). In this reaction,is concomitantly converted tousing the hydrogen molecules removed from succinate by succinate dehydrogenase. This reaction was not listed in the answer choices though, and therefore none of the reactions listed produced.

Each of the reactions listed did produceotherside products. The conversions of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate all result in the production ofas a side product, but not. The conversion of succinyl-CoA to succinate results in the production of ATP or GTP and CoA-SH as side products, but not.