Energy output per Acetyl co A by one round of TCA cycle

The total energy generated by one round of TCA cycle starting with 1 Acetyl co A is approximately 12 ATP. Of this total energy output, what percentage is contributed by oxidation of niacin dependent coenzymes in the electron transport chain?

A. 38%

B. 42%

C. 86%

D.75 %

E. 100%

The correct answer is D- 75%.

The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) includes a series of oxidation-reduction reactions in mitochondria that result in the oxidation of an acetyl group to two molecules of carbon dioxide and reduce the coenzymes that are reoxidized through the electron transport chain, linked to the formation of ATP (figure-1).

The function of the citric acid cycle is the harvesting of high-energy electrons from carbon fuels.

The citric acid cycle itself neither generates a large amount of ATP nor includes oxygen as a reactant. Instead, the citric acid cycle removes electrons from acetyl CoA and uses these electrons to form NADH and FADH2 (Figure-2). In oxidative phosphorylation, electrons released in the reoxidation of NADH and FADH2 flow through a series of membrane proteins (referred to as the electron-transport chain) to generate a proton gradient across the membrane.

These protons then flow through ATP synthase to generate ATP from ADP and inorganic phosphate (Figure-1)

TCA and ETC

Figure-1-Oxygen is required for the citric acid cycle indirectly in as much as it is the electron acceptor at the end of the electron-transport chain, necessary to regenerate NAD+ and FAD.

TCA cycle overview

A four- carbon compound (oxaloacetate) condenses with a two-carbon acetyl unit to yield a six-carbon tricarboxylic acid (citrate). An isomer of citrate is then oxidatively decarboxylated. The resulting five-carbon compound (α-ketoglutarate) also is oxidatively decarboxylated to yield a four carbon compound (succinate).

Oxaloacetate is then regenerated from succinate. Two carbon atoms enter the cycle as an acetyl unit and two carbon atoms leave the cycle in the form of two molecules of carbon dioxide. Three hydride ions (hence, six electrons) are transferred to three molecules of nicotinamide adenine dinucleotide (NAD+), whereas one pair of hydrogen atoms (hence, two electrons) are transferred to one molecule of flavin adenine dinucleotide (FAD) (Figure-2).

TCA overview

Figure-2- TCA cycle overview

Energy yield per Acetyl co A per turn of cycle

Three molecules of NADH and one of FADH2 are produced for each molecule of acetyl-CoA catabolized in one turn of the cycle. These reducing equivalents are transferred to the respiratory chain, where reoxidation of each NADH results in formation of 3, and 2 ATP of FADH2. Consequently, 11 high-transfer-potential phosphoryl groups are generated when the electron-transport chain oxidizes 3 molecules of NADH and 1 molecule of FADH2; In addition, 1 ATP (or GTP) is formed by substrate-level phosphorylation catalyzed by succinate thiokinase.

Out of the total 12 ATP, 9 ATP are produced from oxidation of 3 NADH, thus contributing to 75 % of the total energy output. (As per the latest concept, 2.5 ATP form NADH and 1.5 ATP from FADH2 are considered to be produced, with the resultant total 10 ATP production from complete oxidation of one Acetyl co A).

 

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