Question

Answer any one and only one of the following:

 a. Describe the mitochondrial electron transport pathway and the synthesis of ATP by mammalian mitochondria.

 b. Provide an overview of fatty acid synthesis and breakdown and their coordinated regulation.

 c. Discuss the regulation of the TCA cycle.

 d. Compare and contrast the KNF and MWC models.

Answer 1

c. TCA cycle regulations.

TCA is so series of reactions in a closed loop that are fundamental for cellular respiration. It is also known as citric acid cycle or tricarboxylic acid cycle or krebs cycle. The citric acid cycle produces the high-energy molecule ATP (adenosine triphosphate) and byproducts that also form ATP in a further process called oxidative phosphorylation.It takes place over eight different steps:

Citric acid cycle - Krebs cycle Condensation Acetyl-COM CH-8-s-can CH2-COO citrate synthase Hoc.coo CH, -COO Citrate 2a. Dehydration aconitase 8. O=C-COO Dehydrogenation CH-COO Oxaloacetate malate COO dehydrogenase Malate HO-CH CH Coo 7. Hydration fumarase CH- S-coo C-COO cis-Aconitate ***** NADH aconitase 2b. Hydration OH-COO H-C-COOTsocitrate HO-C- COO- CH CH coo Fumarate coo succinate dehydrogenase isecitrate dehydrogenase Dehydrogenation CH-COO CH coo -ketoglutarate CH-COO dehydrogenase complex CH 3. Oxidative decarboxylation c=0 Succinate COO succinyl-CoA synthetase CH-COO CH, c-s-coa o (ADP) Succinyl-CoA CASH (-Ketoglutarate 5. Substrate-level phosphorylation Oxidative decarboxylation

Step 1: Acetyl CoA (two carbon molecule) joins with oxaloacetate (4 carbon molecule) to form citrate (6 carbon molecule).

Step 2: Citrate is converted to isocitrate (an isomer of citrate)

Step 3: Isocitrate is oxidised to alpha-ketoglutarate (a five carbon molecule) which results in the release of carbon dioxide. One NADH molecule is formed.

The enzyme responsible for catalysing this step is isocitrate dehydrogenase. This is a rate limiting step as isocitrate dehydrogenase is an allosterically controlled enzyme.

Step 4: Alpha-ketoglutarate is oxidised to form a 4 carbon molecule. This binds to coenzyme A forming succinyl CoA. A second molecule of NADH is produced, alongside a second molecule of carbon dioxide.

Step 5: Succinyl CoA is then converted to succinate (4 carbon molecule) and one GTP molecule is produced.

Step 6: Succinate is converted into fumarate (4 carbon molecule) and a molecule of FADH₂ is produced.

Step 7: Fumarate is converted to malate (another 4 carbon molecule).

Step 8: Malate is then converted into oxaloacetate. The third molecule of NADH is produced.

It is important to be aware that whilst the primary role of the TCA cycle is production of NADH and FADH₂, it also produces molecules that supply various biosynthetic processes. These enter or exit the cycle at various points depending on demand. For example, alpha-ketoglutarate can leave the cycle to be converted into amino acids, and succinate can be converted to haem.

Net Output each cycle produces:

• Two molecules of carbon dioxide.
• Three molecules of NADH.
• Three hydrogen ions.
• One molecule of FADH₂
• One molecule of GTP.

Each molecule of glucose produces two molecules of pyruvate, which in turn produce two molecules of acetyl-coA. Therefore, each molecule of glucose produces double this.

Regulation of the citric acid cycle is important as reactions that are unchecked will lead to large amounts of wasted metabolic energy. The ability to regulate the cycle keeps the cell in a stable state, and this function is maintained by three mechanisms:

1. The availability of substrates.
2. Inhibition of the products formed.
3. Inhibition of enzymes through allosteric feedback.

Regulation of acetyl CoA

The citric acid cycle begins with the reaction that combines the two-carbon acetyl CoA with a four-carbon oxaloacetic acid to produce the six-carbon molecule citrate. Acetyl-CoA is regulated by the controlled amounts of pyruvate that is converted into acetyl-CoA in the pyruvate dehydrogenase complex reaction.

Metabolite flow is allosterically inhibited, where an enzyme is regulated by binding an effector molecule to a non-active site. The pyruvate dehydrogenase complex reaction is allosterically inhibited when there are high ratios of ATP to ADP, NADH to NAD+ and acetyl-CoA to CoA. Allosteric activation occurs when the ratio volumes decrease.

Regulation of enzymes in the citric acid cycle

Three reactions of the cycle are catalyzed respectively by the enzymes:

• Citrate synthase.
• Isocitrate dehydrogenase.
• α-ketoglutarate dehydrogenase

Citrate synthase is responsible for the rate of reaction in the first step of the cycle when the acetyl-CoA is combined with oxaloacetic acid to form citrate. It is inhibited by high concentrations of ATP, acetyl-CoA, and NADH which indicates an already high level of energy supply. The molecule produced in the reaction citrate can also act as an inhibitor of the reaction.

Because citrate synthase is inhibited by the final product of the citric acid cycle as ATP, ADP (adenosine diphosphate) works as an allosteric activator of the enzyme as ATP is formed from ADP. Therefore, the rate of the cycle is reduced when the cell has a high level of ATP.

The enzyme isocitrate dehydrogenase is an important catalyst in the third step of the reaction. It regulates the speed at which the citrate isomer isocitrate loses a carbon to form the five-carbon molecule α-ketoglutarate. The coenzyme NADH is a product of the reaction and at high levels, acts as an inhibitor by directly displacing the NAD+ molecules it is formed from.

The enzyme α-ketoglutarate dehydrogenase is another important catalyst in the fourth step of the cycle where α-ketoglutarate also loses a carbon and combines with Coenzyme A to form succinyl CoA. The two products of the reaction, succinyl CoA and NADH, both work as inhibitors at large concentrations.

Calcium as a regulator of the citric acid cycle

Calcium is also an important regulator of the citric acid cycle an increase in concentrations of both ADP and calcium ions (Ca2+) are a consequence of changes in cellular activity. Therefore, the signal that stimulates muscle contraction is also activating the production of the ATP which sustains it through the citric acid cycle. Calcium ions regulate the citric acid cycle by activating pyruvate dehydrogenase, the first component of the pyruvate dehydrogenase complex reaction that forms acetyl-CoA. Calcium ions also activate the enzymes, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase which catalyze the third and fourth steps of the cycle respectively. The activation of these enzymes, via calcium ions increases the rate of separate reactions within the cycle and therefore increases product production for the whole cycle.