tricarboxylic acid cycle

what is the TCA cycle also known as?

the citric acid cycle or the Krebs cycle

what is the Kreb’s cycle?

a cyclic series of enzymatically catalyzed reactions, carried out by a multienzyme system consisting of 8 enzymes. it is the metabolic pathway that connects carbohydrate, fat and protein metabolism

what are the 8 enzymes of the Kreb’s cycle?

1. citrate synthase

2. aconitase

3. isocitrate dehydrogenase

4. α-ketoglutarate dehydrogenase

5. succinyl-CoA-synthetase (or thiokinase)

where does the Kreb’s cycle take place?

the mitochondrial matrix

what is the purpose of the Kreb’s cycle?

it serves to oxidize the acetyl group in acetyl CoA to CO2 and to generate NADH and FADH2

what type of pathway is the Kreb’s cycle?

an amphibolic pathway as it is involved into both anabolic and catabolic processes

what does the cycle produce anabolically?

it generates precursors for biosynthetic processes

what are examples of precursors generated by the anabolic processes of the TCA cycle?

• α-Ketoglutarate accepts amino group via transamination to form glutamate which is converted into glutamine

• oxaloacetate is transaminated into aspartate, a precursor for nucleotides and other amino acids.

• succinyl CoA serves as the precursor for heme, which is essential for hemoglobin, myoglobin and cytochromes

what does the TCA cycle do catabolically?

it serves to oxidize the acetyl group, providing a common pathway for the final oxidation of all metabolic fuels, such as carbohydrates, fatty acids, amino acids and ketone bodies

what are the sources of acetyl-CoA?

1. proteolysis

2. amino acid metabolism

3. lipolysis

4. β-oxidation

5. glycogenolysis

6. glycolysis

7. oxidative decarboxylation

what is the primary source of acetyl-CoA?

pyruvate arising from glycolysis

what are some of the paths pyruvate arising from glycolysis can take?

1. transaminated to alanine (requires pyridoxal phosphate)

2. reduced to lactate (requires NADH)

3. carboxylated to oxaloacetate (requires biotin)

4. oxidatively decarboxylated to acetyl CoA

what does the oxidatively decarboxylation of pyruvate into acetyl CoA require?

NAD+ (niacin), coenzyme A (pantothenic acid), FAD (riboflavin), thiamine pyrophosphate and lipoic acid

what is the formation of acetyl CoA from pyruvate considered?

it is not a part of the TCA cycle, rather, it is an important preparatory step, and is one of the major routes by which acetyl CoA is generated, with the reaction as follows:

• pyruvate + NAD⁺ + CoA → Acetyl CoA + H⁺ + CO2

how much energy is produce in the conversion of pyruvate to acetyl CoA?

1 NADH is produced per pyruvate, and since the production of glycolysis gives us two pyruvate, 2 NADH is produced, and 2 NADH when converted to ATP is about 6 ATP

how much energy is produce in the conversion of pyruvate to acetyl CoA, when including the energy produced in glycolysis?

2 NADH is produced by the conversion of pyruvate to acetyl CoA, which when converted to ATP is 6. in glycolysis, the amount of net ATP is 2, with 2 NADH produced, when converted to ATP, is 6, totaling 8. combined, 12 ATP is produced

why is the conversion of the pyruvate to acetyl CoA considered irreversible?

because of it’s high exergonic nature, meaning it produces a lot of energy, so the reverse reaction, acetyl CoA to pyruvate, does not happen

what enzyme catalyzes the reaction of pyruvate to acetyl CoA?

pyruvate dehydrogenase, a multienzyme complex, containing 3 different catalytic parts, pyruvate decarboxylase, dihydrolipoyl transacetylase and dihydrolipoyl dehydrogenase

the multienzyme complex, pyruvate dehydrogenase, utilizes what five different coenzymes?

pyrophosphate, lipoic acid, coenzyme A, FAD and NAD+ (i.e. five vitamins: thiamine, lipoic acid, pantothenic acid, riboflavin and niacin). coenzyme A and NAD+ are free in solution whereas the other 3 are bound to proteins, with thiamine pyrophosphate bound to pyruvate dehydrogenase, lipoic acid bound to dihydrolipoyl transacetylase and FAD to dihydrolipoyl dehydrogenase

where does insulin activate in the pyruvate dehydrogenase complex?

in adipose tissue

where does norepinephrine activate in the pyruvate dehydrogenase complex?

in the heart by mechanisms not involving cAMP

the hormonal regulation of the pyruvate dehydrogenase complex involves what?

the activation of phosphatase

how many carbons enter the TCA cycle when acetyl-CoA condenses with oxaloacetate to form citrate, which is catalyzed by citrate synthase?

2 carbons enter

how many carbon atoms released in the TCA cycle?

the first one is released during the conversion of isocitrate into α-ketoglutarate, as catalyzed by isocitrate dehydrogenase, and the second is released during the conversion of α-ketoglutarate into succinyl-CoA, as catalyzed by α-ketoglutarate dehydrogenase, however, the carbon atoms released are not the same as the carbon atoms introduced into the cycle

what both initiates the cycle and is regenerated at the end of the cycle?

oxaloacetate

each Kreb cycle generates what molecules?

• 3 NADH molecules

• 1 FADH2 molecule

• 1 molecule of GTP

based on the electron transport chain, how can we convert the molecules produced during one cycle of the TCA cycle into ATP?

• 1 NADH produces 3 ATP, meaning 6 ATP is generated based on the 3 NADH

• 1 FADH2 is produces to 2 ATP, meaning 2 ATP is generated based on the 1 FADH2

• 1 GTP is equivalent to 1 ATP

how much ATP total is generated during one cycle of the TCA cycle?

12 ATP total

if glycolysis is included, how much ATP is generated in the Krebs cycle?

glycolysis produces 8 ATP, and 2 pyruvate. the conversion of 2 pyruvate into 2 acetyl CoA generates 6 ATP, meaning 2 cycles of the Kreb’s cycle will occur, leading to 24 ATP produced (12×2=24). in total, 38 ATP will be generated

what occurs if the two carbons of acetyl-CoA are (radioactively) labeled?

the label will eventually appear in oxaloacetate at the end of the cycle. however, there is no net synthesis of oxaloacetate from acetyl-CoA, because the two carbons introduced are balanced by the two CO₂ molecules released, since oxaloacetate can enter gluconeogenesis, the labeled carbons from acetyl-CoA may appear in glucose, but again, there is no net glucose synthesis from acetyl-CoA.

when is the TCA cycle regulated?

at its irreversible, rate-limiting step, which are steps 3 and 4, including enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase

what is the main regulatory enzyme of the Krebs cycle?

isocitrate dehydrogenase

what inhibits isocitrate dehydrogenase?

NADH (which also inhibits α-ketoglutarate dehydrogenase) and ATP (which also inhibits citrate synthase)

what activates isocitrate dehydrogenase?

NAD+ and ADP

how are α-ketoglutarate dehydrogenase and pyruvate dehydrogenase similar?

the reaction it catalyzes is very similar to that of pyruvate dehydrogenase, and it is also a multienzyme complex, its reaction involving cofactors derived from the following five cofactors: thiamine, lipoic acid, pantothenic acid (coenzyme A), niacin and riboflavin.

when does phosphorylation of GDP occur?

during step 5, when succinyl-CoA is converted to succinate, a reaction catalyzed by succinyl-CoA synthase

what is GTP equivalent to ATP?

because the energy present in GTP can be used to synthesize ATP from ADP, so the generation of GTP in this reaction is equivalent to the generation of ATP

why can acetyl CoA generated from the oxidation of fatty acids not be converted into glucose via gluconeogenesis?

because of 2 reasons:

1. pyruvate dehydrogenase is irreversible

2. when acetyl CoA is taken through the TCA cycle once, 2 carbon atoms are eliminated, even though they are not the same as the 2 that were originally present in acetyl-CoA, there is no net synthesis of oxaloacetate which can serve as a precursor for gluconeogenesis

the reactions of the cycles converts what?

• 3 equivalents of nicotinamide adenine dinucleotide (NAD+) into three equivalents of reduced NADH

• one equivalent of flavin adenine dinucleotide (FAD) into one equivalent of FADH2

• one equivalent of guanosine diphosphate (GDP) and inorganic phosphate into one equivalent of guanosine triphosphate (GTP)

NADH and FADH2 generated by the citric acid cycle are used by what?

they are used by the oxidative phosphorylation pathway to generate energy-rich ATP