citric acid cycle

Citric Acid cycle

Also known as Krebs cycle (after Sir Hans Krebs) and as the tri-carboxylic acid (TCA) cycle (based on structure of citric acid)

CAC Function

Oxidizes a 2C acetyl group completely to CO2, producing energy directly as GTP, and indirectly as NADH and FADH2

CAC Characteristics

Central biochemical pathway; amphibolic (participates in both Anabolism and Catabolism)

Stage 1 of CAC

Production of acetyl CoA from pyruvate

Pyruvate Conversion

3C pyruvate --> 2C acetyl via pyruvate decarboxylase by oxidative decarboxylation

Energy from PDHase Reaction

Energy is derived as NADH; acetyl product combines with Coenzyme A to make acetyl CoA for entry into the CAC

Pyruvate Dehydrogenase (PDH)

Made of three enzyme units: E1: Pyruvate dehydrogenase, E2: a transacetylase, E3: another dehydrogenase

PDH Complex Structure

Multiple copies of each enzyme plus associated cofactors form an assembly line

PDH Reaction Equation

Pyruvate + CoASH + NAD+ ---> Acetyl-CoA + CO2 + NADH

PDH Reaction Details

Decarboxylation + Redox rxn; Produces 1 NADH molecule and CO2

1. Citrate Synthase

Condensation reaction

Citrate Synthase Function

Adds 2C Acetyl CoA to 4C oxaloacetate to form 6C citrate

Citrate Synthase Enzyme

Acetyl CoA added to the alpha-carbon of oxaloacetate

2. Aconitase

Isomerization Reaction

Aconitase Function

Citrate isomerized to isocitrate

Aconitase Reaction Details

Occurs between C2 and C3: OH and H of C2 and C3 respectively switch positions; Dehydration followed by rehydration

3. Isocitrate Dehydrogenase

FIRST oxidative decarboxylation

Isocitrate Dehydrogenase Function

Reduces number of C's by 1; Isocitrate converted to alpha-ketoglutarate by removing carbonyl group

Isocitrate Dehydrogenase Reaction

Decarboxylation + Redox, as seen in production of Acetyl CoA

4. Alpha-ketoglutarate dehydrogenase

SECOND oxidative decarboxylation

Alpha-ketoglutarate dehydrogenase Function

Final reduction in number of C's; Converts 5C alpha-ketoglutarate to 4C succinyl-CoA; Produces CO2 and NADH

Alpha-ketoglutarate dehydrogenase Enzyme

Enzyme complex using CoA, similar to activity of pyruvate dehydrogenase

Oxidative Decarboxylations

Reactions reducing number of carbons by 1

Dehydrogenases

Oxidative decarboxylation enzymes producing NADH

5. Succinyl-CoA Synthetase

Substrate level phosphorylation

Succinyl-CoA Synthetase Function

Only direct energy producing step, making GTP; GTP later converted to ATP; Succinyl CoA converted to succinate

6. Succinate Dehydrogenase

Dehydrogenation reaction

Succinate Dehydrogenase Function

Succinate oxidized to fumarate; Reduces FAD to FADH2; FADH2 used for later energy production

7. Fumarase

Rehydration reaction

Fumarase Function

Double bond rehydrated in fumarate to form malate

8. Malate Dehydrogenase

Dehydrogenation reaction

Malate Dehydrogenase Function

Malate dehydrogenated to regenerate oxaloacetate to begin cycle again; Produces 3rd NADH for energy

Summary of TCA cycle Products

Acetyl CoA --> 2CO2 + GTP + FADH2 + 3NADH

Summary of TCA cycle rxn types

2 oxidative decarboxylations (not including pyruvate dehydrogenase reaction); 1 Substrate level phosphorylation; 4 dehydrogenase reaction

Site of Citric Acid Cycle

CAC is located in mitochondria. All substrates, cofactors, enzymes and products kept within specific cellular compartment; NADH and FADH2 readily accessible to membrane structure of mitochondrion

Sum of one turn of CAC

2 CO2, 3 NADH, 1 FADH2, 1 GTP; 1 more NADH from Pyruvate dehydrogenase

Summing up Energy from CAC

One GTP is same as 1 ATP; Each NADH ultimately represents 3 ATP and each FADH2 represents 2 ATP; 3 NADH x 3=9; 1 FADH2 x 2=2; 1 GTP =12 ATP per acetyl unit

Energy from 1 glucose molecule

Glycolysis started the process, making 2 NADH and 2 ATP, as well as 2 pyruvate; 2 pyruvate converted to 2 Acetyl CoA via PDHase, making 2 more NADH molecules; Then, 2 times 12 ATP per acetyl unit, since 2 Acetyl CoA were produced; SUM: Glucose-->6CO2 + 38 ATP, which saves about 40% of input glucose energy

Key regulatory steps

Regulation occurs at steps affecting carbon skeleton

Key regulatory ratios

ATP/ADP ratio, ATP/AMP ratio, NADH/NAD+ ratio

Pyruvate Dehydrogenase Regulation

Regulated by phosphorylation. Useful because phosphate comes from ATP, and is indicator of energy state of cell; Active while DEphosphorylated

Regulation via Energy State

Pyruvate dehydrogenase kinase stimulated by ATP and inhibited by ADP acting on E1 with E1-PO4 inactive; If low ATP and high ADP, pyruvate dehydrogenase remains active, supplying acetyl CoA to CAC

Anaplerotic Reactions

Reactions to regenerate TCA intermediates; Primary enzyme is pyruvate carboxylase, making oxaloacetate by substrate feedforward in liver and kidney; Tissue specific because need to replenish one intermediate to supply whole CAC

Regulation by Substrate Flux

Pyruvate carboxylase reaction is controlled by the levels of pyruvate; When pyruvate is high, glycolysis is active but CAC is not; Therefore, make oxaloacetate to engage the CAC and start consuming pyruvate in the form of Acetyl CoA

Citric Acid Cycle is Amphibolic

CAC does more than just make energy. It makes useful compounds that are starting points for synthesis reactions; Thus, CAC is both catabolic and anabolic, which is amphibolic

Amphibolic interchanges: Oxaloacetate

Oxaloacetate: directly converted into aspartate and asparagine, which can be converted to pyrimidines; May also be converted to phosphoenolpyruvate, which then forms serine, glycine, cysteine, phenylalanine, tyrosine, tryptophan, or glucose

Amphibolic interchanges: Succinyl CoA

Can be converted to porphyrins or heme

Amphibolic interchanges: alpha-ketoglutarate

Converted to glutamate, which can be converted to glutamine, proline, arginine, or purines

Amphibolic interchanges: Citrate

Converted to fatty acids, sterols

Interactions with other pathways

CAC generates useful compounds that intersect with other pathways to extend amphibolic nature of these reactions; Important uses include: Amino acid metabolism, including nitrogen use; Nucleotide metabolism; Fatty acid metabolism; Most other pathways

CAC and Ketone Bodies

Fatty acid converted to acetyl-coA, which forms ketone bodies (hepatocyte, acetoacetate, d-Beta-hydroxybutyrate, acetone); Acetoacetate and d-beta-hydroxybutyrate exported as energy source for heart, skeletal muscle, kidney, and brain;