the krebs cycle

Convergent and divergent cyclic pathway

The type of metabolic pathway that the Krebs cycle is

Acetyl-CoA

3 NADH + H+ , 1 FADH2, 1 GTP

The Krebs cycle uses … to yield

• reduction to form FADH2 and NADH + H+

• Oxidation of acetyl CoA to form CO2

• Synthesis of GTP (then ultimately ATP)

The thee major reaction components of the Krebs cycle

Mitochondrial matrix

Succinct dehydrogenase, bound to inner mitochondrial membrane

All enzymes within the Krebs cycle are within the … except …

Acetyl-CoA and oxaloacetate

N/A

Citrate synthase

H2O to CoA-SH

Citrate

Irreversible

Step 1: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

Acetyl-CoA and oxaloacetate go through condensation reaction. These 2 molecules bind and the CoA-S portion is removed by water, replacing it on the structure with an OH group forming citrate

What happens in step 1

Citrate

N/A

Aconitase

Removal and addition of water

Intermediate cis-Aconitate then Isocitrate

Reversible

Step 2: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

A water molecule is removed from citrate forming intermediate cis-Aconitate. Water is then added back into the structure forming isocitrate

What happens in step 2

Isocitrate

N/A

Isocitrate dehydrogenase

NAD(P)+ to NAD(P)H + H+ and CO2 removal

Intermediate oxalosuccinate then alpha-ketoglutarate

Irreversible

Step 3: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

There is a redox reaction where isocitrate is oxidized to oxalosuccinate and NAD(P)+ is reduced to NAD(P)H + H+. CO2 is then lost from oxalosuccinate to form alpha-ketoglutarate. The initial reactant has 6 C and final molecule has 5

What happens in step 3

Alpha-ketoglutarate

N/A

Alpha-ketoglutarate dehydrogenase complex

CoA-SH added, NAD+ to NADH

Succinyl-CoA and CO2

Irreversible

Step 4: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

NAD+ is reduced to NADH while alpha-ketoglutarate is oxidized.

CoA-SH is added displacing a CO2

What happens in step 4

Succinyl-CoA

N/a

Succinyl-CoA synthetase

GDP + P to GTP and removal of CoA-SH

Succinate

Reversible

Step5: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

GDP is phosphorylated to GTP and the CoA-SH group is removed from succinyl-CoA to form succinate

What happens in step 5

Because the liberation of a phosphate from GTP yields the same free energy as a phosphate from ATP

Why is GTP equivalent to ATP

Nucleotide triphosphate kinase

Transfer the phosphate on GTP to ADP forming ATP

ATP is still preferred over GTP so it is converted using … to …

Succinate

N/A

Succinate dehydrogenase

FAD to FADH2

Fumarate

Reversible

Step 6: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

FAD is reduced to FADH2 and succinate is oxidized to fumarate

What happens in step 6

Fumarate

N/A

Fumarase

Addition of OH- and H+

L-malate

Reversible

Step 7: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

The enzyme fumarase spits water into OH- and H+. OH- is added forming a transition state then the H+ is added to form final product L-malate. A hydration reaction

What happens in step 7

L-malate

N/A

L-malate dehydrogenase

NAD+ to NADH + H+

Oxaloacetate

Reversible

Step 8: Reactant, cofactor, enzyme, enzyme reaction, product, reversibility

NAD+ is reduced to NADH + H+ and L-malate is oxidized to oxaloacetate which is reformed as it was added in step 1 to bind with acetyl-CoA

What happens in step 8

Because oxaloacetate is rapidly used in step 1 causing there to be none within the cell so it must be regenerated

Why is step 8 reversible even though it has a high positive delta G

• citrate

• Alpha-ketoglutarate

• Succinyl-CoA

• Oxaloacetate

Intermediates in the Krebs cycle also used for anabolic pathways that can cause deficiency reducing Krebs cycle efficacy

• high citrate yields more NADH that translates to more ATP

• In the liver PFK is inhibited by excess citrate

PFK-1 and citrate regulation

Anaplerosis

The process of replenishing diminished levels of metabolic intermediates

• 2 use pyruvate

• 1 uses phosphoenolpyruvate

In mammals the three anaplerotic reactions to replenish KC intermediates

Oxaloacetate, where in high levels that can fill gaps as it fuels the Krebs cycle to continue and make more of the diminished intermediates (used in first step)

Anaplerosis in the KC uses reaction of PEP and pyruvate to form

• pyruvate → oxaloacetate And PEP → oxaloacetate

• Pyruvate → malate

The 2 conversion of anaplerotic reactions that will directly convert to oxaloacetate … in this conversion the product will drive production of oxaloacetate so acts indirectly

Making acetyl-CoA, this reaction is bypassed

When there is an oxaloacetate deficiency, pyruvate will focus on making it to reduce the deficiency instead of …

Pyruvate carboxylase

The reaction of pyruvate to oxaloacetate during deficiency uses enzyme

PEP carboxykinase

The reaction of phosphoenolpyruvate (PEP) to oxaloacetate during deficiency uses enzyme

Malic enzyme

The reaction of pyruvate to malate during deficiency uses enzyme

Liver and kidneys

Conversion of pyruvate to oxaloacetate occurs in the

Heart and skeletal muscle

Conversion of PEP to oxaloacetate occurs in the

Widely distributed in eukaryotes and bacteria

Conversion of pyruvate to malate occurs

• pyruvate dehydrogenase complex

• Entry of acetyl-CoA into the cycle

• Within the cycle itself (allosteric and covalent mechanisms)

The Krebs cycle is regulated at these three levels

• When there is high levels of ATP, acetyl-CoA and NADH PDH complex is inhibited

• High acetyl-CoA and NADH will feedback and inhibit as it will further made more ATP

• When there is high ADP and pyruvate the PDH complex is stimulated

Regulation of KC within at the pyruvate dehydrogenase complex (converts pyruvate to acetyl-CoA)

• High ATP and NADH inhibit isocitrate dehydrogenase so that no more ATP is produced

• High ADP stimulates isocitrate dehydrogenase

Regulation of KC within at the conversion of isocitrate to alpha-ketoglutarate

• only inhibitory

• High ATP, succinyl-CoA and NADH inhibit alpha-ketoglutarate dehydrogenase complex so no more ATP is made. Succinyl-CoA and NADH feedback to inhibit

Regulation of KC within at the conversion of alpha-ketoglutarate to succinyl-CoA