The Citric AcidyC cle

Citric Cycle Part 2

• We have seen that the Citric Acid Cycle begins when a two – carbon acetyl group from acetyl CoA combines with oxaloacetate to form a citrate. In part 1, two carbon atoms are removed from citrate to yield two CO2 and four – carbon compound that

undergoes more reactions in part 2 to regenerate oxaloacetate. The figure below illustrate the Citric Cycle.

• In part 2, this is the start of Reaction 5 which is the four oxidation reactions, energy is released that reduces three NAD+ and one FAD. In one reaction, GTP produced by a direct phosphorylation is used to form ATP from ADP.

Reaction 5: Hydrolysis of Succinyl CoA – by the hydrolysis of thioester in CoA is used to add a phosphate group (Pi) directly to GDP (guanosine diphosphate). The products are succinate and GTP, which is high-energy compound to ATP.

COO- COO-

I ICH2 CH2

I ICH2 + GDP + Pi + H+ CH2 + GTP + HS -- CoAI IC = O COO-

IS – CoA

Succinyl CoA Succinate

5.1— The hydrolysis of GTP is used to add phosphate group to ADP, which regenerates GDP. This is the only time in citric acid cycle that a direct substrate phosphorylation is used to produce ATP

GTP + ADP GDP + ATP

Guanosine Triphosphate + Adenosine Diphosphate C10H16N5O14P3 C10H15N5O10P2

GTP + ADP

GDP + ATP

Reaction 6: Dehydration of Succinate -- In this oxidation, the hydrogen is removed from succinate, to produce fumarate, a compound with a trans-double bond. This is the only place in the citric acid cycle where FAD is reduced to FADH.

COO- -OOC HI CH2 CI IICH2 + FAD CI COO- H COO-

Succinate Fumarate

Flavin Adenine Dinucleotide – Is a redox cofactor involved in several important reactions in metabolism. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecules consists of a riboflavin moiety (vitamin B2) bound to the phosphate group of an ADP

molecule.FAD can be reduced to FADH2 whereby it accepts

two hydrogen atoms.

Flavin Adenine Dinucleotide

Reaction 7: Hydration – In a hydration reaction, water adds to the double bond of fumarate to yield maltate.

COO- COO-

I IC – H HO -- C – HI IC – H + H2O H – C – HI ICOO- COO-

Furmate Maltate

Reaction 8: Dehydration forms oxaloacetate – In this last step of citric acid cycle, the hydroxyl (-OH) group in malate is oxidized to yield oxaloacetate. The coenzyme NAD+

Is reduced to NADH + H+.

COO- COO-

I I HO – C – H C = O

I ICH2 + NAD+ CH2 + NADH + H+

I I COO- COO-

Malate Oxaloacetate