PYRUVATE OXIDATION &

KREB’S CYCLE

PRESENTED BY:NOOPUR JOSHIM.Sc. Biotechnology

CELLULAR RESPIRATION

• Under aerobic conditions, the cells obtain energy from ATP, produced as a result of breakdown of glucose.

• The aerobic organisms oxidize their organic fuels completely to CO₂ and H₂O.

• In such conditions, the pyruvate, instead of being reduced to lactate, ethanol and CO ₂, gets completely oxidized in to CO₂ and H₂O.

• This is termed as Cellular respiration.

Thus Cellular respiration can be defined as:“A sequence of molecular processes involved in

O₂ consumption and CO₂ formation by the cells.”

3 STAGES OF CELLULAR RESPIRATION

STAGE 1:“Oxidative decarboxylation of Pyruvate to Acetyl

CoA and CO₂.”This conversion is catalyzed by a highly organized

multienzyme “pyruvate dehydrogenase complex.”In the overall reaction, the carbooxylic group of

pyruvate is lost as CO₂, while the remaining 2 carbons form the acetyl moeity of acetyl-CoA.

The reaction is highly Exergonic and is essentially irreversible, in vivo.

Pyruvate

CoA

Acetyl CoA

CO ₂

NAD⁺ NADH

• STAGE 2:“Citric acid Cycle or Acetyl CoA catabolism”In this stage, the acetyl group so obtained is fed into

citric acid cycle/Kreb’s Cycle which then degrades it to yield energy rich hydrogen atoms and to release CO₂; the final product of organic fuels.

It is the final common pathway for oxidation of fuel molecules.

This cycle also provides intermediates for biosynthesis.

• STAGE 3:“Electron transport chain and oxidative

phosphorylation”In this final stage of respiration, the hydrogen atoms

are separated into protons (H⁺) and energy rich electrons.

The electrons are transferred via chain of electron-carrying molecules, the respiratory chain, to molecular oxygen, which is reduced by electrons to form water.

PYRUVATE OXIDATION

• The oxidative decarboxylation of pyruvate to form Acetyl CoA Is the link between glycolysis and kreb’s cycle.

• It occurs in mitochondrial matrix.• Here pyruvate from Glycolysis is dehydrogenated to

form Acetyl CoA and CO₂ by the enzyme pyruvate dehydrogenase complex.

• The reaction is irreversible and can be represented as follows:

COO ‾ S CoA

C O + CoA SH + NAD⁺ C O + CO₂ + NADH

CH₃ CH₃

Pyruvate dehydrogenase complex

Mg²⁺

Pyruvate Acetyl CoACoenzyme A

This conversion is catalyzed by a highly organized multienzyme “pyruvate dehydrogenase complex.”

In the overall reaction, the carbooxylic group of pyruvate is lost as CO₂, while the remaining 2 carbons form the acetyl moeity of acetyl-CoA.

The reaction is highly Exergonic and is essentially irreversible, in vivo.

KREB’S CYCLE

• Also known as Citric acid cycle was discovered by H.A.Kreb, German born British Biochemist.

• This cycle occurs in mitochondrial matrix in eukaryotes and in cytosol in prokaryotes.

• The net result for this cycle is that for each acetyl group entering the cycle as Acetyl CoA, 2 molecules of CO₂ are produced.

Citric acid

Cis-aconitic

acid

Iso- citric acid

Oxalosuccinic

acid

α-Ketogluerate

Succinyl-CoA

Succinic acid

Fumaric acid

Malic acid

Oxaloacetic acid

Acetyl CoA

CoA-SH

Aconitase + H₂O

Aconitase + H₂O

IsocitricDehydrogenase

+ Mn²⁺

CO₂

NADH+H⁺ NAD⁺

Succinyl CoA Synthetase

+ Mg²⁺

Succinic dehydrogenase

NADP⁺

NADPH+ H⁺

Fumerase

Malic dehydrogenase

NADH+ H⁺

NAD⁺

FADH₂

FAD

+CO₂

H₂O

STEP WISE EXPLAINATION OF THE CITRIC ACID/TRICARBOXYLIC/KREB’S CYCLE

STEP1: Condensation OF Acetyl-CoA with Oxaloacetate

• The cycle begins with the condensation of a 4 carbon unit, the oxaloacetate, and the acetyl group of the Acetyl CoA, which is a 2 carbon unit.

• Oxaloacetate reacts with Acetyl-CoA and H₂O to yield citrate and CoA.

• This reaction is an aldol condensation reaction and is followed by hydrolysis.itis catalyzed by the enzyme: “ citrate synthetase”.

CITRATE SYNTHETASE

H₂O

CONDENSATION

HYDROLYSIS

Citryl-CoAOxaloacetate Acetyl-CoA

Citrate Coenzyme-A H+ +

+

STEP 2: ISOMERIZATION OF Citrate INTO Iso-citrate

• In this reaction, water is first removed and then added back, moves the hydroxyl group from one carbon atom to its neighbor.

• The enzyme catalyzing this reaction is aconitase.

CITRATE

Aconitase-H₂O

Cis-ACONITATE

+H₂O

ISOCITRATE

Aconitase-H₂O

+H₂O

STEP 3: Oxidative Decarboxylation of Isocitrate

• Isocitrate is oxidized and decarboxylated into α-ketogluterate .• This reaction is catalyzed by the enzyme “isocitrate

dehydrogenase.”

ISOCITRATE

OXALO-SUCCINATE

(enzyme bound)

α-KETOGLUTERATE

NAD⁺ NADH+H⁺ H⁺ CO₂

Isocitrate dehydogenase

Isocitrate dehydogenase

STEP 4: Oxidative decarboxylation of α-ketogluterate

• This second oxidative decarboxylation results in formation of “Succinyl CoA” from α-ketogluterate.

• “α-ketogluterate dehydrogenase” catalyzes this oxidative step and produces NADH, CO₂ and a high-energy thioester bond to coenzyme-A (CoA).

α-ketogluterate + CoA—SH NAD⁺+

α-ketogluterateDehydrogenase

complex

Succinyl-CoA

STEP 5: Conversion of Succinyl-CoA into Succinate

• The cleavage of the thioester bond of Succinyl-CoA is coupled to the phosphorylation of a purine nucleoside diphosphate, usually GDP (substtrate level phosphorylation).

• It is catalyzed by “succinyl CoA synthetase/ succinyl thiokinase”.

• This is the only step in the Kreb’s Cycle that directly yields a compound with high phosphoryk transfer potential through a substrate level phosphorylation

Succinyl-CoA

Succinyl phosphate(enzyme bound)

Succinate

Pi

CoA—SHMg²⁺

Mg²⁺

STEP 6: Dehydrogenation of Succinate to form Fumerate

• In this third oxidation step, FAD removes 2 hydrogen atoms from succinate.

• This reaction is catalyzed by the enzyme “succinate dehydrogenase.”

• This reaction is the only dehydrogenation in the citric acid cycle in which NAD⁺ doesn’t participate. Rather, hydrogen is directly transferred from the substrate to falvoprotein enzyme (succinate dehydrogenase).

Succinate FumerateE—FAD+

Succinatedehydrogenase

STEP 7: Hydration of Fumerate to Malate

• Fumerate is hydrated to form L-malate in the presence of “fumerate hydratase”.

• It involves hydration i.e. addition of water to fumerate which places a hydroxyl group next to the carbonyl carbon.

Fumerate L-malate

Fumerate hydratase

+H₂O

-H₂O

STEP 8: Dehydrogenation of Malate to Oxaloacetate

• This is the 4th oxidation-reduction reaction in the citric acid cycle where L-malate is dehydrogenated to oxaloacetate.

• This reaction takes place in the presence of “l L-malatae dehydrogenase”.

• The NAD⁺ which remains linked to the enzyme molecule acts as the hydrogen acceptor and gets reduced to NADH and H⁺

• This reaction is a reversible reaction.

• Although the equilibrium of this reaction favours formation of malate again, but the reaction proceeds forward since the oxaloacetate and the NADH so formed are removed rapidly and continuously in the further reactions.

• The generated Oxaloacetate allows repetition of the cycle and NADH precipitates in oxidative phosphorylation

• This reaction completes the cycle.

COO‾

HO C H

H C H

COO ‾ L-Malate

+ NAD⁺

COO ‾

C O

CH₂

COO ‾

++ NADH H⁺

OXALOACETATE

L-malatedehydrogenase

Thus, the complete cycle so obtained can be represented as follows:

Citric acid

Cis-aconitic acid

Iso- citric acid

Oxalosuccinic acid

α-KetogluerateSucciny

l-CoA

Succinic acid

Fumaric acid

Malic acid

Oxaloacetic acid

Acetyl CoA

CoA-SH

Aconitase + H₂O

Aconitase + H₂O

IsocitricDehydrogenase

+ Mn²⁺

CO₂

NADH+H⁺ NAD⁺

Succinyl CoA Synthetase

+ Mg²⁺

Succinic dehydrogenase

NADP⁺

NADPH+ H⁺

Fumerase

Malic dehydrogenase

NADH+ H⁺

NAD⁺

FADH₂

FAD

+CO₂

H₂O

THANK YOU