Production of Acetyl-CoA (Activated Acetate)

• Coenzyme A has a reactive thiol group that is critical to its role as an acyl carrier.

  • The thiol group forms a thioester with acetate in acetyl CoA
• Coenzyme A:

  • Forms thioester acyl groups
  • Has lipoic acid as one of its components
  • Forms esters with relatively small standard free energies of hydrolysis
  • Can have pyruvate linked to it through an ester linkage

  Forms thioester acyl groups. The thiol group of the mercaptoethylamine moiety forms a thioester with acetate in acetyl coenzyme A (acetyl CoA).
• Pyruvate is oxidized to acetyl CoA and CO2
• Mitochondrial pyruvate carrier (MPC) is an H+ coupled pyruvate specific symporter in the inner mitochondrial membrane
• Pyruvate dehydrogenase (PDH) complex is a highly ordered cluster of enzymes and cofactors that oxidizes pyruvate in the mitochondrial matrix to acetyl CoA and CO2.

  • The series of chemical intermediates remain bound to the enzyme subunits
  • Regulation results in precisely regulated flux
• Pyruvate is produced in glycolysis and used by the citric acid cycle in the mitochondrial matrix. How does pyruvate get into the matrix?

  • It moves through the membrane by simple diffusion.
  • Diffusion is facilitated through a specific uniport
  • It transforms into acetate, which moves through a facilitated transporter
  • It moves through the malate shuttle system

  Diffusion is facilitated through a specific uniport.
• In eukaryotes, pyruvate may diffuse into mitochondria first through larger openings in the outer mitochondrial membrane, and then into the matric via a H+ coupled pyruvate specific symporter in the inner mitochondrial membrane, the mitochondrial pyruvate carrier (MPC)
• Oxidative decarboxylation is an irreversible oxidation process in which the carboxyl group is removed, forming CO2.
• Pyruvate is oxidized to form CO2, NADH and acetyl CoA.

  • Facilitated by the pyruvate dehydrogenase compels, CoA-SH, NAD+, TPP, lipoate, and FAD
  • delta G'degree = -33.4 kJ/mol
• The three enzymes that form the PDH complex are:

  • Pyruvate dehydrogenase, E1
  • Dihydrolipoyl transacetylase, E2
  • Dihydroliopyl dehydrogenase, E3
• The five coenzymes of the PDH complex are:

  • Thiamine pyrophosphate, TPP
  • Lipoate
  • Coenzyme A, CoA or CoA-SH
  • Flavin adenine dinucleotide, FAD
  • Nicotinamide adenine dinucleotide, NAD
• Lipoate is a coenzyme with two thiol groups that can undergo reversible oxidation to a disulfide bond

  • Serves as an electron (hydrogen) carrier and an acyl carrier
  • Covalently linked to E2, via a lysine residue.
• Which vitamin is a coenzyme to the pyruvate dehydrogenase (PDH) complex?

  • Pantothenate
  • Thiamine pyrophosphate
  • Niacin
  • Riboflavin
  • All of the above answers are correct

  All the above answers are correct. Four different vitamins required in human nutrition are vital components of the PDH complex: thiamine (TPP), pantothenate (CoA), riboflavin (FAD), and niacin (NAD)
• The PDH complex contains multiple copies of pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).
• The PDH complex has an E2 core of 24 to 60 copies and is surrounded by variable numbers of E1 and E3 copies.
• The structure of the PDH complex is similar to other enzymes that catalyze oxidations, such as alpha-ketoglutarate dehydrogenase, and branched chain alpha-keto acid dehydrogenase.

  • In a given species. E3 is identical in all three complexes
  • Similarities reflect a common evolutionary origin, thus they are paralogs
• Oxidative decarboxylation of pyruvate


  1. In E1 bound to TPP, forms decarboxylation of pyruvate to the hydroethyl derivative

  2. In E1 bound to TPP, forms oxidation of the hydroethyl derivative to an acetyl group

  3. E2 catalyzes the esterification of the acetyl moiety to one of the lipoyl thiol groups, followed by transesterification to CoA to form acetyl CoA

  4. E3 catalyzes the electron transfer to regenerate the oxidized form of the lipoyllysyl group

  5. E3 catalyzes the electron transfer to regenerate oxidized FAD cofactor, forming NADH
• Rate limiting stepof oxidative decarboxylation


  Decarboxylation of pyruvate to the hydroethyl derivative
• In step 2 of oxidative decarboxylation, the electrons and the acetyl group are transferred from TPP to the lipoyllysyl group E2
• Which two chemical mechanisms change pyruvate to acetyl-CoA in the pyruvate dehydrogenase complex?

  • dehydrogenation and oxidation
  • Decarboxylation and condensation
  • Condensation and dehydrogenation
  • Dehydrogenation and decarboxylation
  • Condensation and oxidation

  Dehydrogenation and decarboxylation. The overall reaction catalyzed by the PDH complex is an oxidative decarboxylation, an irreversible oxidation process in which the carboxyl group is removed from pyruvate as a molecule of CO2 and the two remaining carbons become the acetyl group of acetyl CoA.
• Substrate channeling is the passage of intermediates from one enzyme directly to another enzyme without release.
• The long lipoyllysyl arm of E2 channels the substrate from the active site of E1 to E2 to E3.

  • Tethers intermediates to the enzyme complex
  • Increases the efficiency of the overall reaction
  • Minimizes side reactions.
• What is the advantage to having an enzyme complex, as in the PDH complex?

  • Multiple steps can be regulated at one point
  • Products do not need to diffuse to become substrates for the next enzymatic reaction
  • Products cannot be scavenged by other enzymes or pathways
  • All of the answers are correct

  All of the answers are correct.