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Organic Chemistry 6 th Edition Paula Yurkanis Bruice

Organic Chemistry 6 th Edition Paula Yurkanis Bruice. Chapter 25 The Organic Chemistry of the Coenzymes, Compounds Derived from Vitamins. Many enzymes catalyze a reaction with the help of a cofactor. Cofactors can be metal ions or organic molecules.

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Organic Chemistry 6 th Edition Paula Yurkanis Bruice

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  1. Organic Chemistry 6th Edition Paula Yurkanis Bruice Chapter 25 The Organic Chemistry of the Coenzymes, Compounds Derived from Vitamins

  2. Many enzymes catalyze a reaction with the help of a • cofactor. • Cofactors can be metal ions or organic molecules. • An enzyme that has a tightly bound metal ion is called a metalloenzyme. • Cofactors that are organic molecules are coenzymes. • Coenzymes are derived from vitamins.

  3. The Pyridine Nucleotide Coenzyme Is Needed for Many Redox Reactions • NAD(P)+ are oxidizing agents. • NAD(P)H are reducing agents.

  4. NAD+ is composed of two nucleotides linked together through their phosphate groups The adenine nucleotide portion of NAD+ is derived from ATP

  5. Many enzymes that catalyze oxidation reactions are called dehydrogenase: NAD+ and NADH are catabolic coenzymes, whereas NADP+ and NADPH are anabolic coenzymes

  6. Mechanisms for the pyridine nucleotide coenzymes: All the chemistry of the pyridine nucleotide coenzymes takes place at the 4-position of the pyridine ring

  7. Glyceraldehyde-3-phosphate dehydrogenase uses NAD+ as an oxidizing coenzyme:

  8. The mechanism for reduction by NAD or by NADPH: NADH and NADPH are hydride donors

  9. The structural complexity of a coenzyme is needed for • enzyme recognition. • Molecular recognition allows the enzyme to bind the • substrate and the coenzyme in the proper orientation for reaction. • Biological redox reactions are equilibrium reactions driven in the appropriate direction by the removal of reaction products.

  10. A Biological Redox Reaction Is Highly Selective

  11. A reducing enzyme can distinguish between the two hydrogens at the 4-position of the nicotinamide ring:

  12. FAD and FMN are coenzymes used to oxidize substrates A flavoprotein is an enzyme that contains either FAD or FMN

  13. The structure of the oxidized FMN cofactor:

  14. Biological Redox Reactions Involving FAD or FMN

  15. FAD and FMN are oxidizing agents, whereas FADH2 and FMNH2 are reducing agents:

  16. Two-electron transfer to the flavin ring Thiolate addition to 4aposition Mechanism for dihydrolipoyl dehydrogenase:

  17. Mechanism for D- or L-amino acid oxidase:

  18. Unlike NAD+ and NADH, FAD and FADH2 do not dissociate from the enzyme: But NAD+ is required to reoxidize the reduced cofactor

  19. Thiamine pyrophosphate (TPP) is the coenzyme required by enzymes that catalyze the transfer of a two-carbon fragment:

  20. The enzyme pyruvate decarboxylase requires thiamine pyrophosphate (TTP) as a coenzyme: TTP affords a nucleophilic ylide carbanion that facilitates decarboxylation:

  21. Enamine intermediate Mechanism for pyruvate decarboxylase:

  22. The conversion of pyruvate to acetyl-CoA requires coenzymes TPP, lipoate, coenzyme A, FAD, and NAD+

  23. Mechanism for acetyl-CoA formation:

  24. Biotin is required by enzymes that catalyze the carboxylation of a carbon adjacent to a carbonyl group:

  25. Enzymatic reactions utilizing the biotin coenzyme:

  26. In addition to requiring bicarbonate, biotin-requiring enzymes require Mg2+ and ATP:

  27. Mechanism for carboxylation of acetyl-CoA by acetyl-CoA carboxylase:

  28. Pyridoxal phosphate (PLP) is required by enzymes that catalyze certain transformations of amino acids:

  29. Amino Acid Transformations That Require PLP Coenzyme

  30. The first step of the reactions involves the breakage of the bond attached to Ca

  31. Mechanism for transimination:

  32. Mechanism for PLP-catalyzed decarboxylation of an amino acid:

  33. Mechanism for PLP-catalyzed racemization of an L-amino acid:

  34. Mechanism for PLP-catalyzed transamination of an amino acid:

  35. Mechanism for PLP-catalyzed Ca—Cb bond cleavage:

  36. The bond cleaved in the first step depends on the conformation of the enzyme-bound amino acid

  37. Enzymes that catalyze certain rearrangement reactions require coenzyme B12

  38. Enzymatic Reactions Requiring B12

  39. In a coenzyme B12–requiring reaction, a group (Y) bonded to one carbon changes places with a hydrogen bonded to an adjacent carbon:

  40. Mechanism for a coenzyme B12–requiring enzyme-catalyzed reaction:

  41. Tetrahydrofolate (THF) is the coenzyme required for one-carbon transfer reactions:

  42. The Six Different THF-Coenzymes

  43. GAR transformylase is an example of an enzyme that requires a THF-coenzyme:

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