L23 Cofactors and vitamins

• • Cofactors are inorganic ions (Fe2+, Mg2+, Zn2+) or organic molecules (coenzyme)
  • that are required for enzyme's activity as a catalyst
• Define coenzyme

  • Specific type of organic cofactor that binds to an enzyme
  • Many cofactors are derived from vitamins (NAD+, FAD, Coenzyme A)
• Apoenzyme

  • Inactive protein portion of an enzyme
  • That requires a cofactor or coenzyme to become active
• Holoenzyme

  • Fully active enzyme
  • formed when apoenzyme binds to its necessary cofactor or coenzyme
• Riboflavin is converted to coenzymes - FMN

  • Phosphorylation of riboflavin forms Flavin mononucleotide (FMN)
  • This helps with enzyme binding as it contains an additional phosphate group

  
• Riboflavin is converted to coenzymes - FAD

  • Formed by attaching an AMP molecule to Flavin mononucleotide
  • This contains an adenosine group, increasing enzyme interaction

  
• Riboflavin is the chemical name of vitamin B2
• Vitamin B3 = niacin

  • This is involved in the formation of NAD and NADP
  • (structure of niacin shown in diagram)

  
• Biotin in Vitamin B7

  • Contains an imidazole ring
  • Acts as a coenzyme in carboxylation reactions
  • It has a role in keratin production

  
• Biotin of vitamin B7 is a coenzyme for pyruvate carboxylase

  • This catalyses the carboxylation of pyruvate to oxaloacetate
• Understand the mechanism of biotin in carboxylation reaction - this reaction is essential for:

  • Gluconeogenesis
  • replenishing Krebs cycle intermediates

  
• Substrate channeling of Biotin Vitamin B7

  • when coenzyme is trapped inside an enzyme
  • used to perform 2 functions in 2 separate reactions
  • without releasing intermediates out
• Two step mechanism of pyruvate carboxylase, biotin as a coenzyme

  STEP 1

  • Uses ATP and bicarbonate (HCO3-) to make an activated Carbon dioxide
  • Biotin grabs the carbon dioxide
  • Biotin is carboxylated and ready to transfer carbon dioxide

  
• Two step mechanism of pyruvate carboxylase, biotin as a coenzyme

  STEP 2

  • Biotin moves onto another active site on the same enzyme - this is called substrate channeling
  • It drops off carbon dioxide onto pyruvate
  • Biotin goes back to BC subunit from CT subunit to repeat the cycle (like dropping off groceries)

  
• Cytochromes

  • Haem containing proteins
  • Has a haem group that has an Fe / iron centre
  • shifts between Fe2+ and Fe3+ states to transfer electron

  
• Learning objectives for L23 cofactors and vitamins

  • Define cofactor, coenzyme, apoenzyme and holoenzyme
  • Recognise the main structural features of vitamins important for coenzyme function
  • Understand the common features and differences between NADH and FMN/FAD in electron transfer
  • Understand the mechanism of biotin in carboxylation reactions
  • Describe the structure and role of cytochromes in electron transfer
• Difference between cofactors and coenzyme

  • Cofactors are either organic molecules (known as coenzymes) or ions (usually metal ions) that are required by an enzyme for activity.
  • A coenzyme is a low-relativemolecular-mass organic molecule which transfers chemical groups, hydrogen or electrons.
  • Often derived from a vitamin.
  • Some coenzymes come in and out of the active site as a cosubstrate (e.g., ATP).