Enzymes

• Charles Caleb Colton: 'Examinations are formidable even to the best prepared, for the greatest fool may ask more than the wisest man can answer.'
• At the end of this section, students should be able to:
• Terms to define


  • apoenzyme
  • coenzyme
  • holoenzyme
  • isoenzyme
  • ribozyme
  • prosthetic groups
  • metalloenzyme
  • metal-activated enzyme
  • active site
  • transition state
  • activation energy

  
• List the assumptions inherent in the derivation of the Michaelis-Menten equation
• Cofactor


  Non-protein component required for enzyme activity

  
• Vmax


  Maximum enzyme velocity
• Coenzyme


  Organic molecule cofactor
• KM


  Michaelis constant
• Zymogen


  Inactive enzyme precursor
• Holoenzyme


  Enzyme with cofactor bound

  
• Proenzyme


  Inactive enzyme precursor
• Ribozyme


  Catalytic RNA

  
• Isoenzyme/Isozyme


  Enzymes with same function but different structure

  
• Metalloenzyme


  Enzyme with tightly bound metal ion cofactor
• Metal-Activated enzyme


  Enzyme requiring a metal ion cofactor
• What are enzymes?

  
• What are they made of?
• Sydney Altman – Nobel Prize in Chemistry in 1989 with Thomas Cech
• Is it necessary to regulate enzyme activity?
• How might enzymes be regulated?

  • Change in enzyme concentration (inducible and constitutive)
  • Change in Enzyme Activity (no change in concentration)
  • Limited access to enzyme (compartmentalization)
  • Covalent modification
  • Non-covalent modification

  
• Ways catalytic power may be regulated (without changing enzyme concentration)

  • Feedback inhibition
  • Allosterism
  • Protein-protein interaction
  • Reversible covalent modification (e.g. phosphorylation)
  • Zymogen activation by proteolysis
• Many* enzymes would be inactive without the presence of some non-protein component referred to as a cofactor
• Apoenzyme


  Inactive protein component of an enzyme
• Holoenzyme


  Apoenzyme + cofactor

  
• Types of cofactors


  • Organic molecule (coenzyme)
  • Metal ion (metalloenzyme or metal-activated enzyme)
• Tightly bound cofactors are sometimes referred to as prosthetic groups
• The Enzyme Commission's Classification

  Uses a 4 digit code and enzymes are divided into six main classes (1st digit)

  
• Why are enzymes necessary for life?
• Specificity of Enzyme Action

  • All enzymes are specific in action
  • Some show group specificity - act on several different but closely related substrates to catalyse a reaction involving a particular group

  
• Examples of group specificity

  • ADH will catalyze the oxidation of a variety of alcohols
  • HK will facilitate the transfer of PO4^2- from ATP to several different hexoses (glucose >> mannose > fructose)

  
• Absolute specificity


  Some enzymes will only act on one substrate
• Stereochemical specificity


  Enzymes will only act on one stereoisomeric form of a substrate, except*

  
• Two distinct types of sites or regions in the active site

  • Binding sites which link to specific groups in the substrate
  • Catalytic sites which promote the reaction

  
• Active sites usually comprise only a small portion of the total enzyme
• Active sites are usually a cleft at or near the surface which excludes water
• Amino acids not involved in binding may contribute to specificity through effects on exclusion & creation of micro-environments
• The serine proteases all have an identical fold with the catalytic triad of Asp, His and Ser at the interface of the two domains
• Collision Theory


  Molecules can only react if they come into contact (bond-forming distance) with each other
• Arrhenius and van't Hoff
• Not all colliding molecules react