Elementary plant Biochemistry and Biotechnology (2+1)

Mechanism of enzyme action

• A chemical reaction such as A → P takes place because a certain fraction of the substrate possesses enough energy to attain an activated condition called the transition state.
  
• This transition state is at the top of the energy barrier separating the reactants and products.
  
• The rate of a given chemical reaction is proportional to the concentration of this transition state species.
• The energy of activation is the amount of energy required to bring all the molecules in 1 mole of a substance at a given temperature to the transition state.
• Enzymes combine transiently with the substrate to produce a transition state intermediate having a lower energy of activation than the uncatalysed reaction.
  
• Thus, they accelerate chemical reactions by lowering the energy of activation
  


Example


H2O2 → H2O + (O)
Catalase


ReactionCondition	Activation energy (KCal mol-1)
Uncatalysed	18
Catalysed by colloidal Pt	13
Catalysed by catalase	7

It is generally believed that the catalytic reactions occur in at least two steps.

• Step 1: A molecule of enzyme(E) and a molecule of substrate(S) collide and react to form an intermediate called the enzyme-substrate complex (ES).
• Step 2: The decomposition of ES complex to give product(s) and the active enzyme

[S] + [E]→ [ES] → P+[E]

• The formation of an ES complex affords a lower activation energy.

Active site

• The substrate binding site in the enzyme is referred as active site.The functional groups that are essential for the formation of ES complex occur at a specific location on the surface of the enzyme molecule.
• This section of enzyme where substrate binding and transformation of substrate to product occurs is called as active site.
• Many attempts have been made to implicate specific amino acid residues (side chain or R groups) as being part of the active site of various enzymes.
• Some of the amino acids occurring at the active site of enzymes are hydroxyl.group of serine, sulfhydryl group of cysteine, imidazole group of histidine and carboxyl groupof aspartic acid.