Enzyme specificity

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1. Enzyme Specificity

2. Enzyme Specificity : Enzyme specificity refers to the tendency for enzymes to catalyze a specific set of chemical reactions. Features of enzyme active site : The active site of an enzyme is the region that binds the substrates (and the cofactor, if any). It also contains the residues that directly participate in the making and breaking the bonds. These residues are called the catalytic groups . In essence, the interaction of the enzyme and substrate at the active site promotes the formation of the transition state . The active site is a three-dimensional cleft (চিড়) formed by groups that come from different parts of the amino acid sequence The active site takes up a relatively small part of the total volume of an enzyme Substrates are bound to enzymes by multiple weak attractions as products needs to be released after the completion of the reactions. Strong bonding inhibits the process. The specificity of binding depends on the precisely( অবিকল) defined arrangement of atoms in an active site Active site has a precise (নির্দিষ্ট) amino acid sequence that is never changed. A change in amino acid sequence in active site generally renders enzyme into a non functional form.

3. Specificity depends on active site orientation based on its atomic configuration Bond breaking and forming reacting groups are in the active site of the enzyme

4. Lock-and-Key Model In the lock-and-key model of enzyme action: - the active site has a rigid (অনমনীয়) shape - only substrates with the matching shape can fit - the substrate is a key that fits the lock of the active site -the amino acid R groups of enzymes help to mediate (মধ্যস্থতা করা) interaction of active site and substrate This is an older model, however, and does not work for all enzymes Limitations: Generally applicable for enzymes that work on single type of substrate It indicates the active site as a rigid shape but it is actually flexible Rigid shape is insensitive to environment modification for substrate binding

5. Induced Fit Model Advantages: Support enzymes which can act on different substrates of different conformations Enhance fidelity of molecular recognition in presence of competitor via conformational proof reading Much accepted as enzymes are not rigid and different conditions promote differential interactions. if it was rigid all the actions were same at always. In the induced-fit model of enzyme action: - the active site is flexible, not rigid - the shapes of the enzyme, active site, and substrate adjust to maximize the fit, which improves catalysis - there is a greater range of substrate specificity This model is more consistent (সংগতিপূর্ণ) with a wider range of enzymes

6. Types of enzyme specificity : Relative, low or bond specificity Moderate, structural or group specificity Absolute, high or substrate specificity Optical or stereo-specificity Dual specificity

7. Bond Specificity Enzymes act on substrate that are similar in structure and have same type of bond. Alpha-amylase can cleave glycosidic bond(alpha 1-4) of starch and glycogen Lipases can hydrolyze the ester bonds in tri-acyl-glycerol

8. Group Specificity Enzyme is not only specific to structure but also specific to surrounding chemical groups. Pepsin hydrolyze peptide bonds where amino groups are from aromatic amino acids-phenylalanine, tyrosine, tryptophan Catalyze same type of reaction for similar substrates . Their action is group specific. e.g ; methyl group, phosphate group. Hexokinases transfer phosphates to hexoses

9. Absolute specificity Enzymes act only on one substrate. Maltase only acts on maltose Sucrase only acts on sucrose Enzyme specific to one substrate and one reaction Products are two alpha D glucose or one alpha and one beta D glucose due to mutarotation

10. Optical specificity Enzyme is not only specific to substrate but also specific to optical configuration. Starch can be digested with alpha glycosidase but cellulose cant be digested by the same enzyme. As the sugars in cellulose are in beta orientation to the cellulose digestion needs beta glycosidase . Specificity is very high. Example: L-Amino Acid Oxidase only acts on L-Amino Acid.

11. Dual Specificity Enzymes act on two substrate by same type of reaction Hypoxanthine Xanthine Uric acid Xanthine oxidase Xanthine oxidase Enzyme acts on substrate with two reaction types. Enzyme is iso -citrate dehydrogensae Oxidatio n D ecarboxylation Removal of Hydrogen is oxidation