Steriospecificity

Enzyme Specificity

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() formed by groups that come from different parts of the amino acid sequenceThe active site takes up a relatively small part of the total volume of an enzymeSubstrates 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() defined arrangement of atoms in an active siteActive site has a precise() amino acid sequence that is never changed. A change in amino acid sequence in active site generally renders enzyme into a non functional form.

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Specificity depends on active site orientation based on its atomic configurationBond breaking and forming reacting groups are in the active site of the enzyme

Lock-and-Key ModelIn the lock-and-key model of enzyme action: - the active site has a rigid() 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( ) interaction of active site and substrateThis is an older model, however, and does not work for all enzymes

Limitations:Generally applicable for enzymes that work on single type of substrateIt indicates the active site as a rigid shape but it is actually flexibleRigid shape is insensitive to environment modification for substrate binding

Induced Fit ModelAdvantages:Support enzymes which can act on different substrates of different conformationsEnhance fidelity of molecular recognition in presence of competitor via conformational proof readingMuch 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 specificityThis model is more consistent() with a wider range of enzymes

Types of enzyme specificity :Relative, low or bond specificityModerate, structural or group specificityAbsolute, high or substrate specificityOptical or stereo-specificityDual specificity

Bond SpecificityEnzymes 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 glycogenLipases can hydrolyze the ester bonds in tri-acyl-glycerol

Group SpecificityEnzyme 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, tryptophanCatalyze same type of reaction for similar substrates. Their action is group specific. e.g; methyl group, phosphate group.Hexokinases transfer phosphates to hexoses

Absolute specificityEnzymes act only on one substrate.Maltase only acts on maltoseSucrase only acts on sucroseEnzyme specific to one substrate and one reaction

Products are two alpha D glucose or one alpha and one beta D glucose due to mutarotation

Optical specificityEnzyme 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.

Dual SpecificityEnzymes act on two substrate by same type of reaction

HypoxanthineXanthineUric acidXanthine oxidaseXanthine oxidase

Enzyme acts on substrate with two reaction types.Enzyme is iso-citrate dehydrogensae

OxidationDecarboxylationRemoval of Hydrogen is oxidation