EnzymesEnzymes are highly efficient, selective biological catalysts whichaccelerate the approach of metabolic reactions to equilibriumwithout changing that equilibrium. Enzymes provide alternativepathways with lower activation energies. Without enzymesmetabolic reactions would not take place quickly enough for cellsto live. Most enzymes are globular proteins.The tertiary structure is important for the enzyme to carry out itsfunctions. A specific area called the active site of the enzymemolecule binds other molecules called substrates to producerequired products. Enzymes are relatively large molecules and form many contactpoints with the substrate. In enzyme catalyzed reactions anintermediate step forms when the enzyme binds to the substratecalled the enzyme substrate complex, the enzyme holds thesubstrate in the required orientation for a reaction to occur, byplacing the substrate next to specific amino acids and co factors.These binding points results from hydrogen bonds, Van der Waalsinteractions and ionic interactions, during these chemical reactionsthe enzymes themselves are not consumed in the end, one enzymecan convert many substrate molecules into product.Enzymes are specific in the types of reactions they catalyze and thetypes of substrate they act on; they have particular shapes whichallow them to be unambiguous with which substrate they bind to.
Fig.I.1 Enzyme ActivityTable: 1.1 Enzyme SpecificityType of Specificity CharacteristicsAbsolute Enzymes that act on a particular substrate onlyGroup Enzymes that act either relatively or absolutely on substrates in close relations with a particular functional groupSteriochemical Enzymes that act on particular sterioisomersLinkage Enzymes that act on a particular bond regardless of the rest of the molecules structure
In metabolism the enzyme usually does not react alone, sometimesthe enzyme cannot convert the substrate to product by itself; this isfacilitated by molecules called co-factors. These enzymes usuallyhave a binding site for the substrate and another for the co-factor,the enzyme only catalyses for these to molecules and no other, theenzyme does this by binding to the substrate first then the cofactorpreventing catalysis of the co factor with anything else.Fig.I.2: Enzyme activity with co factor.Inhibitors are compounds that bind enzymes; they disrupt theiractivity by preventing either the formation of the enzyme substratecomplex or its breakdown to enzyme and product. There are twotypes of inhibition reversible and irreversible. There are differenttypes of reversible inhibition:
Fig.I.3: Reversible Inhibition. a) Competitive Inhibition • Binds to the active site of the free enzyme only, has same shape as substrate. • If the enzyme is allosteric the binding of the inhibitor to one site prevents the binding of substrate at another in, the inhibitor may not have the same shape as the substrate. • Km increases. • Vmax is unchanged. • Both cases prevent the formation of the enzyme substrate complex. b) Noncompetitive Inhibition • Binds to either the active site of the free enzyme or the active site of the enzyme substrate complex, preventing either the formation of the ES complex or the of the ES complex to E+P. • the substrate still binds to the enzyme
• The inhibitor alters the enzymes shape in such a way that the enzyme becomes inactive. The inhibitor does not have the same shape as the substrate. • Vmax decreases. • Km is unchanged. • c) Uncompetitive inhibition • Binds to the active site of the enzyme substrate complex only. • does not compete with the substrate • Prevents breakdown to enzyme and product. The inhibitor does not have the same shape as the substrate. • Km and Vmax both decrease. • Ratio of Vmax/Km is unchanged. d) Mixed Inhibition • Binds to free enzyme • Binding of either the inhibitor or substrate to the enzyme decreases the enzymes affinity to the other. • Km may increase or decrease. • Vmax always decreases.Irreversible inhibitors bind to enzymes by covalent bonds.
Fig.I.3: Graphs of factors affecting rate of enzyme catalyzedreactions. a) Effect of increasing substrate concentrationAs concentration of substrate increases collision frequencybetween substrates and active sites of enzymes increases, in turnincreasing rate. Because the number of enzyme molecules is fixed,eventually all the enzyme active sites become filled with substrateand the rate reaches a maximum without any further increase. b) Effect of increasing temperature.Increasing temperature increases the kinetic energy in the systemcausing more molecules to have sufficient energy to get over theenergy barrier, increasing collisions between substrate and activesites, further increase in temperature causes the bonds keeping theprotein together to vibrate, when the enzyme reaches optimum
temperature these bonds vibrate so much that they break. Theenzyme’s tertiary structure denatures and changes the shape of theactive site making the enzyme inactive. This denaturation iscooperative; as soon as the optimum temperature is surpassed theentire enzyme breaks down, it does not break down bond by bond.Table :1.2 Enzyme Classification.Group number Class Function Examples1. Oxidoreductases catalyze oxidation Cytochrome reduction reactions. oxidase, lactate dehydrogenase2. Transferases Transferases Acetate kinase, catalyze group alanine deaminase transfer reactions- the transfer of a functional group from one molecule to another.3. Hydrolases In hydrolysis Lipase, sucrase reactions, C-O, C-N, and C-S bonds are cleaved by addition of H2O in the form of OH- and H+ to the atoms forming the bond.4. Lyases Lyases cleave C-C, Oxalate C-O, C-N, and C-S decarboxylase, bonds by means isocitrate lyase other than hydrolysis or oxidation.5. Isomerases Isomerases just Glucose-phosphate rearrange the isomerase, alanine existing atoms of a racemase molecule, that is, create isomers of the starting material.6. Ligases Acetyl-CoA synthetase, DNA ligase
Ligases synthesizeC-C, C-S, C-O, andC-N bonds inreactions coupled tothe cleavage of highenergy phosphatebonds in ATP