Which enzyme joins two substrates-

Ans. is 'b' i.e., Ligase* An enzyme which joins two substrates in an energy dependent process is called ligase# Examples includeLigases, Synthetases, Carboxylases* Enzymes are classified into 6 classes:i) Oxidoreductasesii) Transferasesiii) Hydrolasesiv) Lyasesv) Isomerasesvi) LigasesI. Oxidoreductases catalyse oxidation or reduction reactions of the primary substrate. Hence they are in turn classified as those which mediate:# Oxidation reactions# Reduction reactions* Those which mediate oxidation reactions can be classified as those which mediate oxidation by:# Removal of hydrogen ion or# Addition of oxygen# Those which cause REMOVAL OF HYDROGEN can again be classified depending upon the acceptor* NAD/FAD/NADP as acceptor - DehydrogenasesExamples - Lactate Dehydrogenase, Pyruvate Dehydrogenase both remove hydrogen ion from Lactate and Pyruvate respectively and donate the hydrogen ions to NAD forming NADH and H+* Oxygen as acceptor - OxidasesExample - L Aminoacid oxidase removes hydrogen atoms from aminoacid and that is donated to Oxygen molecule forming hydrogen peroxide# Those which add oxygen can be classified depending upon whether one or both the atoms are used asMonooxygenases* Monooxygenase allows oxygen molecule to react with a substrate. However, it incorporates only one oxygen atom into the substrate. The substrate thereby gets converted into hydroxylated form.* RH + O2 - ROH + H2O* So the other name for monooxygenases is hydroxylases* Example Phenylalanine hydroxylase, tyrosine hydroxylaseDioxygenases* Dioxygenase allows oxygen molecule to react with a substrate. It incorporates both the oxygen atoms into the substrate.* E.g., Homogentisate oxidase# Those which mediate REDUCTION REACTIONS can be given various namesPeroxidases* If hydrogen peroxide is getting reduced, it is called as peroxidase* E.g., glutathione peroxidaseCatalases* For reduction of hydrogen peroxide, if hydrogen peroxide is used as a source of hydrogen, it is catalase.Reductases* A substance is reduced using reducing equivalentsII. TRANSFERASES transfer a group from one substrate to another. Depending upon the group that is transferred, they are classified as -Kinases /Phosphotransferases# They transfer phosphate group from one substrate to another# Example hexokinase or glucokinase transfers phosphate group from ATP to glucose or any hexose respectivelyTransketolases# Transfers ketone group from one substrate to anotherTransaldolase# Transfers aldehyde group from one substrate to anotherAcyltransferase# Transfers acyl group or fatty acid from one substrate to another# Example Glycerol phosphate acyl transferase transfers acyl group or fatty acid from acyl CoA to glycerol phosphate forming triacylglycerolGlucosyltransferase - glycogen synthaseIII. HYDROLASES catalyse cleavage of a covalent linkage by addition of water# These enzymes can cleave only ester (phosphate/peptide) / ether (glycosidic) linkages# Examples include:a) Amidasese) Phospholipasesb) Peptidasesf) Glycosidasesc) Esterasesg) Ribonucleasesd) PhosphatasesIV. LYASES catalyse cleavage of a C-N /C-C linkage without adding water# Since all reactions are reversible, they can also synthesise the same linkage back# Examples include:a) Decarboxylasesd) Aldolaseb) Hydratasee) Lyasesc) Dehydratasef) SynthasesV. ISOMERASES catalyse the conversion of one form of a substance to another form of substance without altering the molecular formula# Examples include:a) Isomerasesc) Epimeraseb) Racemased) Mutase - shifts a group from one carbon atom to anotherVI. LIGASES catalyse the Synthesises a covalent linkage using ATP as a source of energy# Examples includea) Synthetasesb) Carboxylases