Ligases: Enzyme class 6

Hi this Dr. Vijay. In this presentation we’re going to learn
about ligases, the enzyme class 6. Prior to 1984 these ligases are also known
as synthetases. So, these enzymes, what is the meaning of
ligation? Ligation that means joining of two molecules. Suppose this is a molecule 1 and this is a
molecule 2, another molecule, then there will be formation of ligation. So, this is catalyzed by enzymes ligases. So, ligases catalyze joining of two molecules
and always this joining process coupled with breakdown or cleavage or hydrolysis of ATP,
not only ATP, in some reaction’s other similar cofactors. So, we can summarize reaction catalyzed by
ligases like this. Suppose a molecule A combines with molecule
B and this coupling reaction give a product A-B and this reaction coupled with hydrolysis
of ATP. ATP converted to ADP and inorganic phosphate,
and another hallmark of this particular enzyme catalyzed reaction, there will be, invariably
formation of new bonds. So, depending upon the new bond formation,
they are subclassified that is 2nd digits of enzyme nomenclature. These new bonds could be C-O or C-S or C-S
or C-C or even C-P. So, depending upon these new bond formations,
the 2nd digit will be determined. So, now this will become EC 6.1. So, this will become EC 6.2, like this 6.3,
6.4, 6.5 like this another2 more subclassifications are there. Further the 3rd digit in this enzyme nomenclature,
IUB system of enzyme nomenclature determines or describes the new bond formation. So, if the C-N, the new bond formation is
amide bond, then it will be 6.3.1 series. If it is a peptide then it will become 6.3.2
series. So, like this. So, this is the subclassification of ligases. So, we will see specific examples. So, very good example is glutamate synthetase,
don’t confuse for glutamate synthase, so, synthase and synthetase are different. This particular enzyme catalyzes conversion
of glutamic acid or glutamate, so we know that there will be always 2 molecules in the
enzyme catalyzed reaction by ligases. You need to remember there will be always
2 molecules, so the second molecule will be ammonia. Now this glutamate and ammonia combines, joined
or ligated to form glutamine. We know that this particular reaction coupled
with hydrolysis of ATP. So, ATP undergo cleavage to give ADP plus
inorganic phosphate. The EC number of, this is the common name,
glutamate synthetase, it is EC class 6, subclass 3, sub-sub class 1 and it is the second enzyme
in this EC 6.3.1 series. The systematic name for this glutamate synthetase
glutamate: ammonia ligase. So, we can give many many examples for ligases,
like pyruvate carboxylase, acetyl-CoA carboxylase, as the name suggests second molecule would
be here carbon dioxide or bicarbonate, another example is in DNA replication that is DNA
ligase, joining Okazaki fragments. So, pyruvate carboxylase, the EC number is
EC of course they belong to class 6, Acetyl-CoA carboxylase is EC DNA ligase the EC number is So, if the cofactor is different in DNA ligase,
again the EC number will change, if it is ATP it is EC 6.1.1 and the new bond formed
in this reaction, the enzyme catalyzed by pyruvate carboxylase is C-C and even in Acetyl-CoA
carboxylase it is C-C because they have same 2nd digit number and in DNA ligase the 2nd
digit number is 5, so the new bond formed is carbon-phosphoric acid. So, this particular enzyme pyruvate carboxylase
is an enzyme for conversion of pyruvate to oxaloacetate and the second substrate would
be bicarbonate or carbon dioxide. Similarly, acetyl-CoA carboxylase, this enzyme
helps joining acetyl-CoA and bicarbonate to form malonyl-CoA. This is with respect to the class 6 enzymes
ligases. Remember, ligate, to join. So, these enzymes catalyze reactions helps
in joining of two molecules coupled with cleavage of ATP or similar cofactors and while doing
so there will be invariably formation of new bonds like carbon-oxygen, carbon-carbon, carbon-sulfur,
carbon-nitrogen or carbon-phosphoric acid. Thanks for watching.

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