Classification of Enzymes – Enzyme Classification

by Kevin Ahern, PhD

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    00:01 The last thing I wanna talk about here are the classification of enzymes.

    00:05 Enzymes, according to a systematic scheme that has been developed by the EC commission, The Enzyme Commission, have broken all reactions that enzymes catalyze into 6 categories.

    00:14 And this 6 category scheme is used to organize and name all enzymes that are in biology.

    00:23 The first category scheme is that of an oxidoreductase, that is an oxidation and a reduction is happening in the reaction that is catalyzed by the first category of enzymes.

    00:35 In this case you can see malate which is shown on the left that is being oxidized. It is donating its electrons to NAD to form oxaloacetate and NADH.

    00:45 So oxidoreductases will always have transfers of electrons and it will always have an electron carrier involved.

    00:54 You can see the NAD and the NADH here.

    00:57 The second category of enzymes are those called transferases.

    00:59 And transferases grab a part of one molecule and move it to another. So we can see here for example that we are starting with glucose.

    01:09 We are taking a phosphate off of ATP and we are putting it onto glucose. This enzyme hexokinase catalyzes the first reaction in glycolysis and it's a transferase.

    01:22 The next reaction involves hydrolases and as the name is suggest these enzymes use water to break bonds.

    01:29 So we can see in this schematic reaction here, a molecule on the left that has a peptide bond is actually combining with water to break that peptide bond.

    01:37 That's what happens with the serine protease for example.

    01:40 Water is being used to split a peptide bond.

    01:47 In the fourth category we have enzymes called lyases.

    01:50 And a lyase is an enzyme that uses a non-hydrolytic meaning no water, non-oxidative way of breaking bonds.

    02:00 So on the left we see for example isocitrate.

    02:03 The enzyme isocitrate lyase which is found in plants breaks this 6 carbon molecule into a 2 carbon piece called glyoxylate and 4 carbon piece called succinate; because, water is not involved and because there is no oxidation involved this reaction is a lyase.

    02:23 The 5th category of enzyme is an isomerase.

    02:26 And isomerases are enzymes that catalyze rearrangements without doing other anything else to the structure of the molecule that they are acting on.

    02:35 So in this case we see another reaction from glycolysis the enzyme converts glucose-6-phosphate, which is a sugar with a phosphate on it, to fructose-6-phosphate, which is the different sugar with the phosphate on it.

    02:46 But all that is happened has been simply the rearrangement of the molecule.

    02:50 That's an isomerase.

    02:52 The last category of reactions are those of ligases. And ligases are molecules that put things together. So instead of breaking things apart ligases are making covalent bonds to join things together.

    03:06 So in this case we are seeing ATP energy being used to join urea and bicarbonate to make urea-1-carboxylate.

    03:15 Ligases join things together.

    03:18 Well, we have seen in the reactions here that enzymes have some pretty amazing abilities in terms of flexibility for catalyzing reactions, flexibility as they affect the mechanism that they use to catalyze things and we have also learned about the different categories of enzymes that are there.

    03:34 In other lectures I will talk about the ways that enzymes become inhibited.

    About the Lecture

    The lecture Classification of Enzymes – Enzyme Classification by Kevin Ahern, PhD is from the course Enzymes and Enzyme Kinetics.

    Included Quiz Questions

    1. Ligase — methyltransferase
    2. Ligase — argininosuccinate synthetase
    3. Oxidoreductase — alcohol dehydrogenase
    4. Hydrolase — sucrase
    5. Isomerase — alanine racemase
    1. Lyases catalyze the breaking of bonds
    2. Transferases transfer energy to create high energy molecules
    3. Ligases break bonds
    4. Hydrolases use water to put together molecules

    Author of lecture Classification of Enzymes – Enzyme Classification

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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