Covalent Modification – Metabolism and Regulation

by Kevin Ahern, PhD

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    00:00 Covalent modification can be used to control enzymes and the example I wanna talk about here is the control of the enzymes involved in glycogen metabolism.

    00:09 Glycogen can be made and broken down at the same place in a cell.

    00:14 So, it's very important that cells only be making glycogen or only be breaking down glycogen at any given time and covalent modification of the enzymes provides a way for that to happen.

    00:26 We start this process by looking in this slide, at protein kinase A.

    00:31 Protein kinase A is an enzyme that phosphorylates other proteins.

    00:35 When it's active, as shown in the red form, it puts phosphates on the things, and as you can see here, one of the things that protein kinase A puts a phosphate onto is an enzyme known as phosphorylase kinase.

    00:48 That converts it from the inactive or unphosphorylated form on the blue side to the phosphorylated and active form on the red side.

    00:56 Phosphorylase kinase can, in turn, phosphorylate another important enzyme in the process known as glycogen phosphorylase B.

    01:04 Now, glycogen phosphorylase B is a relatively inactive form of the enzyme.

    01:08 Again, it contains no phosphate and the phosphorylation of it produces the form known as glycogen phosphorylase A.

    01:15 This is the more active form of the enzyme and as you can see in the slide, it catalyzes the breakdown of glycogen to produce glucose 1-phosphate, which can readily be used to make glucose that's important for cellular energy.

    01:30 Now, the reciprocal regulation or the control of this process is accomplished by virtual effect that the enzymes that make glycogen are inactivated by the phosphorylation that activates the enzymes that break down glycogen.

    01:46 You can actually see it in this slide because protein kinase A, in addition to phosphorylating phosphorylase kinase, will also phosphorylate glycogen synthase.

    01:56 Now, glycogen synthase is the enzyme that's necessary for making glycogen.

    02:01 The active form of glycogen synthase is dephosphorylated. It contains no phosphate.

    02:08 When protein kinase A is activated to break down or stimulate the breakdown of glycogen, it also phosphorylates glycogen synthase, causing it to become inactive and thus, stopping the synthesis of glycogen at the same time as it's stimulating the breakdown of glycogen.

    02:28 This phenomenon is known as reciprocal regulation and it's a very important principle in understanding the control of metabolic pathways.

    02:38 With reciprocal regulation, the same action is having opposite effects on catabolic and anabolic process.

    02:47 The catabolic process here being glycogen breakdown, the anabolic process being glycogen synthesis.

    02:52 Phosphorylation of the respective enzymes is having opposite effects on those two processes.

    About the Lecture

    The lecture Covalent Modification – Metabolism and Regulation by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. It has opposite effects on catabolic and anabolic pathways.
    2. It is the same as feedback inhibition.
    3. It always involves ATP, ADP, or AMP.
    4. It reverses the direction of all enzymatic reactions in a pathway.
    5. It up-regulates the production of the enzymes on the pathway that is being inhibited.
    1. Glycogen synthase
    2. Phosphorylase kinase
    3. Glycogen phosphorylase
    4. Glycogen branching enzyme
    5. Mutase

    Author of lecture Covalent Modification – Metabolism and Regulation

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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