Futile Cycle

by Kevin Ahern, PhD

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    00:01 Now one of the considerations that we have with metabolic pathway is that metabolic pathways that breakdown and make things, if they are occurring at the same time, it can cause problems for cells and we refer those problems as a futile cycle.

    00:15 We see on the screen the schematic pathway for glycolysis and gluconeogenesis.

    00:19 Now these two pathways overlap a lot and that one pathway is partly the reverse of the other pathway.

    00:27 We see the glycolysis direction moving down, as you can see here, and the gluconeogenesis pathway starts at the bottom and moves up.

    00:35 And with the exception of the oxaloacetate, shown in the bottom right of the gluconeogenesis pathway, there is really no difference in the molecules that are present in either pathway.

    00:45 The pathway on the left, glycolysis generates ATP energy.

    00:50 It also, as a result of oxidation, generates 2 reduced electron carriers known as NADH.

    00:56 Gluconeogenesis by contrast to start with pyruvate and move upwards, requires 4 ATPs, 2 GTPs and the 2 NADHs; because, a lot of the reactions of glycolysis and gluconeogenesis occurring at the same place, there is concerned that if they are both happening at the same time. If that happens then the products of gluconeogenesis are made and then burned in glycolysis in the pathway where the glycolysis intermediates are made are burned and feed into gluconeogenesis.

    01:28 When this happens we have what's referred to as a futile cycle.

    01:30 The cell is making things and breaking things down at the same time without any productive output of energy. In fact, this is energy consuming.

    01:40 So the futile cycle is something for a cell to avoid.

    01:43 The consequences of running a futile cycle are shown in this slide.

    01:47 The output of glycolysis is 2 pyruvates, 2 ATPs, and 2 NADHs plus some other molecules.

    01:55 The outputs of gluconeogenesis are glucose plus 2 NADs plus 4 ADPs and 2 GDPs and 6 phosphates.

    02:03 You notice that the output of glycolysis becomes the input of gluconeogenesis.

    02:06 But similarly the output of gluconeogenesis can become the input of glycolysis.

    02:12 Well this would be fine and dandy except for the fact that when these two are running together the gluconeogenesis pathway is taking more triphosphate energy than the glycolysis pathway is generating.

    02:24 So this futile cycle if it runs goes in the circle and the output of that circle is simply heat and loss of triphosphate energy. This is something for a cell to avoid.

    About the Lecture

    The lecture Futile Cycle by Kevin Ahern, PhD is from the course Metabolic Control.

    Included Quiz Questions

    1. They generate energy
    2. They include gluconeogenesis and glycogen breakdown
    3. They require NADH, FADH2, or NADPH
    4. They make big molecules from smaller ones
    1. It occurs when a catabolic and corresponding anabolic pathway are occurring simultaneously in a cell
    2. It is the primary source of ATP energy
    3. It is favored by reciprocal regulation
    4. It is common in cells

    Author of lecture Futile Cycle

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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