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Metabolism – Oxidation and Reduction in Metabolism

by Kevin Ahern, PhD
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    00:00 Now let's take this idea of the equation and put into practice okay. So let's imagine that we have a couple of reactions, we have a reaction, A going to B, and we have a reaction B going to C. Now this is not just an imaginary scenario, but in fact it's exactly what cells have to do. Cells have within them thousands of reactions that are going on at any given time and those reactions are moving by a connected path, that is for example, if I were following the oxidation of glucose, I would see how glucose goes and becomes glucose-6-phosphate in the first step of the oxidation of glucose. In the next step, glucose-6-phosphate is converted into fructose-6-phosphate. Well these are A, B and C. A is glucose, B is glucose-6-phosphate, but then glucose-6-phosphate which is produced in the first reaction is now the substrate for the second reaction, because glucose-6-phosphate becomes fructose-6-phosphate. Metabolic pathways work like this in an interconnected fashion. So we have our scenario A going to B and B going to C. Now what happens for example if I were to take the reaction B going to C and I increase the quantity of C, what will happen to the delta G equation for that. Well if I make the concentration of C increased, let's say greater than B, then what that means is that the log term itself is going to increase. And as we said before, when the log term for a reaction increases, then the value of delta G will increase. Remember that I said that if the delta G for a reaction is positive, the reaction will move backwards. So if we make this reaction positive enough, the reaction B going to C will start going backwards and producing B. Well that wont happen in isolation because B becomes affected in the first equation, so now an excess of B is going to affect the reaction A going to B. So we can see how the production of C is actually favoring the movement backwards all the way back to A.

    02:13 Now what I've illustrated for you with this principle is that these processes are interconnected with each other. Alright. So the interconnectedness of processes means that the delta Gs themselves are also linked, right. So as I said, if I move the other way, let's say I decided to increase the A concentration, what's going to happen? Well we could turn this around, say if we make more A, then a greater amount of substrate that is A, the starting material, is going to cause the log term to decrease, mean we're going to favor production of B.

    02:48 If B's concentration increases, then that means that B is now the substrate for the following reaction going to C that means that C will be favored as well. Now I've just described a process to you of increasing A and pushing a reaction all the way down. That pushing the reaction is caused by increasing the amount of reactant. So if I increase the amount of reactant, I push a reaction. If I decreased the C, what's going to happen? Well as I decrease the C, that means the B to C reaction is going to move in the direction of C. If I move the reaction in the direction of C that's goings to be less B, which means that A is going to start moving to the right and making more B. What I've done in this example is what I call 'pulling a reaction'. So pushing reaction refers to increasing the amount of reactant and pulling a reaction refers to removing a certain amount of product.

    03:53 So the equation at the top, the delta G equation, relates to the pushing and pulling that I've talked about. Remember that pulling means removing product and pushing means increasing reactant, that means B would get smaller and A would get larger. We remember that means that the log term will get more negative, meaning the delta G will decrease and as delta G decreases and gets more negative, the forward reaction is favored. So pulling and pushing a reaction will favor a reaction moving in the forwards direction.

    04:24 Now let's consider another problem. Let's take a problem where we have a reaction that has a known value for ΔG0' and then see what that value actually tells us about the reaction.

    04:36 In this case I'd like to consider a situation where ΔG0', it has a negative value, we know that the start with. So the ΔG0' is negative. And let's also assume that we take the reaction A going to B and we start with equal amounts of A and B. Now then I say, let's let that reaction go and get to the place where it finds equilibrium. So we start with equal amounts, but those are going to react according to the energy needs of the system and they're going to come to some final concentration of A and B at equilibrium and the question I'm asking is, what is the concentration of them at equilibrium? Well let's set up the equation. We know that at equilibrium delta G is equal to zero, and I've said that the ΔG0' is a negative number, so that means that zero is equal to a negative number, plus RT times the natural log of the B over A. If I take the negative number to the other side of the equation that means a positive number is equal to RT times the natural log of B over A. Now why is that significant? The significance is, that means for that log term to be positive, there must be more B than A, that's one of the properties of logarithms, there must be more B than A. Well if there's more B than A and we started with equal amounts of B and A, what does that mean? It means that the reaction started with equal amounts and it moved forwards, it means that the reaction that had a negative ΔG0' was favorable moving forwards when we had equal concentrations. Now people commonly say what that means is that when this happens, that the reaction is favorable, a ΔG0' means a reaction is favorable. Now we can't predict the exact direction of a reaction without knowing all the concentrations of A and B, but the tendency for a reaction that has a negative value of ΔG0' is favorable. Now we also can say the opposite, that if the ΔG0' for a reaction is positive, we can do the same analysis and what we would find is that the equilibrium concentrations would be greater amount of A than B. This would mean that the reaction was unfavorable, because if we started with equal amounts and we ended up with more A than B that meant the reaction had to go backwards.


    About the Lecture

    The lecture Metabolism – Oxidation and Reduction in Metabolism by Kevin Ahern, PhD is from the course Biochemistry: Basics.


    Included Quiz Questions

    1. It does not affect the standard Gibbs free energy change
    2. It involves removal of products
    3. It causes the change in the standard Gibbs free energy to decrease
    4. It causes the change in Gibbs free energy to increase
    1. It tells us that whether a reaction is favorable or not, e.g., ΔG = negative means the reaction will move in a forward direction, whereas the positive value indicates that the reaction is unfavorable
    2. It tells us about the changes in the concentrations of the reactant molecules during a reaction at a particular temperature
    3. It tells us about the changes in the concentrations of the product molecules during a reaction at a particular temperature
    4. It tells us about the changes in the gas pressure during a reaction at a particular temperature
    5. It tells us the changes in the concentrations of the protons during a reaction at a particular temperature
    1. The reaction is unfavorable
    2. The reaction is favorable
    3. The reaction is in equilibrium
    4. The change in ΔG0’ value is positive
    5. The change in temperature and pressure are needed
    1. The pushing-pulling mechanism
    2. Newton's law of gravity
    3. Quantum mechanics
    4. The theory of gravity
    5. The push-turn mechanism

    Author of lecture Metabolism – Oxidation and Reduction in Metabolism

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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    By Schola A. on 04. October 2017 for Metabolism – Oxidation and Reduction in Metabolism

    Professor Dr. Kevin Ahern is very clear in presenting. Love the short but very informative lectures. Would recommend to everyone, those with or without biochemistry understanding. Thank you very much Professor and Thanks to the maker of Lecturio.