Are All Metabolic Reactions at Equilibrium? (a)...

Are All Metabolic Reactions at Equilibrium? (a) Phosphoenolpyruvate (PEP) is one of the two phosphoryl group donors in the synthesis of ATP during glycolysis. In human erythrocytes, the steady-state concentration of ATP is $2.24 \mathrm{mM}$, that of ADP is $0.25 \mathrm{mM}$, and that of pyruvate is $0.051 \mathrm{mM}$. Calculate the concentration of PEP at $25^{\circ} \mathrm{C}$, assuming that the pyruvate kinase reaction (see Fig. 13 - 13) is at equilibrium in the cell. (b) The physiological concentration of PEP in human erythrocytes is $0.023 \mathrm{mM}$ Compare this with the value obtained in (a). Explain the significance of this difference.

Are All Metabolic Reactions at Equilibrium?
(a) Phosphoenolpyruvate (PEP) is one of the two phosphoryl group donors in the synthesis of ATP during glycolysis. In human erythrocytes, the steady-state concentration of ATP is $2.24 \mathrm{mM}$, that of ADP is $0.25 \mathrm{mM}$, and that of pyruvate is $0.051 \mathrm{mM}$. Calculate the concentration of PEP at $25^{\circ} \mathrm{C}$, assuming that the pyruvate kinase reaction (see Fig. 13 -
13) is at equilibrium in the cell.
(b) The physiological concentration of PEP in human erythrocytes is $0.023 \mathrm{mM}$ Compare this with the value obtained in (a). Explain the significance of this difference.

Key Concepts

Chemical Equilibrium in Metabolic Reactions

This concept refers to the state in which the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products as dictated by the equilibrium constant. In metabolic pathways under equilibrium conditions, the ratio of substrate and product concentrations is set by the mass action law, which depends solely on the reaction’s thermodynamics.

Steady-State Concentration

In living cells, metabolites are maintained at steady-state concentrations, a condition where the levels remain constant over time despite continuous flux through the pathway. This does not necessarily indicate that each reaction is at chemical equilibrium, as cells control metabolite levels via regulated enzyme activities and substrate fluxes, often resulting in some reactions being far from their equilibrium conditions.

Mass Action Law and Equilibrium Constants

This principle forms the basis for calculating the concentrations of reactants and products at equilibrium. It states that at equilibrium, the ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficients, equals a constant specific to the reaction under given temperature and pressure conditions.

Regulation of Metabolic Flux

Metabolic pathways are often regulated by enzymes that operate far from equilibrium to allow rapid and flexible control of flux through the pathway. This regulatory mechanism is crucial because it permits distinct control points within the cell, ensuring that substrates are efficiently allocated between competing pathways, even if this means that not all steps reflect equilibrium conditions.

Transcript

00:02 Pep, which is fossil in all pyruvate, and it's one of the major phosphoryl group donors to atp during glycolysis.

00:13 It's asking about with given the certain amount of molarity for atp, atp, the pyruvate.

00:24 What would be the calculation for the concentration of the pep? and this is in sort of like driving the question is, are all reactions at equilibrium? and this is especially as we're looking at metabolic reactions within the cell.

00:43 So the first thing that we need to do is look at pep, which is an intermediate factor involved in glycolysis.

00:49 And it aids the adp and to make an atp.

00:51 The first thing is writing out the equation.

00:54 We've got pep plus h2o yields pyruvate plus a phosphate, the delta g, the amount of energy is negative 61 .9 kilojoules per mole.

01:04 And then the second reaction that's happening, of course, the whole point is that the phosphate is being donated and given to adp.

01:13 So adp plus the phosphate yields atp plus h2o, and the delta g in that situation is 30 .5 kilojoules per mole.

01:21 When we do a summation between the two reactions we have and we cross out the ones that are in common, we're going to have pep plus adp yields parruvate plus.

01:31 Atp and yielding as a summary negative 31 .4 kilojoules per mole.

01:37 Now assuming that we have 25 degrees celsius we're told to calculate the concentration of pep.

01:45 In order to calculate the concentration of pep first, sorry, calculate the k sub eq, in other words the equilibrium value.

01:56 So the equation that we're going to use is delta g prime equals negative rt, natural log case vq.

02:04 Then moving the things around so that we can put our knowns on one side and put our unknowns on the other side, just like basic algebra.

02:11 We're going to have delta g on the left side, divided by rt, because rt is multiplying to the natural log.

02:20 So if we divide both sides by that, we can bring it over to where the delta g is located.

02:25 So what we end up having then is a negative delta g divided by rt is equal to the natural log of, of, you know, k -e -q.

02:35 And then once we do that, then we just simply plug and play to put the numbers in terms of the various concentrations provided that that's in the problem as well as the one that we figured for the summation of the reductions.

02:46 So the top number is the delta g.

02:48 What's the negative of the delta g...

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