Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431

Biochemistry

Fall 2008

Lecture Notes: 3 November

© R. Paselk 2008
 
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Thermodynamics in Metabolism

Remember also that for chemists and biologists the thermodynamic term generally of most interest is the Free Energy for a reaction, that is the energy available to do work.

Since free energy depends on conditions, chemists tabulate free energies under Standard Conditions, (DeltaG°): 298 K, 1 atm., with all concentrations at 1 M.

For biological systems we define a slightly different standard free energy with [H+]= 10-7 M (pH=7), G° '.

For non-standard conditions we can find the free energy of a reaction using:

DeltaG = DeltaG° ' + RT lnQ.

For the special case of equilibrium, the free energy is zero, so

DeltaG° ' = -RT lnK',

DeltaG° ' = -5700 log K (in joules).

Thus free energy is related to the equilibrium constant, K. To provide a quantitative feeling for this relationship some values are tabulated below:

 K log K DeltaG°' (calories) DeltaG°' (joules)
10-3 -3 4,089 17,100
10-1 -1 1,363 5,700
1 0 0 0
103 3 -4,089 -17,100

For non-equilibrium situations we can find the energy available for work using DeltaG = DeltaG° ' + RT lnQ, where Q is the mass action expression, Q = ([C][D])/([A][B]) for the reaction A + B equilibrium arrows C + D.

One advantage of using free energy is that it is easier to evaluate the overall equilibrium/energy for a series of sequential reactions (its additive instead of multiplicative): DeltaGtot = Sum[DeltaG]. Often use to predict feasibility of pathways, possible energy yields, and to determine when individual reactions are not at equilibrium (important for determining potential control steps etc.).

Note that the overall free energy determines spontaneity of the reaction - the pathway doesn't matter! As noted above thermodynamics is pathway independent. Thus can drive unfavorable reactions by linking with favorable reactions. This can be done:

    1. Sequentially: ,
      1. A equilibrium arrows B, DeltaG = +
      2. B equilibrium arrows C, DeltaG = - -
        where the reaction of B to C, with a larger absolute value of free energy change, pulls A to B; or
    2. In parallel: ,
      1. A equilibrium arrows B, DeltaG = +
      2. M equilibrium arrows N, DeltaG = - -
        where the two reactions are linked, for example by a common enzyme, and the reaction of M to N, with a larger absolute value of free energy change, pulls A to B.

Example: glucose + phosphate to G-6-P (DeltaG= +3300 cal) and ATP + water to ADP + Pi (DeltaG= -7600 cal); mix together, no G-6-P (DeltaG= -4300 cal). But link with enzyme, Glu + ATP G-6-P + ADP (DeltaG= -4300 cal). All of metabolism depends on such coupled reactions. In essence catabolic reactions drive anabolic reactions etc. via direct, and more commonly, indirect, multi-step, coupling.

Again, metabolism would be extremely complex if coupled processes directly. Instead use ATP. Thus catabolic processes make ATP which can then be used for anabolic processes, locomotion, pumping ions across cell membranes (major contribution to basal metabolic rate or BMR), etc. Note that ATP is not used to store energy however. (Often compared to electricity's role in our culture).

Carbohydrate Metabolism

Glycolysis

Glycolysis is our first pathway, and it is arguably the most important and universal of the metabolic pathways. Thus we will spend extra time on it, exploring it in some detail from a variety of perspectives. But before we begin glycolysis let's take a brief look at how glucose (and carbohydrate generally) gets to the tissue from food intake.

Digestion

Glycolysis

First let's look at Glycolysis to get an overview, then we will look at the reactions and enzymes of this pathway individually. We will then come back and look at the overall regulation and control of this pathway. If we look at the Glycolysis Pathway (overhead), we can break it into three phases:


Pathway Diagrams

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Lecture Notes

Last modified 4 November 2008