James Richard Fromm
A living organism requires energy simply to survive as an organized structure, let alone to perform any useful functions. In this section we explore the energy usage of organisms in a particular area of chemistry, biochemical electrochemistry.
Electrochemistry plays a significant role in biological reactions. The number and variety of electrochemical reactions in biological systems is large but the available potential range is limited, as shown in the following Table.
|Electrode Couple||E0 (V)||dE/dT (mV/K)|
|2e- + 2H+ H2(g)||-0.4141||0.000|
|2e- + H+ + NAD+ NADH||-0.320||---|
|2e- + 2H+ + HOOCCOCH3 HOOCCHOHCH3||+0.19||---|
|e- + Fe3+ Fe2+||+0.769||-0.846|
|4e- + 4H+ + O2(g) 2H2O||+0.8147||+1.170|
Note to Table: HOOCCOCH3 is pyruvic acid; HOOCCHOHCH3 is l-lactic acid.
The strongest oxidizing agent generally available in biological systems is molecular oxygen. Consider the oxidation of reduced pyridine nucleotide NADH (also called reduced nicotinamide-adenine-dinucleotide) by molecular oxygen. This reaction is 2NADH + 2H+ + O2 2NAD+ + 2H2O. The half-reactions can be written as:
NADH NAD+ + H+ + 2e-; E0' = -0.320 V
4e- + 4H+ + O2 2H2O; E0' = +0.8147 V The driving force (potential difference) DE0'is 1.136 V; the electron flow would have to be from the most negative system, which is the couple NAD+/NADH, to the least negative system which is the couple O2/H2O. This requires that the NADH/NAD+ couple be oxidized. In other words, the reaction will go as written, while the O2/H2O couple is reduced, as written. For this spontaneous direction, then, DG0 = -zFE0' = -2F(1.136 V) = -2.272FJ, or -219.2 kJ/mole NADH formed. Thus the oxidation of the reduced form, NADH, by molecular oxygen is a spontaneous process. It is nevertheless a slow reaction, unless enzymes are present to catalyze the reaction.
The reactions of the NAD+/NADH system are of great biochemical importance. An example of this is the oxidation of ethanol to ethanal (acetaldehyde) by means of the yeast enzyme alcohol dehydrogenase. The overall reaction is:
CH3CH2OH + NAD+ CH3CHO + NADH + H+.
The oxidation half-reaction is:
CH3CH2OH CH3CHO + 2H+ + 2e-
and the reduction half-reaction is:
NAD+ + H+ + 2e- NADH.
The equilibrium constant for the reaction is K = [CH3CHO][NADH][H+]/[CH3CH2OH][NAD+] which has the value of about 10-4 at pH = 7.0. This is linked to the production of energy-rich phosphates (bodily energy sources).
The structure of NAD+, is an abbreviation for nicotinamide-adenine-dinucleotide.
The electrochemical reaction is at the nicotinamide end of the molecule. The R group notation stands for the rest of the molecule.
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