BioPHYSICAL chemistry

bacterial reaction center (Chapter 5). Notice that hydrogen has a zero

midpoint potential since it serves as a standard, but only under standard

conditions. At pH 7, the midpoint potential is decreased to 0.42 V due to

the 0.059 decrease per pH unit expected for reactions coupled to proton

transfer. Ferredoxin is a small protein that contains an iron–sulfur cluster

that becomes oxidized or reduced during different metabolic processes. In

some cases, enzymes that catalyze oxidation/reduction reactions transfer

electrons into universal electron carriers. Several compounds have low mid-

point potentials and so serve as good electron carriers, including flavin

mononucleotide (FMN), flavin adenine dinucleotide (FAD), and glutathione.

In some cases the electron carrier readily moves between enzymes, as found

for NAD+and NADP+, whereas in other cases the cofactor is tightly bound,

as usually found for FMN and FAD. As Table 6.1 indicates, many but not

all proteins that participate in oxidation/reduction reactions contain metals

that serve as the electron donor or acceptor.

In many biological reactions, the oxidation/reduction reaction involves

the transfer of two electrons and two protons. Such reactions are termed

dehydrogenations and the enzymes that catalyze them are called dehydro-

genases. For example, the conversion of lactate to pyruvate involves the

removal of two protons from the ketone group at the second carbon

position in addition to the removal of two electrons (Figure 6.2). The net

transfer of two protons with two electrons is common but not required.

For example, the oxidation of NAD involves the release of two protons

in the dehydrogenation reaction (Figure 6.3). One of these protons is

released into the aqueous solution but the oxidized form of the molecule

accepts a hydride ion, yielding a net release of one proton.

The values reported in Table 6.1 have been determined experimentally by

one of two means. One approach is to poise the ambient potential at a series

of values with the use of chemical reductants and oxidants (Figure 6.4).

Alternatively, the potential can be established using an electrochemical

cell. For each potential, the oxidation state of a particular cofactor must

then be measured by spectroscopic means, such as by monitoring the

changes in an optical absorption spectrum. From the spectra, the fraction

reduced at each potential is fitted using the Nernst equation (eqn 6.8)

and the midpoint potential is determined. Since the cofactors in proteins

are usually buried inside the protein, special mediator compounds for these

measurements may be used to facilitate the transfer of electrons between

the electrodes and the cofactor.

In addition to factors such the pH and ionic strength of the solution

surrounding the protein, the midpoint

potential of a cofactor in a protein can

differ by up to 0.5 V compared to the

value in solution due to cofactor–protein

interactions. The most critical factor is

the ligation of the cofactor that will

preferentially stabilize the reduced or

118 PARTI THERMODYNAMICS AND KINETICS

O

O

CH C

OH

CH 3

O

O

C

2H  2e

2H  2e

Lactate

dehydrogenase

Lactate Pyruvate

C

O

CH 3

Figure 6.2
The oxidation of
lactate to pyruvate
requires the loss of
two protons.