Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

6 Enzymatic Redox Chemistry
6.1 Introduction
Oxidation and reduction involves the transfer of electrons between one
chemical species and another. Electron transfer processes are widespread in
biological systems and underpin the production of biochemical energy in all
cells. The ultimate source of energy for all life on Earth is sunlight, which is
utilised by plants for the process of photosynthesis. Photosynthesis involves a
series of high-energy electron transfer processes, converting sunlight energy into
high-energy reducing equivalents, which are then used to drive biochemical
processes. These biochemical processes lead ultimately to the Wxation of carbon
dioxide and the production of oxygen. Oxygen in turn serves a vital role for
mammalian cellular metabolism as an electron acceptor.
Cells contain a number of intermediate electron carriers which serve as
cofactors for enzymatic redox processes. In this chapter we shall examine
some of these redox cofactors, and we shall also examine metallo-enzymes
which utilise redox-active metal ions to harness the oxidising power of molecular
oxygen.
First of all we need to deWne a scale to measure the eVectiveness of these
diVerent electron carriers as oxidising or reducing agents. How do we measure
quantitatively whether something is a strong or weak oxidising/reducing agent
We can measure the strength of an oxidising agent electrochemically by dissolving
it in water and measuring the voltage required to reduce it (i.e. add
electrons) to a stable reduced form. The voltage measured under standard
conditions with respect to a reference half-cell is known as the redox potential.
The reference half-cell is the reaction 2H þ þ 2e ! H 2 , whose redox potential
is 0.42 V at pH 7.0. A strong oxidising agent will be reduced very readily,
corresponding to a strongly positive redox potential; whilst a weak oxidising
agent will be reduced much less readily, corresponding to a less positive or a
negative redox potential. A scale showing the redox potential of some important
biological electron carriers is shown in Figure 6.1. One obvious point is that
the strongest available oxidising agent is oxygen, hence the large number of
enzymes which use molecular oxygen either as a substrate (oxygenases) or as an
electron acceptor (oxidases).
We can use redox potentials to work out whether a particular redox reaction
will be thermodynamically favourable. For example, the enzyme lactate dehydrogenase
catalyses the reduction of pyruvate to lactate, using nicotinamide
adenine dinucleotide (NADH) as a redox cofactor. NAD þ (the oxidised form
of NADH) has a redox potential of 0.32 V, whereas pyruvate has a redox
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