Microbes and Metabolism 41electron donors, molecules which have a more negative reduction potential than
water and so are easier to oxidise. These include hydrogen, hydrogen sulphide,
elemental sulphur and a variety of organic compounds including sugars, and
various organic acids such as amino acids, and succinate.
There are many ways in which green and purple nonsulphur bacteria may
produce NADH. For example, direct reduction is possible if they are growing
in the presence of dissolved hydrogen gas due to the fact that hydrogen has a
more negative reduction potential than NAD+. In addition, purple nonsulphur
photosynthetic bacteria may use ATP or the proton gradient established during
photosynthesis to reverse the electron flow such that the direction is from one of
the electron donors noted above to NAD+.
Green and purple sulphur bacteria are rather different in that in addition to
having a cyclic system broadly similar to that of purple bacteria, they have an
additional enzyme activity which allows the nonlinear transfer of electrons to
ferredoxin linked to NAD+resulting in the production of NADH. One of the
sources of electrons to replace those used in this reduction is the oxidation of
hydrogen sulphide to sulphate or elemental sulphur, in a process comparable to
the oxidation of water in oxygenic organisms. Other electron donors which may
be used in this way are hydrogen and elemental sulphur. Both of these nonsulphur
bacteria are strict anaerobes.
Photosystem in a halophile
A photosystem which is different again from those described so far is that found
in the halophileHalobacterium salinariumwhich has previously been classified
asHalobacterium halobium.Under normal conditions this organism obtains its
energy by respiration, but in order to survive conditions of low oxygen concentra-
tions, it can photosynthesise provided there is sufficient light. The pigment which
has been developed for this purpose is bacteriorhodopsin, which is very similar to
the rhodopsin pigment found in vertebrate eyes. The part of the molecule which
absorbs light is retinal. When this occurs, changes in the bond formation of this
chromophore result in expulsion of protons across the membrane thus producing
a proton gradient. As described for other systems, this proton gradient may then
be used to drive ATP synthesis.
The dark reactions
The result of illumination of a photosynthetic organism is to stimulate electron
transport which leads to the production of NADPH or NADH, and synthesis of
ATP. Both are required for the next stage which in eukaryotes and cyanobacteria
(blue-green algae) is the synthesis of sugar from carbon dioxide involving the
Calvin cycle. Many biochemistry textbooks give excellent descriptions of this
process and so only a summary is given in Figure 2.10.
In brief, ribulose diphosphate is carboxylated with carbon dioxide catalysed by
the enzyme rubisco to form an unstable six-carbon sugar which is then cleaved