In this context, it is interesting to note that it has been pointed
out that enzyme catalysis driven by light represents photosyn-
thesis in its simplest biological form, and that photoenzymes
probably played an important role in the early biotic environ-
ment on earth ( 39 ). Today, the only metalloenzyme directly
involved in photochemical processes under natural conditions is
the visible light-driven water–plastoquinone oxidoreductase, bet-
ter known as photosystem II (PS II) of oxygenic photosynthetic
organisms ( 40 ). However, the functional properties of many other
metal sites in bioinorganic systems are also known to be
modified by irradiation. Some of these examples are given below
(Table II).
The reasons for light sensitivity in these enzymes are not
always well understood, and several other examples will proba-
bly be discovered when the nature of the light-absorbing species
is clarified. It therefore seems appropriate to provide a brief over-
view on the basic types of inorganic chromophores occurring in
biological metal sites.
A.2. Bioinorganic chromophores
Excitation of proteins, nucleic acids, or organic cofactors acting
as ligands in biological systems frequently involves individual
functional groups or isolated conjugated p-systems. When a
metal is coordinated to such moieties, the resulting molecular
orbitals are usually classified as predominately located at the
FIG. 5. Electronic spectra of a mixture of ferric cytochrome cand
enhanced green fluorescent protein EGFP before (—) and after (----)
490 nm photolysis indicating the formation of the ferrous form of hemec
( 6 ) with maxima at 413, 520, and 550 nm. Adapted from Ref. ( 37 ).
PHOTOSENSITIZATION AND PHOTOCATALYSIS 243