The route th at an electron takes in the reaction centre will depend on the resistance to its passage. In the Q-type reaction centres there are two possible routes of electron transfer due to the symmetry of the structure (see sections 1.5.3 and 1.5.4). The electrons however, m ust have some way of moving between the cofactors. In biological systems it is believed th a t there is a preferred hierarchy of bond types for electron transport.
resistant route, followed by through hydrogen bonds, via orbital overlap and van der Waals contact and finally jumping across space being the least preferred route. Where there are more hydrogen honds connecting the pigments to the protein there is more scope for the route of least resistance heing on th a t branch.
There appears to he a considerable disparity of hydrogen bonds between the pigments (chlorophylls and pheophytins) and the protein on the two branches of possible electron transfer. Table 3.3 shows the
possible hydrogen bond interactions of the two branches in the PS II model and the Rps. viridis structure. It is clear th a t there are more possible hydrogen bonds on the pigments associated with the branch th a t is active in electron transport.
T ab le 3.3 The number of hydrogen bonds possible between the peripheral groups on the chlorin macromolecular rings and the local protein in the PS II model and the Rps. viridis.
Number of possible hydrogen honds
Pigment PS II Rps. viridis
A branch B branch L branch (active) M branch
Chi 3 1 5 1
Acc.Chl 0 0 0 0
All of the groups capable of forming hydrogen bonds on the
tetrapyrrole pigments are either carbonyl or ether oxygen atoms. These groups have no protons of their own and therefore this means th a t the pigment will take part in the bonding by acting as a proton acceptor. The withdrawal of a proton from a surrounding group makes the pigment moiety slightly positive. The cumulative effect of several hydrogen bonds on a pigment may allow a significant build up of positive charge. This charge on an electron acceptor may assist in generating a greater potential difference between an electron donor and an electron acceptor. If the alternative pathway has a smaller potential difference then th a t electron pathway is less likely to be favoured. This observation may be p art of the reason for the unidirectional flow of electrons within reaction centres. With a slightly greater potential difference between one pheophytin and the primary donor th at route of electron donation will be favoured.
It is interesting to note th at the D l protein is the subunit th a t has the most pheophytin/protein interactions like the L protein in the bacterial structure, further reinforcing analogy between the eubacterial and
oxygenic Q-type reaction centres.
Within the special pair there is also a similar trend. The pigment associated with the active branch has the possibility of forming more
hydrogen bonds to the protein environment than the second pigment in the dimer. This disparity may assist in the symmetry break by causing a slight polarity across the dimer thereby attracting more of the delocalized electrons towards the pigment associated with the active branch. If this polarity is formed it will also locate the electron donor pigment closer to
the acceptor with the bigger potential difference again leading to a unidirectional route for electron transfer.
Besides their possible role in changing the distribution of electrons around the pigments, the hydrogen bonds will contribute to the structural stability of pigment binding. The orientation of and distance between cofactors involved in electron transport are im portant (Moser et al. 1992). Electron transfer from donor to acceptor is likely to occur when the two cofactors are in the most favourable orientation. The pigments on the D1 branch have more possible hydrogen bonds with the protein th an those on the D2 branch (see table 3.3) so it is likely th a t these pigments will be more stably bound to the reaction centre than their D2 counterparts. This stability of binding may reduce the movement of the cofactors within the dynamic, functioning structure. It is therefore probable th a t stably bound cofactors will be in the correct orientation for electron transfer more often than pigments th a t are less stably bound. This increase in stability of cofactor binding could be another contributory factor for the dominance of one route of electron transport.
This proposal of a reduction of the vibration in the pigments leading to an increase in electron transport rate suggests th a t the pigments are arranged to give their maximal rate of electron transport in vivo.
Reducing the reaction centres to cryogenic tem peratures and therefore reducing vibration would not significantly affect the speed of electron transfer.
An alternative explanation of factors involved in determining rates of electron transport has been proposed by Moser et al. (1992). They suggest
th a t the rates of electron transport and route of electron transfer in the reaction centre of the eubacteria are dominated by several factors
including the delta G°. An increase in the number of hydrogen bonds between the cofactor and the protein will decrease the Gibbs free energy and increase the rate of electron donation.
The observation th a t there are more possible hydrogen bonds on the active branch can be interpreted in several different ways but it certainly appears to be significant.
A further proposed difference between the eubacterial and higher plant systems is the rate of initial charge separation. The rate constant for the primary charge separation between the special pair and the first acceptor (bacteriopheophytin) in purple non-sulphur bacteria is about 3 ps (Martin et al. 1986; Breton et al. 1986 and Holzapfel et al. 1990). There is considerable debate as to the precise time constant for this reaction in higher plants. A reaction time of about 3 ps has been proposed (Schatz et al. 1988 and Wasielewski et al. 1989) but an alternative of 21 ps has been
suggested by D urrant et al. (1992).
It is interesting to note th at in the eubacterial structure there is a tyrosine (Tyr210 in M) between the centre of the dimer and the acceptor. In the homologous position in the model is a leucine (Leu206). A site directed m utant in Rh. sphaeroides has been reported for the change Tyr2 1 0 to Leu, making it analogous to the situation in PS II. In this
modified bacterium the rate of primary charge separation is about 2 0 ps
(Weiss, 1993), close to the 21 ps value proposed in PS II. However it m ust be noted th a t the genetic modification may have an effect on the bacterial
system which modifies the electron transfer rate by some novel mechanism not found in PS II, but giving the same time constant. Also the protein structure around the accessory chlorophylls appears to be different
between PS II and the eubacteria and this may have a considerable effect on the rate of electron transport.
The non-haem iron
In the model structure the non-haem iron is liganded to the sigma- nitrogen atoms of four histidine residues. His215 and His272 in D1 and His215 and His269 in D2 minimized to bond distances within 0 . 2 2 nm of
the non-haem iron (figure 3.10). These ligands are predicted fi*om the alignment since they are totally conserved in oxygenic and anoxygenic species.
In the purple non-sulphur bacteria the remaining two iron binding ligands are provided by Glu232 of the M subunit. It has been suggested th a t the structure around the iron in the higher plant protein is different because D2 does not have an equivalent residue to Glu232 in the
appropriate region of the alignment. However, one or more of these ligands may be provided by Glu231 in D1 which is aligned, in the
template, with Glu232 of the two M subunits and Lys204 in Rh.
sphaeroides and Arg204 in Rps. viridis L subunits (figure 3.3).
In the proposed model structure the delta-carbonyl oxygen atom of D1 Glu231 models to be 0.23 nm from the non-haem iron (figure 3.10). The overall orientation of the side-chain is similar to th a t described in Rb.
term inal oxygen atoms is likely to form a ligand to the iron. This may be the co-ordination in reaction centres th at are depleted of bicarbonate (see also section 3.1.5).
F ig u re 3.10 The non-haem iron (P:7_FE sphere) and surroimding
residues (His215 and 272 and Glu231 in D1 and His215 and 269 in D2).
The view is in the plane of the membrane.
1:231_GLU
:269_HIS
1:272.
,1:215_HIS