Could chemically- or enzymically-modified chlorophylls be
present and have a role in photosynthetic reaction centre(s)?
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I have not been active in chloroplast work for some 25 years, so am not
fully conversant with the latest research. I should nevertheless like to
offload the following thoughts, just in case anyone out there were to
consider them worth pursuing.
It has been shown (1,2,3,4) that 'changed' chlorophylls having, for example,
an -OH group at the 13^2 position (4), are artifacts arising from various
leaf treatments or chromatographic procedures.
I think, nevertheless, that this would not preclude the possibility of them
being naturally present in chloroplasts as a very small percentage of the
total chlorophyll complement, and thus not easily detectable by
chromatographic methods, but nevertheless fullfilling an important role
in reaction centre(s). Indeed, one might almost expect any exposed
chlorophylls in the chloroplast structure to be modified chemically or
enzymically, given the relative ease of formation of 'changed'
chlorophylls in vitro, and thus to legitimately exist as such in the
choroplast (note that the in vitro absorption spectrum of the 13^2 -OH
form is virtually unchanged from that of the parent molecule).
Although, if there is any validity in this assumption, more than one type
of 'changed' chlorophyll could be present (1,2), the 13^2 -OH form might
certainly be a candidate for an active role in the reaction centre,
given that the electron-donating potential of carbonyl oxygen is activated
by an -OH in the alpha position (5) (as is the case with
13^1 carbonyl/13^2 -OH). Such a strong electron-donor may also be a very
good acceptor, once an electron as been donated (see 5). The chemical
reactivity of the 13^2 -OH itself may also be interesting, given that it
has two carbonyl groups adjacent to it.
The question then arises -- does recent work UNAMBIGUOUSLY preclude the
possibility of e.g. a 13^2 -OH being present in one or more of the reaction
centre chlorophylls? If not, the idea might be worth pursuing by someone.
(I must confess that I do not know if the Deisenhofer et al. X-ray
crystallography work would clearly distinguish an -OH from an -H,
nor whether reaction centre chlorophylls can be extracted totally from
reaction centre preparations for chromatographic or mass spectroscopic
analysis).
Thank you for your attention. I would welcome any comments, either via
this news group (bionet.photosynthesis), or by email, care of
pbacon at catep.demon.co.uk
References:
(1) M.F. Bacon, Biochem. J., 1966, 101, 34c-36c
(2) M.F. Bacon & M. Holden, Phytochemistry, 1967, 6, 193-210
(3) H.H. Strain, J. Sherma & M. Grandolfo, Anal. Chem., 1967, 39, 926-932
(4) M. Senge, A. Struck, D. Dornemann, H. Scheer & H. Senger,
Z. Naturforsch., C., BioSci., 1988, 43, 515-518
(5) A. Szent-Gyorgyi, Proc. Nat. Acad. Sci, 1967, 58, 2012-2014
--
Michael Bacon.
"Ojo del Sol"
San Antonio,
La Herradura, (Granada)
SPAIN
Tel: (34) 58 827608
Fax: (Attention M. Bacon) (34) 58 640422
Email: (Attention M. Bacon) pbacon at catep.demon.co.uk
