Spectroscopic Analysis of Genetically Modified Photosynthetic Reaction
Centers.
Coleman WJ, Youvan DC
Department of Chemistry, Massachusetts Institute of Technology, Cambridge 02139.
The photosynthetic reaction center (RC) from the purple bacteria is a membrane-bound
pigment-protein complex containing four bacteriochlorophylls (Bchl), two
bacteriopheophytins (Bphe), two quinones (Q), and a ferrous nonheme iron atom (Fe).
These components are attached to two symmetrically arranged protein subunits (L and M),
which along with a third subunit (H), comprise the basic RC core structure. The L
and M subunits each contain five transmembrane a-helices (designated A through E) and two
short, connecting helices (designated cd and de). Together, the L
and M subunits form a dimeric complex in which the chromophores are arranged in pairs
around an approximate C2 axis of symmetry. The axis runs from the special
pair dimer of Bchls on the periplasmic side of the RC to the Fe on the cytoplasmic
side. Excitation energy is transferred into the RC complex from the surrounding
light-harvesting (LH) pigment-protein complexes, whose three-dimensional structures are
not yet known.