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61.
We present studies on a series of photosynthetic reaction center (RC) mutants created in the background of the Rhodobacter capsulatus D(LL) mutant, in which the D helix of the M subunit has been substituted with that from the L subunit. Previous work on the D(LL) mutant in chromatophore preparations showed that RCs assembled without the bacteriopheophytin H(L) electron acceptor and performed no charge separation following light absorption. We have successfully isolated poly-His-tagged D(LL) RCs by using the detergent Deriphat 160-C and shown that the RCs are devoid of H(L). The excited state of the primary electron donor, P*, is found to have a lifetime of 180 +/- 20 ps and to decay exclusively (>95%) via internal conversion to the ground state, with no evidence for formation of any charge-separated intermediates. By additional mutation in the D(LL) background of two residues that affect the P/P+ oxidation potential and one that facilitates M-side electron transfer, we achieve an unprecedented 70% yield of P+ H(M)-, more than doubling the highest yield of this state achieved previously. This result underscores the importance of the relative free energies of P* and the charge-separated states in governing the rates and yields of electron transfer in bacterial RCs and provides a basis for systematically investigating M-side electron transfer without any competition from the native L-side pathway. 相似文献
62.
An assessment of the mechanism of initial electron transfer in bacterial reaction centers 总被引:4,自引:0,他引:4
Subpicosecond time-resolved photodichroism measurements on Rhodobacter sphaeroides R26 reaction centers are reported in the key region between 620 and 740 nm, where the anions of both bacteriopheophytin and bacteriochlorophyll (BChl) have their most diagnostic absorption bands. These measurements fail to resolve clearly the formation of a reduced BChl species. The implications of this for elucidating the role of the accessory BChl in the initial stage of charge separation are discussed. 相似文献
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Summary Recent results from several laboratories suggest that complex interactions between hormones and dietary carbohydrate may be
responsible for regulating the induction of several hepatic lipogenic enzymes. Elucidation of these interactions requires
the ability to culture hepatocytes for several days in serum-free medium where the hormones or carbohydrate or both present
is strictly controlled. The functional response of primary adult rat hepatocytes was examined in a medium without exposure
to serum, hormones, or carbohydrates and on three substrata commonly used to culture cells in a defined medium. Hepatocytes
cultured on a floating collagen gel in which is embedded a nylon mesh possess cell attachment and morphologic characteristics
superior to either cells cultured on a collagen-coated or fibronectin(Fn)-coated substratum. Cells cultured on the gel-mesh
system retain insulin responsivity, as measured by protein synthesis rates and glucose-6-phosphate dehydrogenase (G6PD) induction,
for at least 6 d in culture. Under these conditions, insulin, dexamethasone, and fructose increase G6PD specific activity
to levels comparble to that seen in an induced animal. Hepatocytes cultured on the gel-mesh system tolerate restricted medium
conditions better than cells cultured on collagen or Fn-coated substratum, and remain viable for sufficient times to allow,
for the first time, full expression and maximal induction (i.e. like in vivo), of G6PD in cultured cells. This system represents
a satisfactory model for in vivo liver metabolism and a superior system for studying the effects of hormones and metabolites
on G6PD levels, as well as other nutritional-hormonally regulated enzymes. 相似文献
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Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions.Abbreviations BChl
bacteriochlorophyll
- P
a dimer of BChl molecules
- BPh
bacteriopheophytin
- QA and QB
quinone molecules
- L, M and H
light, medium and heavy polypeptides of the reaction center 相似文献
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Larry Winberry Roderick Nakayama Ronald Wolfe Darold Holten 《Biochemical and biophysical research communications》1980,96(2):748-755
The glucose-6-P dehydrogenase specific activity in rat hepatocytes increases approximately 10-fold when the cells are placed into culture for three days. The induction requires insulin with maximum enzyme levels occurring at 10?7 M. Pulse-labeling experiments revealed a 10-fold increase in the enzyme's relative rate of synthesis after only 8 hours in culture. 相似文献
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