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Paddock M.L. Senft M.E. Graige M.S. Rongey S.H. Turanchik T. Feher G. Okamura M.Y 《Photosynthesis research》1998,55(2-3):281-291
The structural basis for proton coupled electron transfer to QB in bacterial reaction centers (RCs) was studied by investigating RCs containing second site suppressor mutations (Asn M44 Asp, Arg M233 Cys, Arg H177 His) that complement the effects of the deleterious Asp L213 Asn mutation [DN(L213)]. The suppressor RCs all showed an increased proton coupled electron transfer rate k
AB
(2)(QA
–QB
– + H+ QAQBH–) by at least 103 (pH 7.5) and a recombination rate k
BD (D+QAQB
– DQAQB) 15–40 times larger than the value found in DN(L213) RCs. Proton transfer was studied by measuring the dependence of k
AB
(2) on the free energy for electron transfer (Get). k
AB
(2) was independent of Get in DN(L213) RCs, but dependent on Get in native and all suppressor RCs. This shows that proton transfer limits the k
AB
(2) reaction with a rate of 0.1s–1 in DN(L213) RCs but is not rate limiting and at least 108-fold faster in native and 105-fold faster in the suppressor RCs. The increased rate of proton transfer by the suppressor mutations are proposed to be due to: (i) a reduction in the barrier to proton transfer by providing a more negative electrostatic potential near QB
–; and/or (ii) structural changes that permit fast proton transfer through the network of protonatable residues and water molecules near QB. 相似文献
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Despite their remoteness from sources of atmospheric pollutant emissions, the Afromontane tarns in the Maloti-Drakensberg region are perfect candidates to study the negative effects of acidifying atmospheric pollution, because mountain lakes are widely recognised as sentinel ecosystems, unimpacted by direct human disturbance within their catchments. Thirty-four tarns were sampled in the Maloti-Drakensberg region and most were found to be extremely sensitive to acidic deposition, as indicated by their low acid neutralising capacity. There are very few studies of freshwater critical loads for any region within South Africa. The steady-state water chemistry model (SSWC) was adapted and used to determine critical loads, whereas exceedance was estimated relative to modelled regional deposition data, in order to understand the risk of harmful effects to aquatic ecosystems. Seventy-six percent of sampled sites across the Maloti-Drakensberg would exceed critical loads even at the lowest modelled deposition levels, but there are no current measured deposition data for the region. The sensitivity of the Maloti-Drakensberg tarns needs to be considered in future policy formulation regarding acceptable levels of acidifying atmospheric pollution from South Africa’s energy sector and indicates the need for assessing aquatic ecosystem impacts in other regions of South Africa. 相似文献
79.
Vincent Ranwez Frédéric Delsuc Sylvie Ranwez Khalid Belkhir Marie-Ka Tilak Emmanuel JP Douzery 《BMC evolutionary biology》2007,7(1):241
Background
Molecular sequence data have become the standard in modern day phylogenetics. In particular, several long-standing questions of mammalian evolutionary history have been recently resolved thanks to the use of molecular characters. Yet, most studies have focused on only a handful of standard markers. The availability of an ever increasing number of whole genome sequences is a golden mine for modern systematics. Genomic data now provide the opportunity to select new markers that are potentially relevant for further resolving branches of the mammalian phylogenetic tree at various taxonomic levels. 相似文献80.