Imasheva Eleonora S, Balashov Sergei P, Wang Jennifer M, Dioumaev Andrei K, Lanyi Janos K
Department of Physiology and Biophysics, University of California, Irvine, California 92697, USA.
Biochemistry. 2004 Feb 17;43(6):1648-55. doi: 10.1021/bi0355894.
Similarly to bacteriorhodopsin, proteorhodopsin that normally contains all-trans and 13-cis retinal is transformed at low pH to a species containing 9-cis retinal under continuous illumination at lambda > 530 nm. This species, absorbing around 430 nm, returns thermally in tens of minutes to initial pigment and can be reconverted also with blue-light illumination. The yield of the 9-cis species is negligibly small at neutral pH but increases manyfold (>100) at acid pH with a pK(a) of 2.6. This indicates that protonation of acidic group(s) alters the photoreaction pathway that leads normally to all-trans --> 13-cis isomerization. In the D97N mutant, in which one of the two acidic groups in the vicinity of the retinal Schiff base is not ionizable, the yield of 9-cis species at low pH shows a pH dependence similar to that in the wild-type but with a somewhat increased pK(a) of 3.3. In contrast to this relatively minor effect, replacement of the other acidic group, Asp227, with Asn results in a remarkable, more than 50-fold, increase in the yield of the light-induced formation of 9-cis species in the pH range 4-6. It appears that protonation of Asp227 at low pH is what causes the dramatic increase in the yield of the 9-cis species in wild-type proteorhodopsin. We conclude that the photoisomerization pathways in proteorhodopsin to 13-cis or 9-cis photoproducts are controlled by the charge state of Asp227.
与细菌视紫红质类似，通常含有全反式和13 - 顺式视黄醛的视紫质在低pH值下，于波长> 530 nm的连续光照下会转变为含有9 - 顺式视黄醛的物种。该物种在430 nm左右有吸收，在数十分钟内会热回复到初始色素，也可通过蓝光照射重新转变。9 - 顺式物种在中性pH值下的产量极小，但在酸性pH值下（pK(a)为2.6）会增加许多倍（> 100）。这表明酸性基团的质子化改变了通常导致全反式→13 - 顺式异构化的光反应途径。在D97N突变体中，视黄醛席夫碱附近的两个酸性基团之一不可电离，低pH值下9 - 顺式物种的产量显示出与野生型类似的pH依赖性，但pK(a)略有增加，为3.3。与这种相对较小的影响相反，用天冬酰胺取代另一个酸性基团天冬氨酸227会导致在4 - 6的pH范围内，光诱导形成9 - 顺式物种的产量显著增加，超过50倍。似乎低pH值下天冬氨酸227的质子化是导致野生型视紫质中9 - 顺式物种产量急剧增加的原因。我们得出结论，视紫质中向13 - 顺式或9 - 顺式光产物的光异构化途径受天冬氨酸227的电荷状态控制。
