Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0847097 (acidity)
 15,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The biologically active photoproduct of rhodopsin, metarhodopsin II (M II), exists in a pH-sensitive equilibrium with its precursor, metarhodopsin I (M I). Increasing acidity favors M II, with the midpoint of the pH titration curve at pH 6.4. To test the long-standing proposal that histidine protonation regulates this conformational transition, we characterized mutant rhodopsins in which each of the 6 histidines was replaced by phenylalanine or cysteine. Only mutants substituted at the 3 conserved histidines showed abnormal M I-M II equilibria. Those in which His-211 was replaced by phenylalanine or cysteine formed little or no M II at either extreme of pH, whereas mutants substituted at His-65 or at His-152 showed enhanced sensitivity to protons. The simplest interpretation of these results is that His-211 is the site where protonation strongly stabilizes the M II conformation and that His-65 and His-152 are sites where protonation modestly destabilizes the M II conformation.
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PMID:Histidine residues regulate the transition of photoexcited rhodopsin to its active conformation, metarhodopsin II. 153 20

Glu-113 serves as the retinylidene Schiff base counterion in bovine rhodopsin. Purified mutant rhodopsin pigments were prepared in which Glu-113 was replaced individually by Gln (E113Q), Asp (E113D), Asn (E113N), or Ala (E113A). E113Q, E113N, and E113A existed as pH-dependent equilibrium mixtures of unprotonated and protonated Schiff base (PSB) forms. The Schiff base pKa values determined by spectrophotometric titration were 6.00 (E113Q), 6.71 (E113N), and 5.70 (E113A). Thus, mutation of Glu-113 markedly reduced the Schiff base pKa. The addition of NaCl promoted the formation of a PSB in E113Q and E113A. An exogenously supplied solute anion replaced Glu-113 to compensate for the positive charge of the PSB in these mutants. The lambda max values of the PSB forms of the mutants in NaCl were 496 nm (E113Q), 506 nm (E113A), 510 nm (E113D), and 520 nm (E113N). To evaluate the effect of different types of solute anions on lambda max values, mutants were prepared in sodium salts of halides, perchlorate, and a series of carboxylic acids of various sizes and acidity. The lambda max values of E113Q and E113A depended on the solute anion present and ranged from 488 nm to 522 nm for E113Q and from 486 nm to 528 nm for E113A. The solute anion affected the lambda max values of E113N and E113D to lesser degrees. The reactivities of the mutants to hydroxylamine were also studied. Whereas rhodopsin was stable to hydroxylamine in the dark, E113N reacted slowly and E113Q reacted rapidly under these conditions, indicating structural differences in the Schiff base environments. The lambda max values and solute anion dependencies of the Glu-113 mutants indicate that interactions between Schiff base and its counterion play a significant role in determining the lambda max of rhodopsin.
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PMID:The role of the retinylidene Schiff base counterion in rhodopsin in determining wavelength absorbance and Schiff base pKa. 201 28

The proposal that the absorption maximum of the visual pigments is governed by interaction of the 11-cis-retinal chromophore with charged carboxylic acid side chains in the membrane-embedded regions of the proteins has been tested by mutating five Asp and Glu residues thought to be buried in rhodopsin. Changing Glu113 to Gln causes a dramatic shift in the absorption maximum from 500 nanometers to 380 nanometers, a decrease in the pKa (acidity constant) of the protonated Schiff base of the chromophore to about 6, and a greatly increased reactivity with hydroxylamine. Thus Glu113 appears to be the counterion to the protonated Schiff base. Wavelength modulation in visual pigments apparently is not governed by electrostatic interaction with carboxylate residues, other than the counterion.
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PMID:Effect of carboxylic acid side chains on the absorption maximum of visual pigments. 257 54

1. Unlike rhodopsin, the extracted 521-pigment of the Tokay gecko (Gekko gekko) is pH-sensitive and changes its spectral absorbance in the pH range of 4.5-7.3. The colour change is reversible and pH can be employed to adjust the spectral maximum anywhere between 490 nm and its native location at 521 nm.2. The hypsochromic shift with increasing acidity is opposite to that expected for the protonation of the Schiff base nitrogen and suggests an action on the secondary system of interacting charges that have long been postulated to adjust vertebrate visual pigment colour within the visible spectrum.3. Chloride ions modulate this pH effect in a systematic and significant manner. For the pigment extracted in the chloride-deficient state the colour change occurs in the pH range of 6.0-7.0, the midpoint being close to 6.5, suggesting the possible participation of the imidazole group of histidine as the functional moiety. With added NaCl the colour shifts to the region below pH 6.2.4. The modulating action of chloride is postulated to be a conformational change of the opsin leading to a shift of the secondary interacting site from one functional group to another or else to a change in pK of a single group due to the conformational alteration of the electrostatics of the system.5. At pH values between 7.5 and 9.0 a different mechanism becomes apparent. In this region a decrease occurs in the photopigment density as well as a shift in absorbance toward the blue. This alkaline effect is readily reversed either by adding NaCl or else by lowering the pH. Along with the other protective effects of chloride these ions serve to reduce or prevent this alkaline loss in density.6. Associated with this reversible photopigment loss is a reversible appearance of a product with a maximum at about 366 nm. The spectrum of this product is like that produced by the addition of 11-cis retinal to the extract. Acidification of the alkaline preparation leads to a restitution of the photopigment as well as to a reduction of the 366-product.7. Addition of hydroxylamine to the alkaline extract in appropriate concentration inhibits the restitution of pigment-521 with acid or NaCl, but adding 11-cis retinal to the system leads to restoration of the photopigment after acidification. All the evidence suggests that product-366 is either free 11-cis retinal or else held to the opsin in a form that does not alter its spectral absorbance. The alkaline effect is therefore a disruption of the aldimine bond of the visual pigment.8. In many respects the gecko 521-pigment behaves like the chicken cone pigment, iodopsin, suggesting that an investigation of the latter in terms of pH may be a worthy project for future study.9. With its ability to change colour with pH, with chloride, with nitrate, etc. the extractable gecko pigment offers possibilities for the investigation of mechanisms responsible for adjusting visual pigment absorbance throughout the visible spectrum. The techniques of circular dichroism, Raman spectroscopy, infra-red spectroscopy, etc. may find here a suitable material for these studies.
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PMID:The gecko visual pigment: a pH indicator with a salt effect. 733 18

Rhodopsin is a member of a family of G protein-coupled receptors which share structural and functional homologies. A tripeptide sequence (Glu or Asp/Arg/Tyr) at the cytoplasmic border of the third transmembrane segment is conserved among most of these receptors. This region is involved in G protein activation in rhodopsin as well as in other receptors. The role of the conserved Glu-134 was studied by site-specific mutagenesis of rhodopsin in combination with a real-time fluorescence assay of G protein (transducin) activation. Assay conditions were chosen under which the transducin activation rate was determined either by rhodopsin-transducin complex formation or by GTP gamma S-induced complex dissociation. Glu-134 was replaced by Gln in order to mimic the protonated state of the carboxylic acid group. This mutation caused the pH dependency of complex formation to extend to the alkaline range as compared with rhodopsin. Replacement of Glu-134 by Asp had an opposite but less pronounced effect on the pH dependency and lowered the overall efficiency of transducin activation. The acidity constant (pKa) of the residue at position 134 did not directly determine the pH sensitivity of complex formation, indicating that other amino acid residues contribute to a titratable binding domain that includes Glu-134. In contrast, the pH sensitivity of GTP gamma S-induced complex dissociation was not changed by the mutations, although absolute rates were affected. The data suggest that the protonated state of Glu-134 favors binding of rhodopsin to transducin and that Glu-134 is not titratable in the rhodopsin-transducin complex.
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PMID:Regulation of the rhodopsin-transducin interaction by a highly conserved carboxylic acid group. 834 12

The environment of the retinylidene Schiff base in bovine rhodopsin has been studied by movement of its carboxylic acid counterion from position 113 to position 117 by site-specific mutagenesis. Replacement of the counterion at position 113 by a neutral amino acid residue has been shown to produce a lowering of the Schiff base acidity constant (pKa) from > 8.5 to about 6. The aim of the present work was to change the position of the counterion without causing a significant effect on the Schiff base pKa. A triple replacement mutant (Glu113-->Ala/Ala117-->Glu/Glu122-->Gln) was designed to move the position of the counterion by one helix turn in the third putative transmembrane helix (helix C). The mutant bound 11-cis-retinal to form a chromophore with a visible absorbance maximum (lambda max) of 490 nm which was independent of pH in the range of about 5-8.5. Upon illumination under conditions in which rhodopsin was converted to the active metarhodopsin II (MII) photoproduct, the mutant was converted to a metarhodopsin I (MI)-like species (lambda max = 475 nm). Furthermore, the effect of pH on the photobleaching behavior of the mutant was the reverse of that reported for rhodopsin. In the mutant, acidic pH favored the formation of the MI-like photoproduct, and basic pH favored the formation of an MII-like photoproduct (lambda max = 380 nm). The MII-like photoproduct of the mutant pigment was able to activate the guanine nucleotide-binding protein, transducin. We conclude that the Schiff base counterion in rhodopsin can be repositioned to form a pigment with an apparently unperturbed Schiff base pKa. Furthermore, a specific amino acid residue that acts as a Schiff base proton acceptor is not strictly required for photoconversion of rhodopsin to its active MII form.
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PMID:Movement of the retinylidene Schiff base counterion in rhodopsin by one helix turn reverses the pH dependence of the metarhodopsin I to metarhodopsin II transition. 844 40