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 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

The guanine nucleotide-binding protein Ras occurs in solution in two different states, state 1 and state 2, when the GTP analogue GppNHp is bound to the active center as detected by (31)P NMR spectroscopy. Here we show that Ras(wt).Mg(2+).GppCH(2)p also exists in two conformational states in dynamic equilibrium. The activation enthalpy DeltaH(++)(12) and the activation entropy DeltaS(++)(12) for the transition from state 1 to state 2 are 70 kJ mol(-1) and 102 J mol(-1) K(-1), within the limits of error identical to those determined for the Ras(wt).Mg(2+).GppNHp complex. The same is true for the equilibrium constants K(12) = [2]/[1] of 2.0 and the corresponding DeltaG(12) of -1.7 kJ mol(-1) at 278 K. This excludes a suggested specific effect of the NH group of GppNHp on the equilibrium. The assignment of the phosphorus resonance lines of the bound analogues has been done by two-dimensional (31)P-(31)P NOESY experiments which lead to a correction of the already reported assignments of bound GppNHp. Mutation of Thr35 in Ras.Mg(2+).GppCH(2)p to serine leads to a shift of the conformational equilibrium toward state 1. Interaction of the Ras binding domain (RBD) of Raf kinase or RalGDS with Ras(wt) or Ras(T35S) shifts the equilibrium completely to state 2. The (31)P NMR experiments suggest that, besides the type of the side chain of residue 35, a main contribution to the conformational equilibrium in Ras complexes with GTP and GTP analogues is the effective acidity of the gamma-phosphate group of the bound nucleotide. A reaction scheme for the Ras-effector interaction is presented which includes the existence of two conformations of the effector loop and a weak binding state.
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PMID:Conformational states of Ras complexed with the GTP analogue GppNHp or GppCH2p: implications for the interaction with effector proteins. 1569 48