Quantized visual awareness.

The proposed model holds that, at its most fundamental level, visual awareness is quantized. That is to say that visual awareness arises as individual bits of awareness through the action of neural circuits with hundreds to thousands of neurons in at least the human striate cortex. Circuits with specific topologies will reproducibly result in visual awareness that correspond to basic aspects of vision like color, motion, and depth. These quanta of awareness (qualia) are produced by the feedforward sweep that occurs through the geniculocortical pathway but are not integrated into a conscious experience until recurrent processing from centers like V4 or V5 select the appropriate qualia being produced in V1 to create a percept. The model proposed here has the potential to shift the focus of the search for visual awareness to the level of microcircuits and these likely exist across the kingdom Animalia. Thus establishing qualia as the fundamental nature of visual awareness will not only provide a deeper understanding of awareness, but also allow for a more quantitative understanding of the evolution of visual awareness throughout the animal kingdom.

Effects of site-specific mutagenesis of tyrosine 105 in a class A beta-lactamase.

Tyr-105 is a conserved residue in the Class A beta-lactamases and is in close proximity to the active-site. Tyr-105 in beta-lactamase from Bacillus licheniformis was converted into Phe by site-directed mutagenesis. This mutation caused no significant effect on the structure of the enzyme and had only small effects on the catalytic properties. In particular, in comparison to the wild-type, kcat. for benzylpenicillin was increased slightly, whereas it was decreased slightly for several other substrates. For each substrate examined, Km increased 3-4-fold in the mutant compared with the wild-type enzyme. Examination of the effect of pH on the catalytic reaction revealed only small perturbations in the pK values for the acidic and basic limbs of the kcat./Km pH profiles due to the mutation. Overall effects of the Y105F substitution on the catalytic efficiency for different penicillin and cephalosporin substrates ranged from 14% to 56% compared with the wild-type activity. We conclude that Tyr-105 is not an essential residue for beta-lactamase catalysis, but does contribute to substrate binding.

Site-directed mutagenesis of glutamate-166 in beta-lactamase leads to a branched path mechanism.

Glutamate-166 of the Bacillus licheniformis beta-lactamase was specifically mutated to aspartate and cysteine in order to probe the function of this residue in catalysis. In both cases, a large decrease in activity (kcat/Km was 3.5 x 10(-5) smaller for E166C and 1 x 10(-3) smaller for E166D than for the wild-type) was observed, although the kinetics for the two mutants were very different. The pH-rate profiles for E166D and E166C reflected the ionization characteristics of the new residue at site 166. This result indicates that the ionization of Glu-166 is responsible for the acidic limb of the kcat/Km-pH profiles, and suggests that the function of Glu-166 is that of a general base catalyst. The kinetics of the E166C mutant were investigated in detail. An initial burst was observed, whose amplitude was stoichiometric with the enzyme concentration, suggesting rate-limiting deacylation of the acyl-enzyme intermediate. However, further study revealed that in the presence of 0.5 M sodium sulfate, which stabilizes the native conformational state, the magnitude of the burst corresponded to 2 equiv of enzyme. This observation, in conjunction with the limited effect of the mutation on Km, indicated that the mutation resulted in a change in the kinetic mechanism from the linear, acyl-enzyme pathway to one with a branch leading to an inactive form of the acyl-enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Side-chain mobility of the beta-lactamase A state probed by electron spin resonance spectroscopy.

beta-Lactamase from Bacillus licheniformis forms a stable compact intermediate state at low pH and moderate salt concentration (the A state), with properties consistent with a molten globule. A single cysteine residue was introduced into this class A beta-lactamase by site-directed mutagenesis at position 166. A spin label was attached to the thiol of this cysteine residue via a disulfide bond as a probe of the side-chain mobility. The mutant protein and the spin-labeled derivative exhibited similar conformational properties to the wild-type enzyme at acidic pH. The A state induced by chloride or trichloroacetate (TCA) anions was characterized by circular dichroism and esr. The A state at pH 0.5 (0.32 M HCl), or at pH 2 in the presence of 8 mM TCA or 0.4 M Cl-, had comparable amounts of secondary structure to the native state but lacked significant tertiary structure, as judged by the lack of near-UV circular dichroism. Analysis of the esr spectral line widths showed that the mobility of the spin label in the A state was similar to that in the native state and much less mobile than in the unfolded state, indicating significant constraints on the side-chain mobility in this region of the molecule in the A state. The implications of this finding to the structure of the A state are discussed.

A catalytically-impaired class A beta-lactamase: 2 A crystal structure and kinetics of the Bacillus licheniformis E166A mutant.

In the beta-lactamase (penicillinase) of Bacillus licheniformis 749/C, site specific mutation of Glu166 to Ala caused a million-fold reduction of catalytic activity towards both penicillin and cephalosporin substrates and resulted in the stoichiometric accumulation of the acyl enzyme. The rate of deacylation generally slowed by as much as 10(-7) compared to the wild type. The acyl enzyme intermediate was observed by HPLC, but not by X-ray diffraction. The mutant was crystallized from methoxyPEG 5000 at pH 6.2 in space group P2(1) with Z = 4. Molecular replacement based on the wild type structure followed by refinement produced an R factor of 17.2% for 25,800 3 sigma data from 10 to 2 A. Deviations from bond and angle ideals are 0.005 A and 1.5 degrees respectively. The mutant differs very little from the wild type structure, with only 0.25 A (r.m.s.) differences in backbone atoms; the CD spectra and thermal stabilities of the two enzymes are identical. Changes in the positions of the reactive Ser70 and conserved Lys73 are not significant, suggesting that the proposed salt linkage to Glu166 in the wild type enzyme is weak or non-existent. The calculated solvent exposure of Ser70 and Lys73 increases slightly and a buried water molecule is now positioned near Lys73. The hydrolytic water seen in the native active site shifts markedly by 1.6 A, but is held in the active site by Asn170, which possibly becomes an ineffective substitute for Glu166 in activating the water for deacylation.

Site-directed mutagenesis of beta-lactamase leading to accumulation of a catalytic intermediate.

Site-specific mutation of Glu-166 to Ala in beta-lactamase causes a millionfold reduction in catalytic activity toward both penicillin and cephalosporin substrates and results in the stoichiometric accumulation of a normally transient acyl-enzyme intermediate. Kinetic analysis indicated that substitution of Glu-166 by Ala leads to negligible effect on the acylation half of the reaction but effectively eliminates the deacylation reaction. Such differential effects on the rates of formation and breakdown of an enzyme-substrate intermediate have not been previously reported. Thus, unlike the situation for most transfer enzymes, e.g., the serine proteases, acylation and deacylation in beta-lactamase catalysis are not "mirror" images and must involve different mechanisms. The results suggest an explanation for the different catalytic activities between the beta-lactamases and the penicillin-binding proteins involved in bacterial cell-wall synthesis.

The role of lysine-234 in beta-lactamase catalysis probed by site-directed mutagenesis.

Lys-234 has been postulated to participate in beta-lactamase catalysis by acting as an electrostatic anchor for the C3 carboxylate of penicillins [Herzberg, O., & Moult, J. (1987) Science 236, 694-701]. To test this hypothesis, site-directed mutagenesis was used to convert the Lys-234 in Bacillus licheniformis beta-lactamase into Glu-234 or Ala-234. The wild-type, Glu-234, and Ala-234 beta-lactamases have been expressed in Bacillus subtilis and purified to homogeneity. The wild-type, K234E, and K234A enzymes have virtually identical circular dichroism and fluorescence spectra, similar thermal stabilities at neutral pH, and the same susceptibilities to proteolysis, indicating the lack of significant structural perturbation caused by the mutation. At acidic and basic pH the mutant enzymes have the same native circular dichroism as the wild-type enzyme but the thermal stability is significantly different. The mutations cause perturbations of the pK values of the ionizing groups responsible for the pH dependence of the catalytic reaction in both the free enzyme and the E.S complex. As expected, conversion of Lys-234 to Ala or Glu decreased substrate binding (Km) by 1-2 orders of magnitude for several penicillin and cephalosporin substrates at neutral and higher pH. However, at low pH, Km is essentially the same for the K234E and K234A enzymes as for the wild-type enzyme. Furthermore, decreases of 2-3 orders of magnitude in kcat were also observed, indicating substantial effects on the transition-state binding, as well as on ground-state binding. Surprisingly, changing the C3 carboxylate of phenoxymethylpenicillin to a hydroxymethyl group led to little difference in kinetic properties with the K234E or K234A enzyme. The results of this investigation indicate the Lys-234 is an important active-site residue involved in both ground-state and transition-state binding.

Antibodies to poliomyelitis virus in cord blood of neonates born to from mothers in contact or not in contact with the wild virus.

Neutralizing antibodies to polioviruses in cord blood of neonates born from 64 mothers under age 20 and in 53 mothers aged 30 years and over were investigated in order to know and compare the transfer to newborns of antibodies to polioviruses produced by live oral vaccine mainly and those antibodies induced by natural contact with wild poliovirus strains. Total immunity for the two groups was higher than 80% for the three types of polioviruses, with only virus 3 showing an immunity below 80% (77.4%) in mothers aged 30 years and over. Average geometric titers though relatively low may be considered satisfactory. However, there is a statistically significant difference in titers to poliovirus type 2 (24.9) in mothers over age 30 years as compared to those found in mothers below age 20 years (10.8), for which we have found no explanation. It is not deemed necessary for the time being to take special prophylactic measures with these children given the occurrent epidemiologic status quo.