High levels of plasma HHV-6 DNA are associated with higher risk of HHV-6 encephalitis. UCBT is a significant risk factor for HHV-6 encephalitis. HHV-6 encephalitis should be considered if CNS dysfunction develops concomitant to high-level plasma HHV-6 DNA after allogeneic HCT.

␣-Nitrosyl hemoglobin, ␣(Fe-NO) 2 ␤(Fe) 2 , which is frequently observed upon reaction of deoxy hemoglobin with limited quantities of NO in vitro as well as in vivo, has been synthetically prepared, and its reaction with O 2 has been investigation by EPR and thermodynamic equilibrium measurements. ␣-Nitrosyl hemoglobin is relatively stable under aerobic conditions and undergoes reversible O 2 binding at the heme sites of its ␤-subunits. Its O 2 binding is coupled to the structural/functional transition between T-(low affinity extreme) and R-(high affinity) states. This transition is linked to the reversible cleavage of the heme Fe-proximal His bonds in the ␣(Fe-NO) subunits and is sensitive to allosteric effectors, such as protons, 2,3-biphosphoglycerate, and inositol hexaphosphate. In fact, ␣(Fe-NO) 2 ␤(Fe) 2 is exceptionally sensitive to protons, as it exhibits a highly enhanced Bohr effect. The total Bohr effect of ␣-nitrosyl hemoglobin is comparable to that of normal hemoglobin, despite the fact that the oxygenation process involves only two ligation steps. All of these structural and functional evidences have been further confirmed by examining the reactivity of the sulfhydryl group of the Cys ␤93 toward 4,4-dipyridyl disulfide of several ␣-nitrosyl hemoglobin derivatives over a wide pH range, as a probe for quaternary structure. Despite the halved O 2 -carrying capacity, ␣-nitrosyl hemoglobin is fully functional (cooperative and allosterically sensitive) and could represent a versatile low affinity O 2 carrier with improved features that could deliver O 2 to tissues effectively even after NO is sequestered at the heme sites of the ␣-subunits. It is concluded that the NO bound to the heme sites of the ␣-subunits of hemoglobin acts as a negative allosteric effector of Hb and thus might play a role in O 2 /CO 2 transport in the blood under physiological conditions.

Hybrid hemoglobins were prepared in which cobalt was substituted for the heme iron in either the alpha or beta subunits. Transient optical absorption spectra were measured at room temperature for these hybrids at time intervals between 0 and 50 ms following photodissociation of the carbon monoxide complex with 10-ns laser pulses. The cobalt porphyrins do not bind carbon monoxide, making it possible to investigate the time-resolved response of the cobalt-containing subunits to photodissociation of carbon monoxide in the iron-containing subunits. At the same time the response of the iron-containing subunits to the photolysis event can be studied, permitting an independent determination of the kinetics of ligand rebinding and conformational changes in the alpha and beta subunits of an intact tetramer. The data were analyzed by using singular-value decomposition to obtain the kinetic progress curve for ligand rebinding, the deoxyheme and cobalt porphyrin spectral changes, and the time course of these spectral changes. The geminate rebinding kinetics following photodissociation of alpha(Co)2 beta(Fe-CO)2 were very similar to those found unsubstituted hemoglobin, alpha(Fe-CO)2 beta(Fe-CO)2, indicating equivalence of the geminate kinetics for alpha and beta subunits within the R-state tetramer. The results for alpha(Fe-CO)2 beta(Co)2 were consistent with this conclusion, even though the analysis was complicated by the presence of comparable populations of R- and T-state species. Comparison of the deoxyheme spectral changes and relaxation times among the three molecules indicated that both alpha and beta subunits contribute to the deoxyheme spectral changes that signal tertiary and quaternary conformational changes in the unsubstituted tetramer. The response of the cobalt porphyrins to photodissociation was similar in the two hybrids. No structural changes were detected in the cobalt-containing subunits until the second tertiary conformational change in the iron-containing subunits observed at 1-2 microseconds. Much larger structural changes, as judged by the amplitude of the spectral changes, occurred in the cobalt-containing subunits concomitant with the R----T quaternary change at about 20 microseconds.

The technique of femtosecond coherence spectroscopy is applied to a variety of photostable and photochemically active heme protein samples. With the exception of cobalt-substituted myoglobin, strong oscillations are detected near 40 cm -1 in all of the samples studied. Additional modes near 80, 120, and 160 cm -1 are observed in the photochemically active samples. The amplitude and phase behavior of the low-frequency modes are studied by tuning the pump/probe carrier wavelength across the Soret absorption spectrum. A simple harmonic model is not able to account for the observed relative intensities of these modes or the carrier wavelength dependence of their frequency and phase. As a result, we develop an anharmonic model where the oscillatory signal is damped as the result of heterogeneity in the potential surface. The underlying source of the heterogeneity in the anharmonic potential surface is found to be correlated with the inhomogeneous broadening of the Soret band. The presence of the higher harmonics in the photochemically active samples demonstrates that the anharmonic mode is strongly coupled to the ligand photodissociation reaction (i.e., upon photolysis it is displaced far from equilibrium). Moreover, the observation of the ∼40 cm -1 oscillations in all of the ironbased heme protein samples, including porphine and protoporphyrin IX model compounds, suggests that this mode is associated with nuclear motion of the core of the porphyrin macrocycle. Since normal mode calculations and prior kinetic models predict the frequency of the heme "doming mode" to be near 50 cm -1 , we suggest that the reaction coupled oscillations at ∼40 and ∼80 cm -1 are a direct reflection of anharmonic heme doming dynamics. Evidence for coupling between the heme doming dynamics and the Fe-His stretching mode is also presented.

No abstract

No abstract

No abstract

No abstract