Erramilli Shyamsunder

Nonbilayer phases of membrane lipids

Numerous liquid crystalline biomembrane lipids are known to exhibit non-lamellar phases characterized by curvature of their component lipid monolayers. An understanding of the phase stability of these systems begins with analysis of the energy of bending the monolayers, the interactions which lead to the bending energy, and the geometrical constraints which lead to competing energy terms which arise when the monolayers are bent and packed onto lattices with different structures. Diffraction and other techniques suitable for probing lipid phase structure are described. A phenomenological model is reviewed which successfully explains many of the qualitative features of lipid mesomorphic phase behavior. A key result of this model is that lipid bilayer compositions which are close to the non-lamellar phase boundaries of their phase diagrams are characterized by a frustrated elastic stress which may modulate the activity of imbedded membrane proteins and which may provide a rationale for the prevalence of non-lamellar-tending lipid species in biomembrane bilayers. Areas in need of future research are discussed.

Is the Mechanism of General Anesthesia Related to Lipid Membrane Spontaneous Curvature

Lipid bilayers in biomembranes may exist in a state of elastic curvature stress which may couple to the conformation of integral membrane proteins in a logical, and energetically significant, fashion. Many biomembranes contain sufficiently large fractions of nonlamellar-prone lipids to have monolayers under substantial curvature stress. Although very few experiments have been performed that can be used to correlate protein activity with curvature stress, the literature does contain a small number of studies that indicate that some protein function is nonspecifically modulated by the amounts of nonlamellar-prone lipid in the imbedding bilayers. The spontaneous curvature, is altered by the presence of anesthetics in physiologically relevant concentrations. This leads us to suggest that anesthetic action may be coupled to protein function via alteration of the tensions leading to the spontaneous curvature of biomembrane layers. The spontaneous curvature is also sufficiently sensitive to pressure that a mechanism for the pressure reversal of anesthesia follows if the effects of pressure are to counter changes in membrane lateral tension induced by anesthetics. It is emphasized that many more experimental data must be acquired to determine whether the ideas presented in this paper have validity. In particular, there is a need for data on the effects of different anesthetics and pressure on the spontaneous curvature and, more generally, lipid monolayer lateral tensions. Most importantly, experiments must be performed to investigate whether protein function correlates with these quantities.

Recombination of carbon monoxide to ferrous horseradish peroxidase types A and C

The recombination of carbon monoxide to isoenzymes A2 and C of horseradish peroxidase (HRP) was studied as a function of temperature (2 to 320 K) and pH (5 to 8.3) with flash photolysis and infrared difference absorption. At low temperatures three geminate recombination processes are observed. One of these internal processes, denoted by I*, is exponential in time with a rate coefficient that deviates strongly from an Arrhenius behavior below 100 K, implying phonon-assisted tunneling. The two other processes, denoted by I, are non-exponential in time and related to different carbonyl isomers, as shown by the infrared difference spectra. The existence of three internal processes indicates that HRP differs considerably from myoglobin where only one internal process, I, is seen. Moreover, the internal processes in HRP are faster than process I in myoglobin. At 300 K, only one recombination process from the solvent is observed and it is very slow (lambda s approximately 1 s-1 at 1 atm CO (1 atm = 101,325 Pa)), much slower than the corresponding association process in myoglobin. Since process I is fast, but binding from the solvent is slow, the barrier at the heme cannot be responsible for the small association rate. The infrared absorption difference spectra of the amide I/II bands indicate that photolysis and recombination trigger a two-step structural change. The slow recombination rate at 300 K can thus be explained by the large Gibbs energy of the conformational transition that is necessary to let CO move into the heme pocket. The partition coefficient for the CO in the heme pocket and the solvent is extremely small, while bond formation with the heme iron occurs in less than 100 nanoseconds.

Automated pressure and temperature control apparatus for x‐ray powder diffraction studies

A system for performing x‐ray diffraction on biological samples as a function of pressure and temperature is described. It is capable of operating in a pressure range of 1 bar–3 kbar (0.1–300 MPa) and in a temperature range of −30 to 80 °C. The system incorporates microprocessor‐based pressure and temperature controllers which provide automated control with excellent stability characteristics: Fluctuations in pressure and temperature can be maintained within ±1 bar (0.1 MPa) and ±0.05 °C, respectively. Use of the apparatus is illustrated by application to a pressure‐induced phase transition in a lipid‐water liquid crystal.

High-pressure dilatometer

A high‐pressure dilatometer capable of measuring volume changes of 2×10−6 cm3 is described. Fractional volume changes of 1 part in 104 can be measured over a temperature range of −30–90 °C, and a pressure range of 1 bar (0.1 MPa)–3 kbar (300 MPa). The dilatometer is constructed out of a commercial high‐pressure valve and a compact home‐built pressure sensor. The system was specially designed to study lipid‐water biological systems. Preliminary studies on the lipid dioleoylphosphatidylethanolamine (DOPE) revealed the existence of phases not previously reported for this lipid.