The transmembrane domain of the nicotinic acetylcholine receptor (nAChR) from Torpedo californica electric tissue contains both alpha‐helical and beta structures. The secondary structure was investigated by Fourier transform infrared (FTIR) spectroscopy after the extramembrane moieties of the protein from the extracellular and intracellular sides of the membrane were removed by proteolysis using proteinase K. The secondary structure composition of this membrane structure was: alpha‐helical 50%, beta structure and turns 40%, random 10%. The alpha‐helices are shown to be oriented with respect to the membrane plane in a way allowing them to span the membrane, while no unidirectional structure for the beta structures was observed. These findings contradict previous secondary structure models based on hydropathy plots alone.

The secondary structure of the acetylcholinesterase and its temperature behaviour have been investigated using Fourier-transform infrared (FTIR) spectroscopy. The data are compared to the structure obtained by X-ray analysis of the crystalline enzyme. The secondary structure was determined using the spectral features observed in the amide-I band (H,O buffer) and amide-I' band (D,O buffer) at 1600-1700 cm-I, taking advantage of resolution-enhancement techniques along with least-squares band-fitting procedures. The relative amounts of different secondary-structure elements, 34-36% for a-helices, 19-25% for p-sheets, 15-16% for turns and 13-17% for irregular structures, were estimated. These data, obtained with the enzyme in solution, correlate well with X-ray data of the crystalline protein [Sussman, J. L., Hard, M., Frolow, F., Oefner, C., Goldman, A., Toker, L. The relationship between the thermally induced loss of enzyme activity and secondary-structure changes has also been investigated. The decrease in enzyme activity to zero at 30-40 "C was accompanied only by minor changes in the secondary structure. At 55-6OoC, denaturation of AChE occurs. In this temperature range, all bands assigned to the various secondary-structure elements abruptly disappear in a co-operative and irreversible manner, whereas the &aggregation bands (at 1622 cm-' and the corresponding high-frequency band) increase in intensity at the same rate.Acetylcholinesterase (AChE) is a key molecule involved in nerve-impulse transmission at cholinergic synapses. It rapidly hydrolyses the neurotransmitter acetylcholine and thus terminates the signal. As the main point of attack of most insecticides and chemical-warfare agents (nerve gases), AChE is of considerable interest. Moreover, structural research on AChFi is relevant because AChE is the best characterized member of the protein family of serine esterases, as defined on the basis of sequence homology (Myers et al., 1988;Krejci et al., 1991;Ollis et al., 1992). Since disulfide bridges are highly conserved within this protein family, it is plausible that AChE shares secondary-structure and tertiarystructure motifs with these related proteins.The three-dimensional structure of crystallized AChEi has been recently reported (Sussman et al., 1991). The AChE monomer is an ellipsoidally shaped alp-protein, with dimensions of 4.3 nm X 6.0 nm X 6.5 nm, which contains a 12-stranded mixed P-sheet surrounded by 14 a-helices. From the X-ray data, it can be concluded that the a-structure content is approximately 32% and that the p-structure content is 15 %. Even with detailed structural information from X-ray crystallography, spectroscopic methods for investigating secondary structure have not lost their attraction. Firstly, spectroscopic methods allow the direct measurement of proteins in solution (in a more native state) and they have considerable potential for exploring conformational changes, such as temperature-dependent processes, which might not, in principle, be amenable to X-ray analysis. Secondly, ...

No abstract

The binding nnd intcnerion of wrbnmaylcholinc with the nicotinis ncclylcholinc rcccp~or was invcstignkd using photolytiwlly rclczecd carbum-

No abstract