E. Suzuki

Chain conformation in the collagen molecule.

Abstract Quantitative X-ray diffraction data have been collected from stretched kangaroo tail tendon and used to test models for the conformation of the polypeptide chains in the collagen molecule. The magnitude of the unit twist of the molecular helix was estimated to be 107.1 ° ± 0.6 °, which is close to the value expected for a helix with ten units in three turns. The intensity data were used to carry out a linked-atom least-squares refinement of models based on two possible interchain hydrogen bonding schemes suggested by Rich & Crick (1955, 1961). No stereochemically acceptable solution could be found for the hydrogen bonding scheme of model I, but a stereochemically satisfactory solution was found for the scheme of model II which gave a crystallographic R factor of 0.272.

Molecular conformation and packing in collagen fibrils

New X-ray diffraction data have been collected from specimens of tendon collagen stained with phosphotungstic acid. Measurements of the positions of the Bragg reflections associated with the crystalline lattice provide, for the first time, a complete description of the unit cell. A strong band of intensity in the molecular transform associated with the pitch of the molecular helix can be identified and a detailed analysis of the intensities and positions of the Bragg reflections in this band has been carried out. The principal conclusions are that the portions of the collagen molecule that contribute to these reflections have a common direction; that they have a length very much less than that of a complete molecule; that the paths of the individual portions through the crystal are incompatible with a completely straight molecule, and that the molecule is therefore crimped. No evidence was obtained for a second series of Bragg reflections attributable to a second set of molecular portions linking the first set, and it is concluded that the linking set is more mobile and subject to positional variation from cell to cell. The most plausible explanation of our finding is that the first set corresponds to the portions of the molecules in the overlap zone and the second set to the portions in the gap zone. A detailed analysis of the Bragg reflections in the strong band of intensity associated with the pitch of the molecular helix has provided information about the relative azimuthal orientations and the lateral positions in the unit cell of the five molecular segments in the overlap zone. None of the existing models for fibril structure accounts satisfactorily for all the results obtained in the present studies and alternative models are developed and tested.

Structure of the α-keratin microfibril

Quantitative measurements of the intensity of the meridional reflections in the X-ray-diffraction pattern of α -keratin are shown to be consistent with a microfibril structure in which a surface lattice with an axially projected period around 200 A is subject to a periodic interruption with an axially projected period of 470 A. Taken in conjunction with recent evidence on the chemical structure of α-keratin and other inter-mediate filaments this finding enables an elaboration to be made of a model proposed earlier by RDB Fraser, TP MacRae, & E Suzuki (3. Mol. Biol.108, 435–452, 1976.) for the α-helical framework of the microfibrii. The disposition and connectivity of the helical segments suggested here provides a straightforward explanation of a number of recent physicochemical and electron-microscopical observations on intermediate filaments and provides a starting point for the development of models for the framework of other intermediate filaments.

The structure of feather keratin

Abstract When feather rachis is pressed in steam the keratin molecules are partially denatured and the x-ray diffraction pattern, which is greatly simplified, indicates that the microfibrils have a helical structure with four units per turn. Quantitative infra-red and x-ray data have been collected which suggest that this simplified pattern originates from about one third of the material in the form of a central core or framework which is resistant to denaturing agents. A model for this core consisting of a helical array of β-crystallites is shown to give quantitative agreement with the observed x-ray pattern over a wide range of angles. Each crystallite contains two sheets symmetrically disposed about the fibre axis and the sheets are distorted to conform to a ruled surface of opposite sense to the primitive helix.

Poly-L-alanylglycine

The sequential polymer poly- L -alanylglycine has been synthesized and shown to form a β-structure similar to that found in Bombyx mori silk fibroin. The chain arrangement is such that all the glycyl residues project on one side of the sheet and the alanyl residues on the other. The sheets are packed in pairs with the glycyl surfaces in contact. Structure factor calculations suggest that the inter-sheet distance across the glycyl-glycyl contact is significantly greater than in the model for silk fibroin proposed by Marsh, Corey & Pauling (1955) .

Intermediate filament structure: 2. Molecular interactions in the filament

Abstract Molecules of intermediate filament (IF) proteins contain a central rod domain in which the two constituent chains have a predominantly α-helical conformation and are coiled around one another to form segments of two-strand rope. Possible interactions between the two long segments, termed 1B and 2 were investigated by a technique successfully employed in studies of the modes of association of collagen molecules by Miller and coworkers. Prominent maxima were found in all of the six possible modes of association between the rod domain segments in individual IF proteins and certain maxima were found to be common to all IF. The surface lattice of the IF from α-keratin has been determined and possible bonding arrangements between the rod-domain segments are catalogued. A systematic search was carried out for combinations of interaction maxima which were consistent with the dimensions of the surface lattice. By the further application of stereochemical constraints, models for the topological arrangement of the rod-domain segments on the surface lattice were derived and these are illustrated and discussed.

Sequential polypeptides containing L-valyl and γ-methyl-L-glutamyl residues

A series of polypeptides containing ordered sequences of L -valyl (V) and γ-methyl- L -glutamyl (G) residues have been synthesized. The polymers (G x VG) n with x = 0, 1, 2, 3 can all be obtained in a helical conformation in solution. The stability of the helix decreases with decrease in x ; this may be explained in terms of the environment of the V residue on the surface of the α -helix. (G 3 VG) n and the additional polymers (G 3 V 2 G) n and (G 3 ) n gave α -type X-ray diffraction patterns overlaid by β -patterns; the remaining polymers gave β -patterns. α -Helical material could be detected, using infrared spectrometry, in all polymers except (VG) n and (V) n .

The structure of β-keratin

Abstract The structure of β-keratin has been re-investigated. A trial structure obtained from energy calculations was refined on the basis of quantitative X-ray diffraction and infra-red data. If a crystal model with inter-sheet packing disorder similar to that present in the β-form of poly-L-alanine is assumed, the observed data can be satisfactorily accounted for by an antiparallel chain pleated sheet conformation. An alternative model with intra-sheet disorder was also considered. A Fourier synthesis using the observed hk0 intensities and calculated phases yielded an electron density map in which the main chain atoms were clearly resolved.

X-ray diffraction and infrared studies of an α-helical fragment from α-keratin

Fragments of α-keratin were obtained by partial proteolysis of the low-sulphur protein fraction, S-carboxymethylkerateine-A, from wool. X-ray diffraction and infrared absorption studies were made on oriented films formed from these fragments. The results indicate that the fragments have a coiled-coil α-helical conformation similar to that in α-keratins, and that the films consist of particles approximately 160 A in length arranged end-to-end.

Sequential polypeptides containing S-benzyl-l-cysteinyl and γ-ethyl-l-glutamyl residues

A series of polypeptides containing ordered sequences of S-benzyl- L -cysteinyl (C) and γ -ethyl- L -glutamyl (G) residues has been synthesized. Optical rotatory dispersion measurements show that all the polymers adopt some degree of helical conformation in solution. Using polarized infra-red spectrometry α-helical material could be detected in films of all the polymers, as in solution, but always in lower amount than in the parent (G 2 ) n homopolymer † . The polymers (G 3 CG) n , (GCG) n and (GCG 2 ) n gave α -type X-ray diffraction patterns; the remainder gave β -type patterns. In general the stability of the α -helix is lowered by the introduction of C residues. It is suggested that this is due to an unfavourable side chain—main chain interaction.