Globular-chain crystal

Note Globular-chain crystals usually occur with globular proteins. 

Figure 8 displays a transmission electron micrograph of a dried dispersion of fully polymerized PTFE. This micrograph reveals the globular nature of common, commercial PTFE particles. More detailed studies on these particles revealed that these entities are not polycrystalline, but, in fact, are ribbon, or rodlike extended chain crystals that are wrapped around themselves in a more or less random manner (23). 

The hardness of only one type of protein crystal has been found in the literature. It is for lysozyme. This is an enzyme found in egg whites and tears. It destroys bacterial membranes. It is relatively small for a protein molecule, consisting of a chain of 129 amino acids folded into a globule with the volume = 30,000 A3. Its crystals are aggregates of these globular molecules held together by London forces (Stryer, 1988). 

The recommendations embodied in this document are concerned with the terminology relating to the structure of crystalline polymers and the process of macromolecular crystallization. The document is limited to systems exhibiting crystallinity in the classical sense of three-dimensionally periodic regularity. The recommendations deal primarily with crystal structures that are comprised of essentially rectilinear, parallel-packed polymer chains, and secondarily, with those composed of so-called globular macromolecules. Since the latter are biological in nature, they are not covered in detail here. In general, macromolecular systems with mesophases are also omitted, but crystalline polymers with conformational disorder are included. 

The exact disposition of the side chains in a globular protein is difficult to define in solution. Although it is likely that the peptide main chain (backbone) of the protein is relatively rigid, the side chains have been shown to be undergoing motion of several different types (see lysozyme, peroxidase, and carboxypeptidase). This means that the full definition of atomic positions in the structure requires a knowledge of the time dependence of their coordinates. The motion of side chains is likely to be different in the crystal and solution states, but this difference may well... 

View of a modified bovine fibrinogen molecule. The 45-nm-long disulfide-linked dimer is composed of three nonidentical polypeptide chains. The N termini of the six chains from the two halves come together in the center in a small globular "disulfide knot." The C termini form globular domains at the ends. The 340-kDa molecule has been treated with a lysine-specific protease which has removed portions of two chains to give the 285-kDa molecule whose crystal structure is shown. Arrows point to attached oligosaccharides. From Brown et at. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 85-90. Courtesy of Carolyn Cohen. 

In crystal structures the two chains of insulin form highly ordered globular structures (8). Two main structural types form depending on crystallization conditions. In both structures the A-chains form two CC-helical segments, from residues Al—A8 and A13—A19, which are connected by a turn. In the structure referred to as the T-state, the B-chain contains two regions of extended chain, Bl—B8 and B21—B30, connected by an Ot-helix from B9-B19. In the R-state structure, the B-chain helix extends from Bl—B19. The crystallographic T-state structure best matches the solution structure of insulin determined by nmr (9), although the R-state can be induced in solution under the appropriate conditions. The surface of insulin which interacts with the insulin receptor includes the N- and C-termini of the A-chain and the C-terminus of the B-chain. 

Studies of the myosin head, the globular part of the heavy chain together with the ELC and RLC, were dramatically transformed when the isolated myosin head from chicken skeletal muscle myosin was crystallized and its structure solved using protein crystallography by Rayment et al. (1993b Fig. 4A). This showed that the head consists of a globular... 

Two crystalline forms of ascorbate oxidase from zucchini (Messerschmidt et al., 1989) have been analysed at 2.5 A resolution and a model of the polypeptide chain and the copper ions and their ligands has been prepared. The crystal forms M2 and Ml contain a dimer of 140000 Mr and a tetramer of 280000 Mr in the asymmetric unit. Each subunit of about 550 amino acid residues has a globular shape with dimensions of 49 A x 53 A x 65 A. The subunit has three domains arranged sequentially... 

Amino acids of the general form, 1, are the monomeric molecules which are condensed to form the polypeptide chains of the fibrous and globular proteins. The naturally occurring molecules are the L-enantiomers, shown in 1 for chemical formulae see Fig. 19.1. D-amino acids can be synthesized and the individual L- or D-amino acids or the D,Lrracemates can be crystallized. All the common amino acids have been studied by neutron or X-ray crystal structure analysis (see Thble 14.1), in the anhydrous or hydrate forms, as hydrochlorides or hydrochloride hydrates. 

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