Lens, crystallin structure

The frequency of the amide I peak observed in the lens is sensitive to protein secondary structure. From its absolute position at 1672 cm-1, which is indicative for an antiparallel pleated 3-sheet structure, and the absence of lines in the 1630-1654 cm-1 region, which would be indicative of parallel (1-sheet and a-helix structures, the authors could conclude that the lens proteins are all organized in an antiparallel, pleated 3-sheet structure [3]. Schachar and Solin [4] reached the same conclusion for the protein structure by measuring the amide I band depolarization ratios of lens crystallins in excised bovine lenses. Later, the Raman-deduced protein structure findings of these two groups were confirmed by x-ray crystallography. 

Lack of congruence of sequence and structure with function Common sequence and structure, indeed identity of the protein itself, do not imply a unique function Each pair, o-succinylbenzoate synthase (OSBS)-N-acetylamino acid racemase, and lens crystallin-lactate dehydrogenase, share sequence and structure but differ in function. 

Figure 7.9. Thermal stabilities of eye lens crystallins of differently thermally adapted vertebrates. The temperature (°C) at which 50% loss of secondary structure occurred, as measured using CD spectroscopy, is given as a function of the maximal body temperature of each species. Species (1) Pagothenia borchgrevinki (Antarctic fish), (2) Coryphaenoides armatus (deep-sea fish), (3) Coryphaen-oides rupestris (deep-sea fish),...

The single analytical instrument that is appropriate for analysis of waterborne asbestos, then, is transmission electron microscopy (TEM). This high-resolution instrument clearly displays the narrowest asbestos fibers on its bright phosphor screen at magnifications of 10,000 X to 20,000 X. When the intermediate lens is focused on the back focal plane of the image, electron-diffraction (ED) patterns can be produced. Amphibole or chrysotile crystalline structures can be positively identified by measuring these patterns. Finally, specific species of amphiboles can be differentiated by EDX detectors inserted near the specimen holder [18]. 

The major structural protein components of the lens are the crystallins whose function is to maintain transparency and refractive power. They constitute about 90% of the soluble proteins in the lens and consist of two superfamilies a and (/3H/3L and y) with molecular masses of between 900 and 21 kD (Table 9.1). 

Small angle X-ray-scattering studies and light-scattering studies of lens extracts show that the transparency of the lens is the result of the short-range spatial order of lens proteins (Delaye and Tardieu, 1983). The molecular structure of the individual crystallins is well ordered even though the overall pattern in the lens may appear chaotic. 

Ji, X., von Rosenvinge, E.C., Johnson, W.W., Tomarev, S.I., Piadgorsky, J., Armstrong, R.N., and Gilliland, G.L. (1995) Three-dimensional structure, catalytic properties, and evolution of a sigma class glutathione transferase from squid, a progenitor of the lens S-crystallins of cephalopods. Biochemistry 34(16), 5317-5328. 

Perng, M. D., Muchowski, P. J., van Den, I. P., Wu, G. J., Hutcheson, A. M., Clark, J. I., and Quinlan, R. A. (1999b). The cardiomyopathy and lens cataract mutation in alphaB-crystallin alters its protein structure, chaperone activity, and interaction with intermediate filaments in vitro. J. Biol. Chem. 274, 33235-33243. 

Some crystallins are (as far as we know) ubiquitous in vertebrates. Of these aA-crystallin is the most abundant and has been used in large-scale phylogenetic analyses, using classic techniques of isolation and sequencing.6 However, it has recently been appreciated that other major crystallins show remarkably taxon-specific patterns of expression (Fig. 1). Furthermore, these taxon-specific crystallins are all identical to enzymes, or else rather recently derived from enzymes (Table I). Some modification in a single functional gene led to the acquisition by the protein product of a dual function as both enzyme and structural lens protein. In this model gene recruitment comes first duplication may or may not follow.7... 

Lens The lens is the transparent biconvex structure situated behind the iris and in front of the vitreous. It plays an important role in the visual function of the eye and also enables accommodation together with the ciliary muscle. The lens is made up of slightly more than 30% protein (water-soluble crystallins) and therefore has the highest protein content of all tissues in the body [20], The lens receives its nutrients from the aqueous humor and its transparency depends on the geometry of the lens fibres. 

The roles of the extensions in /3-crystallin subunit structure and function are matters of debate. It would seem, however, that they are involved in regulating interactions between the /3-crystallin subunits and their interactions with the Gland 7-crystallin subunits. As the concentration of proteins within the lens is very high, these interactions are of great importance in ensuring crystallin protein solubility and hence the maintenance of lens transparency. However, the specifics of which /3-crystallin subunits interact with each other, and the role of the extensions in this process, are not known, and extensive NMR analysis is required to elucidate the hierarchy of subunit interactions. Furthermore, the extensions in the /3-crystallin subunits undergo extensive post-translational modification, especially proteolysis, and these age-related changes alter the electrostatic interactions between the subunits. 

This condition can be obtained in a defocused condition of the objective lens. As a result, an image of phase contrast may be interpreted in terms of periodic structures of a crystalline solid. Such an image of phase contrast is called the interpretable structure image. 

A diffraction pattern is formed on the back-focal plane of the objective lens when an electron beam passes through a crystalline specimen in a TEM. In the diffraction mode, a pattern of selected area diffraction (SAD) can be further enlarged on the screen or recorded by a camera as illustrated in Figure 3.16. Electron diffraction is not only useful to generate images of diffraction contrast, but also for crystal structure analysis, similar to X-ray diffraction methods. SAD in a TEM, however, shows its special characteristics compared with X-ray diffraction, as summarized in Table 3.4. More detailed SAD characteristics are introduced in the following section. 

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