Perfect crystals proteins

The next step is for a protein crystallographer to mount a small perfect crystal in a closed silica capillary tube and to use an X-ray camera to record diffraction patterns such as that in Fig. 3-20. These patterns indicate how perfectly the crystal is formed and how well it diffracts X-rays. The patterns are also used to calculate the dimensions of the unit cell and to assign the crystal to one of the seven crystal systems and one of the 65 enantiomorphic space groups. This provides important information about the relationship of one molecule to another within the unit cell of the crystal. The unit cell (Fig. 3-21) is a parallelopiped... 

Most protein crystals do not diffract to true atomic resolution. Due to an inherent lack of perfection in protein crystals the high angle component of the diffraction pattern, which contains the fine detail, is lost. For this reason the protein sequence must generally be known. 

Once conditions for nucleation and growth have been identified and the investigation of variables more or less complete, the concentration of the protein should be gradually reduced in increments to moderate the growth of the crystals. As a rule, the largest and most perfect crystals result when the rate of incorporation of molecules is slow and orderly. Reduction of macromolecule concentration is an effective means for controlling this. 

One enzyme form that has received considerable attention is based on enzyme crystals. Production of protein microcrystals from aqueous solution is often quite easy, and is increasingly used as a step in the manufacture of enzymes on an industrial scale. (Many people have the impression that protein crystallization is very difficult, but this stems from the problems in growing large near-perfect crystals for... 

Three different forms of biological iron oxides appear to have distinct relationships to the proteins, lipids, or carbohydrates associated with their formation and with the degree of crystallinity. Magnetite, on the one hand, often forms almost perfect crystals inside lipid vesicles of magneto-bacteria. Ferrihydrite,... 

Certain proteins have been fouhd which form nearly perfect crystals, and some of these melt sharply under appropriate conditions. 

The experimental technique used to determine the tertiary structure of a protein is X-ray crystallography. Perfect crystals of some proteins can he grown under carefully controlled conditions. In such a crystal, aU the individual protein molecules have the same three-dimensional conformation and the same orientation. Crystals of this quality can he formed only from proteins of very high purity, and it is not possible to obtain a structure if the protein cannot be crystallized. 

Furthermore, membrane crystallisation also presents a viable and practical route to present molecules of pharmaceutical interest, such as proteins, in the form of perfect crystals. This is an important target for research, as the identification of protein-activity relationships can accelerate the design of new generation drugs, as well as assist in the development of advanced diagnostic tools and effective therapies for the treatment of human diseases. 

A growing number of protein crystal structures has provided solid evidence that in many phosphoesterase enzymes, two and sometimes even three, di- or trivalent metal ions are involved in substrate transformation. Consequently, the high catalytic efficiency is, in part, the result of a perfectly coordinated catalytic cooperation of the metal ions. Dinu-clear phosphoiyl transfer enzymes have been discussed thoroughly in recent reviews [1-3]. Therefore, this chapter (Section 2) only gives a brief description of enzymes for which two-metal promotion of phos-phoester hydrolysis was proposed on the basis of detailed mechanistic or crystallographic studies (Table 1). 

New glycolipids have to be synthesized to get further insights into liquid crystal properties (mainly lyotropic liquid crystals), surfactant properties (useful in the extraction of membrane proteins), and factors that govern vesicle formation, stability and tightness. New techniques have to be perfected in order to allow to make precise measurements of thermodynamic and kinetic parameters of binding in 3D-systems and to refine those already avalaible with 2D-arrays. Furthermore, molecular mechanics calculations should also be improved to afford a better modeling of the conformations of carbohydrates at interfaces, in relation with physical measurements such as NMR. 

Sander and Schneider performed an empirical determination of homology thresholds by studying thousands of sequence alignments within the PDB database [19]. One has to keep in mind that perfect sequence similarity not always implies perfect structural agreement a protein crystal structure may vary, typically in loop regions or in domain orientation, as a result of different substrate or cofactor interaction, complex formation or crystal contacts. The curve in Figure 4, however, provides a reliable rule of thumb for those who wonder if their enzyme can be modeled with any degree of confidence. 

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