Chemical modifications denaturation

Chemical modification of proteins — This can lead to denaturation and aggregation and reduce both specificity (increased nonspecific binding) and sensitivity (decreased ligand affinity). 

The delipidated serum lipoprotein proteins exhibit solubility differences in aqueous media. The polypeptides of HDL and the D polypeptides of VLDL are readily soluble in aqueous media, particularly in slightly alkaline low-ionic strength buffers (S28, S30). In contrast, the LDL protein does not dissolve in such buffers and, like many other water-insoluble proteins, requires denaturing agents, detergents, or suitable chemical modification. The many techniques for the solubilization of apo LDL have been reviewed recently (G15). A thorough assessment of such techniques is not possible since not all the solubilized products have been characterized. The choice of the method presently depends on the investigator s preference and experimental needs. 

Any chemical or enzymatic process that modifies a therapeutic protein in the body is known as protein metabolism. Unfolded proteins are, in general, more susceptible to metabolism via proteolysis because of increased access to critical peptide sequences. Chemical modification of a protein can mark it for further degradation. This is thought to occur by altering the folding equilibrium in favor of the denatured conformation [3]. 

The sulfur atom of methionine residues may be modified by formation of sulfonium salts or by oxidation to sulfoxides or the sulfone. The cyanosulfonium salt is not particularly useful for chemical modification studies because of the tendency for cyclization and chain cleavage (129). This fact, of course, makes it very useful in sequence work. Normally, the methionine residues of RNase can only be modified after denaturation of the protein, i.e., in acid pH, urea, detergents, etc. On treatment with iodoacetate or hydrogen peroxide, derivatives with more than one sulfonium or sulfoxide group did not form active enzymes on removal of the denaturing agent (130) [see, however, Jori et al. (131)]. There was an indication of some active monosubstituted derivatives (130, 132). 

There is no single or simple way to purify all proteins. Procedures and conditions used in the purification of one protein may result in the denaturation of another. Further, slight chemical modifications in a protein may greatly alter its structure and thus affect its behavior during purification. Nevertheless, there are certain fundamental principles of protein purification on which most fractionation procedures are based. In all cases, the experimenter takes advantage of small differences in the physical and/or chemical properties of the many proteins in a crude mixture to bring about separation of the proteins from one another. Since the precise nature of these differences is not usually known in advance, the development of a purification procedure necessarily involves considerable trial and error. A procedure leading to a... 

Following translation into the polypeptidyl chain, proteins undergo not only specific chemical modifications but also many nonspecific modifications. Some modifications are the result of continual exposure of the proteins to the potential action of proteolytic enzymes. The level of a specific protein in vivo is the result of a balance achieved between the rate of biosynthesis of that protein and its rate of degradation by proteolytic enzymes. In part, the rate of degradation of a protein is a function of the relative levels of native (N) and reversibly (R) denatured protein (N R) under a given set of conditions. Nutritional state and health of the organism, the extent of posttranslational modification, and environ-... 

To achieve success as protein ingredients for food formulation and fabrication, novel proteins should possess a range of functional properties. Frequently during extraction, refining and drying, plant and yeast proteins, intended for food uses, become denatured or altered and subsequently display poor functional properties which render them of limited use. Chemical modification provides a feasible method for improving the functional properties of plant and yeast proteins and potentially may make it possible to tailor proteins with very specific functional properties. In this review the information on modified plant proteins is reviewed and the use of succinylation for the recovery of yeast proteins with low nucleic acid is described. 

The labilized conformation of mutated thermophilic enzymes results in a significantly higher susceptibility towards chemical modification. Increased susceptibility to thermogenic destruction is also observed after the native conformation is disrupted by preincubation with denaturing reagents [29]. 

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