Macromolecular structures polypeptides

Structural features that make molecules more rigid reduce rotational and vibrational contributions to entropy. Thus the formation of a double bond or ring decreases the entropy even when the molecular weight is unchanged. The formation of comparatively rigid macromolecular structures from flexible polypeptide or polynucleotide chains also requires an entropy decrease, although this can be offset by increases in the entropy of the surrounding water molecules (see chapter 4). 

The selectivity of the reaction also makes the peroxidase-catalyzed io-dination a very good tool for the study of the position of proteins within macromolecular structure such as membranes, ribosomes, and micellular polypeptides. Its use in this way is based on the fact that it is a high molecular weight protein and therefore does not readily penetrate these macromolecular structures. - " If the experimental conditions are correct, it catalyzes the halogenation selectively with those groups on the protein with which the enzyme has access. Thus, when the enzyme has access to proteins on only one side of the macromolecular structure such as the cell membrane, only the accessible proteins will be labeled with iodine. The labeled polypeptide of the macromolecular structure can then be isolated and identified. This provides a general method that can be applied to all macromolecular structures. 

If protein degradation is so quick, clearly it is a process that must be heavily controlled to avoid destruction of the wrong polypeptides. The degradation pathways are restricted to degradative subcellular organelles, such as lysosomes, or to macromolecular structures called proteasomes. Proteius are directed to lysosomes by specific signal sequences, often added in a posttranslational modification step. Once in the lysosome, the destruction is nonspecific. 

Fissi A, Pieroni O, Balestreri E, Amato C. 1996. Photoresponsive polypeptides. Photomodu lation of the macromolecular structure in poly(N((phenylazophenyl)sulfonyl) l lysine). Macromolecules 29(13) 4680 4685. 

The kinetics of cooperative processes in macromolecular structures, synthetic or biological, was developed further with his student R. H. Lacombe [Simha and Lacombe, 1971]. The authors also examined cooperative equilibria in copolymer systems of specified sequence structures. This implied solutions of the classical Ising problem for linear lattices. It had already been treated by the methods of statistical mechanics for homogeneous chains and, most recently, for copolymers. Lacombe and Simha showed how these problems could be dealt with advantageously by the method of detailed balancing of opposing rates [Lacombe and Simha, 1973,1974]. The results were examined for a spectrum of linear structures, chain lengths, and sequential distributions, such as he had computed, for example, with Jack Zimmermann for polypeptides [Zimmerman et al., 1968]. 

More investigations on polypeptides with known structural features will have to be made before exchange measurements can provide fairly specific information about macromolecular structure. Used in conjunction with other physical studies like ORD, CD, X-ray analysis, etc., exchange studies can provide useful information. In the detection of conformational changes, however, exchange studies can play a very useful role and most investigations of exchange reactions have been directed toward this end. 

Upon synthesis insoluble homopol5rmers of poly(L-Tyr) and poly(L-Ala) underwent self-assembly to form macromolecular structures. [51, 54] Polypeptide crystals have been... 

Fissi, A., Pieroni, O., Balestreri, E., and Amato, C., Photoresponsive polypeptides. Photomodulation of the macromolecular structure in poly(N ((phenylazophenyl)sulfonyl)-L-lysine)., Macromolecules, 29, 4680, 1996. 

Protein polymers based on Lys-25 were prepared by recombinant DNA (rDNA) technology and bacterial protein expression. The main advantage of this approach is the ability to directly produce high molecular weight polypeptides of exact amino acid sequence with high fidelity as required for this investigation. In contrast to conventional polymer synthesis, protein biosynthesis proceeds with near-absolute control of macromolecular architecture, i.e., size, composition, sequence, topology, and stereochemistry. Biosynthetic polyfa-amino acids) can be considered as model uniform polymers and may possess unique structures and, hence, materials properties, as a consequence of their sequence specificity [11]. Protein biosynthesis affords an opportunity to completely specify the primary structure of the polypeptide repeat and analyze the effect of sequence and structural uniformity on the properties of the protein network. 

Significant advances have been made in the preparation of discrete macromolecules that include both coenzyme function and a defined polypeptide or protein architecture. Preliminary, but promising, functional studies have been carried out and assay methods developed. While in many cases rather modest effects have been observed, what is significant is that the methodology exists to prepare, characterize, and study defined macromolecular constructs. With new information becoming available on co enzyme-dependent protein catalysts from structural biology and mechanistic enzymology, it should be possible to more fully exploit the remarkable breadth of coenzyme reactivity in tailored synthetic systems. 

This review has tried to present an overview of the control of enzymic activity in complex polyatomic frameworks. The examples discussed are intended to be representative obviously many other examples could be cited. The elementary interactions involved in modulating enzymic activity are well understood in terms of thermodynamics, kinetics, and structure. A considerable amount of information is also available for the simplest type of macromolecular framework, enzymes consisting of a single polypeptide chain, although a considerable amount of work remains to be done. 

Although the size and shape of proteins can have some influence on solubility properties, the chief method of exploiting these properties is gel-filtration chromatography. In addition, preparative gel electrophoresis makes use of differences in molecular size. Proteins range in size from the smallest classified as proteins rather than polypeptides, around 5000 Da, up to macromolecular complexes of many million daltons. Many proteins in the bioactive state are oligomers of more than one polypeptide (see below), and these can be dissociated, though normally with loss of overall structure. Thus many proteins have two sizes that of the native state, and that (or those) of the polypeptides in the denatured and dissociated state. Gel-filtration procedures normally deal only with native proteins, whereas electrophoretic procedures commonly involve separation of dissociated and denatured polypeptides. 

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