Proteins, polypeptide chain

Model analogs of the green type chromophore HBI have been chemically synthe-tized in different forms carrying blocking groups in place of the protein polypeptide chain [21, 24, 68, 69]. However, the covalent structure of HBI does not uniquely define its optical properties, because the molecule undergoes several protonation and conformational equilibria that directly affect its electronic structure. 

Peptides are present naturally in foods, arising mainly from the partial degradation of protein polypeptide chains. Some peptides, like the dipeptides carnosine, anserine, and balenine in vertebrate muscle or glutathione in fruits, are not proteinic in origin. In other cases, peptides are present in foods because they are used as additives (sweeteners, flavor enhancers, bulking agents in light drinks, etc.). 

Linus Pauling and Robert Corey examined the structures of crystals formed by amino acids and short peptides before they ventured into the world of proteins. From their crystallographic investigations of amino acids and peptides, they formulated two rules that describe the ways in which amino acids and peptides interact with one another to form nonco-valently bonded crystalline structures. These rules laid the foundations for our understanding of how amino acids in protein polypeptide chains interact with one another. 

K.G. Mann and W.W. Fish, Protein polypeptide chain molecular weights by gel chromatography in guanidinium chloride, in C.H.W. Hirs and S.N. Timasheff (Eds.), Methods in Enzymology, Vol. XXVI, Enzyme Structure, Part C, Academic Press, New York, 1972, pp. 28-42. 

More recently Lapanje and Tanford (59) have reported osmotic pressure measurements for reduced protein polypeptide chains in 6M guanidine hydrochloride. Second virial coefficient data and intrinsic viscosity data are combined by these authors to yield unperturbed dimensions of randomly coiled proteins. The result is assentially identical with that obtained earlier from viscosity data alone. 

Different polypeptide chains may interact to form more complex multi-polypeptide proteins, wherein each individual polypeptide is known as a subunit. Subunit interactions and interrelationships are illustrated for the tetrameric protein haemoglobin (Figure 1.32). In the case of globular proteins, polypeptide chain association allows functions of individual polypeptide elements to be coordinated or indeed supplemented to give the whole molecule the opportunity to perform multiple biological functions. In the case of hbrous proteins, quaternary structure formation enhances overall molecular strength. 

It is less clear why the cofactor-like subunits of iron enzymes also have to be large proteins. Polypeptide chains of 35,000 to 55,000 dalton predominate here. They usually occur in pairs of two but there are cases where only one subunit of M, 75,000 to 80,000 is found which is not further dissociable. The size could be favorable for Integration of the reductase holoenzyme into even larger multienzyme aggregates, discussed in Sect. E. 

Surface Graft Polymerization of Proteins Polypeptide chains contain free carboxyUc and amine residues at their isoelectric point. These residues are mostly responsible for the polarity of polypeptides and their inclination toward chemical reactions. In addition, presence of sulfur and hydroxyl residues provides active sites for crosslinking reactions by these biopolymers. Peptide bonds are under normal conditions... 

One of the present tasks of biochemistry is the determination of the order in which the amino adds are assembled in the protein polypeptide chains and the description of the structure of that protein. It is first of all necessary to know the total protein nitre en (around 16%) and the total... 

Table IX shows the amino add composition of a collection of proteins whose analysis is complete or almost complete. As can be seen, such a table does not permit any useful conclusion to be drawn from it. Thus attention is focussed on the sequence of the amino acids in the protein polypeptide chains.

F1gur 2.29 Oxidative cieavage of protein polypeptide chain. Stadtman and Levine 2003. With kind permission from Springer Science and Business Media. 

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