Polypeptides definition

Short chains of amino acid residues are known as di-, tri-, tetrapeptide, and so on, but as the number of residues increases the general names oligopeptide and polypeptide are used. When the number of chains grow to hundreds, the name protein is used. There is no definite point at which the name polypeptide is dropped for protein. Twenty common amino acids appear regularly in peptides and proteins of all species. Each has a distinctive side chain (R in Figure 45.3) varying in size, charge, and chemical reactivity. 

Note The formal lUPAC-IUB Commission on Biochemical Nomenclature convention for the definition of the torsion angles polypeptide chain (Biochemistry 9 3471-3479, 1970) is different from that used here, where the atom serves as the point of reference for both rotations, but the result is the same. (Irving Gas)... 

The aldehyde oxidoreductase from Desulfovibrio gigas shows 52% sequence identity with xanthine oxidase (199, 212) and is, so far, the single representative of the xanthine oxidase family. The 3D structure of MOP was analyzed at 1.8 A resolution in several states oxidized, reduced, desulfo and sulfo forms, and alcohol-bound (200), which has allowed more precise definition of the metal coordination site and contributed to the understanding of its role in catalysis. The overall structure, composed of a single polypeptide of 907 amino acid residues, is organized into four domains two N-terminus smaller domains, which bind the two types of [2Fe-2S] centers and two much larger domains, which harbor the molybdopterin cofactor, deeply buried in the molecule (Fig. 10). The pterin cofactor is present as a cytosine dinucleotide (MCD) and is 15 A away from the molecular surface,... 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

A protein, of course, is a polypeptide chain made up of amino acid residues linked together in a definite sequence. Amino acids are handed (except for glycine, in which the normally asymmetric o-carbon has two hydrogens), and naturally occurring proteins contain only L-amino acids. That handedness has far-reaching effects on protein structure, as we shall see, and it is very useful to be able to distin-... 

A stretch of DNA that is transcribed as a single continuous RNA strand is called a transcription unit. A unit of transcription may contain one or more sequences encoding different polypeptide chains (translational open reading frames, ORF) or cistrons. The transcription unit is sometimes termed the primary transcript, pre-messenger RNA or heterogeneous nuclear RNA (hnRNA). The primary transcript is further processed to produce mRNA in a form that is relatively stable and readily participates in translation. In order to understand the primary need for processing of this RNA, the biochemical definition of a gene must be discussed. 

The information available in this polypeptide or these polyeptides is far from definitive. Controversy exists on size and molecular weight of the apo LDL monomers, their number, and chemical properties (Table 8). Also, the question of polypeptide heterogeneity has not been resolved this is not surprising in view of the difficulty of protein solubilization discussed earlier. At the time of this writing, apo LDL is the mostly poorly characterized protein of all serum lipoproteins. This apoprotein is... 

The development of technology for the isolation, fractionation, and purification of the principal polypeptides of VLDL, LDL, and HDL is undoubtedly a major achievement in the area of lipoprotein research. The availability of such polypeptides in pure form has permitted studies on their characterization and the definition of their chemical, physical, and immunological properties. The primary structure of many of these poly-... 

Prerequisites for the use of a protein or polypeptide as calibrator in PAGE are the accessibility in a pure, definite state, its known... 

By analogous considerations, glycoproteins may be described as macromolecules composed of one or more polypeptide chains that are formed by the normal processes of translation at the polysomal level and to which sugar molecules are attached through covalent bonds. There are a number of definitions of glycoproteins, and these have been fully discussed. The definition employed here is wider than those adopted by many workers in the past, as it has a number of advantages in the present discussion. 

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