Polypeptide A polymer composed of amino

Polypeptide A polymer composed of amino acids linked by peptide bonds. 

Polypeptides are polymers composed of amino acids linked by peptide bonds. The order of the amino acids in a polypeptide is called the amino acid sequence. Disulfide bridges, formed by the oxidation of cysteine residues, are an important structural element in polypeptides and proteins. 

Polypeptides, however, are composed of amino acids with side chains that are longer and therefore the area of allowed conformations is reduced when an alanine (Figure 2.12), aspartic acid (Figure 2.13), or a proline (Figure 2.14) is added to the second peptide unit. Finally, the conformational map for a dipeptide of proline-hydroxyproline is dramatically reduced. Rings in the backbone of any polymer reduce the ability of the polymer backbone to adopt numerous conformations and thereby stiffen the structure. 

PEPTIDE BOND FORMATION Polypeptides are linear polymers composed of amino acids linked together by peptide bonds. Peptide bonds (Figure 5.11) are amide linkages formed when the unshared electron pair of the a-ami no nitrogen atom of one amino acid attacks the a-carboxyl carbon of another in a nucleophilic acyl substitution reaction. A generalized acyl substitution reaction is shown ... 

Peptides and proteins are composed of amino acids polymerized together through the formation of peptide (amide) bonds. The peptide bonded polymer that forms the backbone of polypeptide structure is called the a-chain. The peptide bonds of the a-chain are rigid planar units formed by the reaction of the oc-amino group of one amino acid with the a-carboxyl group of another (Figure 1.1). The peptide bond possesses no rotational freedom due to the partial double bond character of the carbonyl-amino amide bond. The bonds around the oc-carbon atom, however, are true single bonds with considerable freedom of movement. 

The absence of side-chain contributions to gross polymer dimensions in 6M GuHCl can be seen by the results obtained with cytochrome-c and the cytochrome-c peptide composed of residues 1-65. Both of these polypeptides contain in addition to a normal complement of amino acids a bulky heme group. However the data of Table III clearly show that even the addition of this large side-chain substituent does not produce anomolous behavior. 

There are three main classes of biopolymers polynucleotides (RNA and DNA), which are long polymers composed of 13 or more nucleotide monomers polypeptides, which are short polymers of amino acids and polysaccharides, which are often linear bonded polymeric carbohydrate structures. Terms such as biopolymers, bioplastics, and biodegradable plastics are used synonymously in certain contexts however,each has a unique meaning [91, 93] (Fig. 3). 

The extension of the term isotactic to condensation polymers was made by Natta in his first article discussing poly-L-a-amino acids (22). In itself the term isotactic is redundant here as the configuration of the repeating unit is sufficient to identify the macromolecular stmcture. It is, however, useful to distinguish a system, racemic or not, in which each macromolecule is composed of only l or only D residues from a mixture of macromolecules made up of random or alternate sequences of L or d units. Similarly the term syndiotactic serves in the identification of oligopeptides or polypeptides composed of alternate sequences of D and L units, like those synthesized by Lorenzi and Tomasic (77). 

Proteins are informational macromolecules, the ultimate heirs of the genetic information encoded in the sequence of nucleotide bases within the chromosomes. Each protein is composed of one or more polypeptide chains, and each peptide chain is a linear polymer of amino acids. The order of the amino acids commonly found in the polypeptide chain is determined by the order of nucleotides in the corresponding messenger RNA template. In this chapter we examine four aspects of protein metabolism (fig. 29.1) (1) The process whereby amino acids are ordered and polymerized into polypeptide chains (2) posttranslational alterations in polypeptides, which occur after they are assembled on the ribosome (3) the targeting process whereby proteins move from their site of synthesis to their sites of function and (4) the proteolytic reactions that result in the return of proteins to their starting material, amino acids. 

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