Peptides and polypeptides

Amino acids can link together by a covalent peptide bond between the a-carboxyl end of one amino acid and the a-amino end of another. Formally, this bond is formed by the loss of a water molecule, as shown in figure 3.9. The peptide bond has partial double-bond character owing to resonance effects as a result, the C—N peptide linkage and all of the atoms directly connected to C and N lie in a planar configuration called the amide plane. In the following chap- 

Formation of a dipepetide from two amino acids, (a) Two amino acids, (b) A peptide bond (CO—NH) links amino acids by joining the a-carboxyl group of one with the a-amino group of another. A water molecule is lost in the reaction. It is conventional to draw dipeptides and polypeptides so that their free amino terminus is to the left and their free carboxyl terminus is to the right. The amide plane refers to six atoms that lie in the same plane. (Illustration copyright by Irving Geis. Reprinted by permission.) 

Any number of amino acids can be joined by successive peptide linkages, forming a polypeptide chain. The polypeptide chain, like the dipeptide, has a directional sense. One end, called the N-terminal, or amino-terminal, end, has a free a-amino group, whereas the other end, the C-terminal, or carboxyl-terminal, end, has a free a-carboxyl group. The sequence of main-chain atoms from the N-terminal end to the C-terminal end is N—Ca—C—N and so on, and in the opposite direction it is C—Ca—N—C and so on. Short polypeptide chains, up to a length of about 20 amino acids, are called peptides or oligopeptides if they are fragments of whole polypeptide chains. A small protein mole- 

In addition to the covalent peptide bonds formed between adjacent amino acids within a polypeptide chain, covalent disulfide bonds can be formed within the same polypeptide chain or between different polypeptide chains (fig. 3.11). Such disulfide linkages have an important stabilizing influence on the structures formed by many proteins (see chapter 4). 

A polypeptide chain, with the backbone shown in color and the amino acid side chains in outline. The polypeptide chain is oriented so that the C-terminal end (not shown) is to the left. (Illustration copyright by Irving Geis. Reprinted by permission.) 

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The amide linkage between monomer units in a protein is called a peptide bond. Peptides and polypeptides, which often exhibit biological activity (see Antibiotics, peptides Neuroregulators), are smaller than proteins. Although the differentiation between polypeptide and protein is somewhat arbitrary, the usual distinction is drawn around 100 monomer units. Proteins are also characterized by higher levels of stmcture resulting from internal interactions. 

Hydrogels Based on [l-Sheet Structures of Peptides and Polypeptides. 146... 

A number of other peptide molecules are currently being explored for delivery via inhalation (6). Very recently, a much smaller peptide (leuprolide, about 9 amino acid residues) has been delivered by metered dose inhaler (MDI) in a characterized fashion to humans (7). This work revealed that about 50% of a dose deposited in the lung could be bioavailable. This value is much greater than those reported for nasal bioavailabilities of this and similar molecules (8). These results, and ours in the rat lung (9), imply that inhalation administration of some peptide and polypeptide molecules is perfectly feasible. 

The words peptide and polypeptide are both used to describe chains of amino acids linked by peptide bonds. Peptides are short chains of amino acids that do not form part of a protein. The number of amino acids is indicated by a prefix (e.g. dipeptide, hexapeptide, oligopeptide). Polypeptides (or polypeptide chains) are longer sequences which can form part of a protein, which may consist of several polypeptides (Appendix 3.1). 

Biologically Active Peptides and Polypeptides Occur in a Vast Range of Sizes... 

The Building Blocks of Proteins Amino Acids, Peptides, and Polypeptides 49 The Three-Dimensional Structures of Proteins 72... 

All Amino Acids except Glycine Show Asymmetry Peptides and Polypeptides... 

Determination of Amino Acid Composition of Proteins Determination of Amino Acid Sequence of Proteins Chemical Synthesis of Peptides and Polypeptides... 

In the first three chapters of part 2 we discuss the basic structural and chemical properties of proteins. In this chapter we concentrate on the structural and chemical properties of amino acids, peptides, and polypeptides—the building blocks of proteins. From our presentation you will learn the following ... 

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