Polypeptides biological activities

Through combined effects of noncovalent forces, proteins fold into secondary stmctures, and hence a tertiary stmcture that defines the native state or conformation of a protein. The native state is then that three-dimensional arrangement of the polypeptide chain and amino acid side chains that best facihtates the biological activity of a protein, at the same time providing stmctural stabiUty. Through protein engineering subde adjustments in the stmcture of the protein can be made that can dramatically alter its function or stabiUty. 

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. 

The hormone peptide YY (PYY) is a 36 amino acid peptide, which is closely related to neuropeptide Y and pancreatic polypeptide. PYY is predominantly synthesised and released by intestinal endocrine cells, and can also coexist with glucagon in pancreatic acini and enteroglucagon in endocrine cells of the lower bowel. It acts on the same receptors as neuropeptide Y. The endogenous long C-terminal PYY fragment PYY3 36 is a biologically active and subtype-selective metabolite. 

In the synthesis of polypeptides with biological activity on a crosslinked polymer support as pioneered by Merrifield (1 2) a strict control of the amino acid sequence requires that each of the consecutive reactions should go virtually to completion. Thus, for the preparation of a polypeptide with 60 amino acid residues, even an average conversion of 99% would contaminate the product with an unacceptable amount of "defect chains". Yet, it has been observed (13) that with a large excess of an amino acid reagent —Tn the solution reacting with a polymer-bound polypeptide, the reaction kinetics deviate significantly from the expected exponential approach to quantitative conversion, indicating that the reactive sites on the polymer are not equally reactive. 

The term polyproteins is used for two different types of entity. The first refers to precursor polypeptides which are cleaved post-translationally into biologically active proteins or peptides of quite different functions. Examples of these include polyproteins of viruses and some prohormones of vertebrates (reviewed in Kennedy, 2000b). The other type is large proproteins which comprise tandem repetitions of identical or similar polypeptides that are post-translationally cleaved into multiple copies of biochemically similar functional entities. The nematode polyprotein allergens/antigens (NPAs) fall into this class (Fig. 16.1). 

Proteins are sometimes classified as simple or conjugated . Simple proteins consist exclusively of polypeptide chain(s) with no additional chemical components present or being required for biological activity. Conjugated proteins, in addition to their polypeptide components(s),... 

Many polypeptides undergo covalent modification after (or sometimes during) their ribosomal assembly. The most commonly observed such PTMs are listed in Table 2.7. Such modifications generally influence either the biological activity or the structural stability of the polypeptide. The majority of therapeutic proteins bear some form of PTM. Although glycosylation represents the most common such modification, additional PTMs important in a biopharmaceutical context include carboxylation, hydroxylation, sulfation and amidation these PTMs are now considered further. 

Phosphorylation Influences/regulates biological activity of various polypeptide hormones... 

Amidation Influences biological activity/stability of some polypeptides... 

Disulfide exchange can also sometimes occur, and prompt a reduction in biological activity (Figure 6.21). Intermolecular disulfide exchange can result in aggregation of individual polypeptide molecules. 

Gel-filtration analysis reveals bands of molecular mass 40-70 kDa. These represent dimers (and some multimers) of the IFN-y polypeptide. Its biologically active form appears to be a homodimer in which the two subunits are associated in an antiparallel manner. 

IL-2 induces its characteristic biological activities by binding a specibc receptor on the surface of sensitive cells. The high-affinity receptor complex consists of three membrane-spanning polypeptide chains (a, P and y Table 9.2). 

IL-ll is a 23 kDa, 178 amino acid polypeptide. Its receptor appears to be a single-chain 150 kDa transmembrane protein. Binding of IL-ll results in tyrosine phosphorylation of several intracellular proteins, which, in turn, somehow promote the observed biological activities of IL-ll. 

Human TNF-a is initially synthesized as a 233 amino acid polypeptide that is anchored in the plasma membrane by a single membrane-spanning sequence. This TNF pro-peptide, which itself displays biological activity, is usually proteolytically processed by a specific extracellular metallo-protease. Proteolytic cleavage occurs between residues 76 (Ala) and 77 (Val), yielding the mature (soluble) 157 amino acid TNF-a polypeptide. Mature human TNF-a appears to be devoid of a carbohydrate component, and contains a single disulfide bond. 

G-CSF is also known as pluripoietin and CSI -fl. Two slight variants are known, one consisting of 174 amino acids and the other of 177. The smaller polypeptide predominates and also displays significantly greater biological activity than the larger variant. 

The biologically active form of M-CSF is a homodimer (two identical subunits). These homodimers can exist as integral cell surface proteins, or may be released from their producer cell by proteolytic cleavage, thus yielding the soluble cytokine. The M-CSF receptor is a single-chain, heavily glycosylated, polypeptide of molecular mass 150 kDa. 

GM-CSF is also known as CSF-a or pluripoietin-a. It is a 127 amino acid, single-chain, glycosylated polypeptide, exhibiting a molecular mass in the region of 22 kDa. It is produced by various cells (Table 10.5), and studies have indicated that its biological activities include ... 

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