Peptide enzyme activities

Nonrepetitive but well-defined structures of this type form many important features of enzyme active sites. In some cases, a particular arrangement of coil structure providing a specific type of functional site recurs in several functionally related proteins. The peptide loop that binds iron-sulfur clusters in both ferredoxin and high potential iron protein is one example. Another is the central loop portion of the E—F hand structure that binds a calcium ion in several calcium-binding proteins, including calmodulin, carp parvalbumin, troponin C, and the intestinal calcium-binding protein. This loop, shown in Figure 6.26, connects two short a-helices. The calcium ion nestles into the pocket formed by this structure. 

Cyclosporine A (CsA) is a water-insoluble cyclic peptide from a fungus composed of 11 amino acids. CsA binds to its cytosolic receptor cyclophilin. The CsA/cyclophilin complex reduces the activity of the protein phosphatase calcineurin. Inhibition of this enzyme activity interrupts antigen receptor-induced activation and translocation of the transcription factor NEAT to the nucleus which is essential for the induction of cytokine synthesis in T-lymphocytes. 

In the periphery, some of the primary triggers for these processes have been identified. Acetylcholine seems to be one such factor because stimulation of preganglionic nerves in vivo increases enzyme activity. However, nicotinic and muscarinic receptor antagonists do not completely prevent this increase. The residual activation is attributed to peptides of the secretin-glucagon subgroup, including VIP and secretin both these peptides activate cAMP synthesis. Purinergic transmitters could also be involved. 

Figure 10.1 ThecyclicAMPsecondmessengersystem.Themostcommonsecond messenger system activated by the protein/peptide hormones and the catecholamines involves the formation of cAMP. This multistep process is initiated by binding of the hormone (the first messenger) to its receptor on the cell surface. The subsequent increase in the formation of cAMP (the second messenger) leads to the alteration of enzyme activity within the cell. A change in the activity of these enzymes alters cellular metabolism.

Steroid hormones and thyroid hormone carry out their effects by way of gene activation. In contrast to the protein/peptide hormones, which alter existing enzyme activity, these hormones induce the synthesis of new enzymes that then influence cellular metabolism. 

Since kinases are not hydrolytic enzymes, a small molecule-based FRET probe does not seem to be a straight forward solution for this enzyme activity. Nevertheless, quite a number of fluorescent probes based on small substrate peptides have been prepared in... 

As already stated, Fischer was deeply intrigued by the phenomenon of enzyme activity. He realized that the substances were proteins and this undoubtedly was why he next undertook the study of amino acids and peptides. He fully appreciated that the specificity of enzyme catalysis depended on the occurrence of a complementarity for interacting dissymmetric surfaces. In this regard, he wrote (3) ... 

J3. Jauhiainen, M., Metso, J., Koskinen, P., and Ehnholm, C., Characterization of the enzyme activity of human plasma lipoprotein(a) using synthetic peptide substrates. Biochem. J. 274, 491-496 (1991). 

Proteolytic enzymes in the respiratory mucosa play important role(s) in the regulation of lung inflammation and remodelling [123, 124], Pulmonary proteolytic enzymes, however, also comprise one of the barriers which pulmonary-administered protein/peptide drugs have to overcome in order to achieve adequate bioavailability [125]. Intriguingly, the pulmonary enzymatic barrier is an aspect that has been little investigated and is poorly understood. Inconsistencies in the data available to date are most likely a result of the use of different techniques (e.g., PCR, immunotechniques and enzyme activity assays), different species and different cell (pheno)types, for example primary cells vs. cell lines. 

This enzyme from E. coli is a tetramer of four identical subunits, each of molecular weight 116,500. Amber and ochre (premature termination) mutants of the enzyme provide a number of enzymically inactive, incomplete peptide chains, identical in sequence with the N-term nal part of the wild-type chains. A subset of these N-terminal peptides, called acceptor peptides, can combine with so-called wild-type chain, to restore enzymic activity (Ullmann et al., 1965, 1967 Ullmann and Perrin, 1970 see also the review by Zabin and Villarejo, 1975). Goldberg (1969) suggested that the acceptor peptides and the independent nucleation centers as evidenced by the following facts ... 

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