Regulation of Protein Activity

Figure 20.31 The principle of interconversion cycles in regulation of protein activity or changes in protein concentration as exemplified by translation/proteolysis or protein kinase/protein phosphatase. They result in very marked relative changes in regulator concentration or enzyme activity. The significance of the relative changes (or sensitivity in regulation) is discussed in Chapter 3. The principle of regulation by covalent modihcation is also described in Chapter 3. The modifications in cyclin concentration are achieved via translation and proteolysis, which, in effect, is an interconversion cycle. For the enzyme, they are achieved via phosphorylation and dephosphorylation reactions. In both cases, the relative change in concentration/activity by the covalent modification is enormous. This ensures, for example, that a sufficient increase in cyclin can occur so that an inactive cell cycle kinase can be converted to an active cell cycle kinase, or that a cell cycle kinase can be completely inactivated. Appreciation of the common principles in biochemistry helps in the understanding of what otherwise can appear to be complex phenomena.

The phosphorylation of proteins on Ser, Thr or Tyr residues is a basic tool for the regulation of protein activity (see 7.1). Many eucaryotic transcriptional activators are isolated as phosphorylated proteins. The phosphorylation occurrs mainly on the Ser and Thr residues, but can also be observed on the Tyr residues. The extent of phosphorylation is regulated via specific protein kinases and protein phosphatases, each components of signal transduction pathways (see ch. 7). The phosphorylation of transcriptio-... 

Flavonoids as Mediators of Transcriptional and Posttranslational Regulation of Protein Activity 512... 

Liu KJ, Gestwicki JE, Crabtree GR. Bringing Small Molecule Regulation of Protein Activity to Developmental Systems. 2007. Taylor Francis, Oxford, UK. 

The versatility of proteins is further enhanced by covalent modifications. Such modifications can incorporate functional groups not present in the 20 amino acids. Other modifications are important to the regulation of protein activity. Through their structural stability, diversity, and chemical reactivity, proteins make possible most of the key processes associated with life. 

In addition to the twenty standard amino acids, many nonstandard amino acids are also found in almost all proteins. Generally, these amino acids arise as a consequence of various chemical modifications after they have been incorporated into protein. Posttranslational modification of amino acids is one basis of the regulation of protein activity, specificity, and stability. 

A FIGURE 3-30 Regulation of protein activity by kinase/phosphatase switch. The cyclic phosphorylation and dephosphorylation of a protein is a common cellular mechanism for regulating protein activity. In this example, the target protein R is inactive (light orange) when phosphorylated and active (dark orange) when dephosphorylated some proteins have the opposite pattern. 

Proteases hydrolyse the amide bonds of proteins. While some proteases have a purely metabolic function, a number of proteases are involved in the post-translational regulation of protein activity and are essential for cellular function. Many pathways such as hormone activation, apoptosis, coagiflation, or viral infection are dependent on the action of specific proteases. As for kinases, there is widespread interest in methods that define the preferred substrate of a protease and in being able to correlate their activity to the cellular state. Three detection methods have been developed based on irreversible inhibitors that selectively label active proteases [52], fluorogenic substrates [55,83], and substrates flanked by two FRETing fluorophores [58]. 

Regulation of enzyme activity is achieved in a variety of ways, ranging from controls over the amount of enzyme protein produced by the cell to more rapid, reversible interactions of the enzyme with metabolic inhibitors and activators. Chapter 15 is devoted to discussions of enzyme regulation. Because most enzymes are proteins, we can anticipate that the functional attributes of enzymes are due to the remarkable versatility found in protein structures. 

Similar to nNOS, Ca2+-activated calmodulin is important for the regulation of eNOS activity. However, several other proteins interact with eNOS and regulate its activity. Heat shock protein 90 (hsp90) is found associated with eNOS and probably acts as an allosteric modulator that activates the enzyme. Caveolin-1 binds eNOS and directs it to caveolae. Caveolin-1 is viewed as an inhibitor of eNOS activity, which is being replaced by CaM upon activation of endothelial cells [2]. 

Shuai K, Liu B (2005) Regulation of gene-activation pathways by PIAS proteins in the immune system. Nat Rev Immunol 5 593-605... 

Booher, R., and Beach, D. (1986). Site-specific mutagenesis of cdc2+, a cell cycle control gene of the fission yeast Schizosaccaromyces pombe. Mol. Cell. Biol. 6 3523-3530. Booher, R. N., Alfa, C. E., Hyams, J. S., and Beach, D. H. (1989). The fission yeast cdc2/cdcl3/sucl protein kinase regulation of catalytic activity and nuclear localization. CeU 58 485-497. 

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