Active subunit

Although in in vivo circumstances an intracellular free calcium increase apparently acts as the primary modulator of contraction, it can be bypassed in highly permeabilized smooth muscle preparations where the active subunit of MLCK can be introduced to phosphorylate myosin and induce contraction. The MLCK catalyzed phosphorylation of serine-19 is seen as the necessary event in the activation of smooth muscle myosin to form crossbridges. Thus, the rising phase of force during an isometric smooth muscle contraction follows an increase in the degree of phosphorylation of myosin, and that in turn follows the transient rise of (a) cytosolic free Ca, (b) Ca-calmodulin complexes, and (c) the active form of MLCK. The regulation of the intracellular calcium is discussed below. The dynam-... 

One should note overall, that while some of these suggested mechanisms may in the future prove to have a role in the control of smooth muscle contraction, in chemically skinned preparations maximum force development follows activation by the MLCK active subunit in extremely low Ca " ion concentrations. The conclusion can hardly be avoided that phosphorylation alone is sufficient for activation, and if another mechanism is involved, it is not necessary for the initial genesis of force. If such mechanisms are operative, then they might be expected to run in parallel or consequent to myosin phosphorylation. A possible example of this category of effect is that a GTP-dependent process (G-protein) shifts the force vs. Ca ion concentration relationship to lower Ca ion concentrations. This kind of mechanism calls attention to the divergence of signals along the intracellular control pathways. 

CcOs from various organisms, as well as CcOs and quinol oxidases, are fairly homologous stmcturally and functionally, and both are distinct from cytochromes ebbs. Most, if not all, CcOs and quinol oxidases require two subunits for catalytic activity (subunits I and II Fig. 18.4), although some, such as mammahan CcO, may contain as many as 11 more subunits of unknown functions [Abramson et al.. 

The occupancy at each subunit is p= [L]/Kd + [L], where [L] is the ligand concentration. If activation of the subunits is independent, as assumed, the number of activated subunits at the receptor complex will follow a binomial distribution that is, the likelihood for activation of n subunits is K4 /F "(1 - p)n. The current will be proportional to ... 

Fig. 10.7. Inh ibitor binding to individual active sites of the yeast 20S proteasome. The inhibitors lactacystin (A), epoxomicin (B) and TMC95A (C) are colored green and are shown in stereo mode together with their unbiased electron densities. The active-site Thrl is highlighted in black. (A) Covalent binding of the Streptomyces metabolite lactacystin to the active site of 5. The SI pockets of the active subunits and differ from that of 5 and are not suitably constructed to bind the inhibitor. As discussed in the text, Met45 (black), which is located at the bottom of the 5-Sl pocket, makes the difference for inhibitor...

Massanz, C., Fernandez, V. M. and Friedrich, B. (1997) C-terminal extension of the H2-activating subunit, HoxH, directs maturation of the NAD-reducing hydrogenase in Alcaligenes eutrophus. Eur. J. Biochem., 245, 441-8. 

Cohen, S., Orian, A., and Ciechanover, A. (2001). Processing of pl05 is inhibited by docking of p50 active subunits to the ankyrin repeat domain, and inhibition is alleviated by signaling via the C-terminal phosphorylation/ ubiquitin-ligase binding domain. J. Biol. Chem. 276, 26769-76. 

Readily measurable fluorescence intensities are found for molecules having aromatic and heteroaromatic rings, in particular when annulated rings are present, and in the case of conjugated 7x-electron systems. If the polymer molecules contain such fluorescence-active subunits they can be characterized by this technique, either directly via their fluorescence spectrum or via fluorescence quenching experiments (for polymers with appropriate quencher groups). It is... 

Stmctmal studies have shown that caspases can form tetramers with two active sites. The catalyticaUy active subunit of a caspase is made up of a large (17—12 kDa) and a small (10—13 kDa) subimit which form a heterodimer with an active site comprised of residues from both large and small subunits. Two heterodimers then align to form a tetramer with two catalytic centers (Fig. 15.4). 

There is very little information on the process by which the enzyme(s) which catalyze the formation of O is activated. Whether activation represents covalent modification of a proenzyme, translocation to the subcellular site where it gains access to its substrate(s), insertion of a necessary cofactor, association of active subunits, or still another process remains to be determined. The rapidity with which activation occurs seems too rapid to represent synthesis of protein de novo. 

The adenylate cyclase pathway of hormone receptor action. When the receptor is unoccupied, the G protein a subunit has GDP bound, and it is complexed with the subunits in this form it cannot activate adenylate cyclase. Binding of hormone activates the receptor, which leads to replacement of GDP by GTP, and the activation subunit then interacts productively with adenylate cyclase to stimulate the synthesis of cAMP. The intrinsic GTPase activity of the a subunit leads to... 

Jiang, R. and Carlson, M., 1997, The Snfl protein kinase and its activating subunit, Snf4, interact with distinct domains of the Sipl/Sip2/Gal83 component in the kinase complex, Mol Cell Biol, 17, pp 2099-106. 

Zhang, X., Malhotra, R., and Guidotti, G. (2000). Regulation of yeast ecto-apyrase yndlp by activating subunit Vmal3p of the vacuolar H+-ATPase. J. Biol. Chem. 275, 35592-35599. 

The organic acylation catalysts currently known are tertiary amines, N-heteroar-omatic compounds (for example pyridine derivatives), or phosphines they can be of central, planar, and axial chirality. Finally, small peptides carrying N-methylhis-tidine as the catalytically active subunit have also been employed they also will be discussed in this chapter. 

When covalently attached to electron transfer active subunits, the DHA-VHF couple can facilitate chemical and physical switching of electronic properties, as a result of photochemically induced rearrangement accompanied by a change in the redox potential. An interesting example of such a switching system is the compound containing a dihydroazulene component and a covalently attached anthraquinone moiety.1311 This system is able to act as a multimode switch, assisted by various processes such as photochromism, reversible electron transfer, and protonation-deprotonation reactions (Scheme 8). 

The major features of equation (1) are 1) the implied requirement for three bound metal ions per active subunit 2) the release of the electrophile Pi (i.e., the phosphoryl group attacked nucleophilically by H2O in the forward direction) prior to the release of the leaving group Pi and 3) the numerical evaluation of the rate constants. We now discuss each of these features in turn. 

Hendriks et al. [106] modeled the structiae of the enzymatically active subunit uf limnaii p.MMuiH CarbuiLypcpuuBK N bused Oil iiic suuCiuTc Oi... 

W. Gebhard, M. Schube, and M. Eulitz. cDNA cloning and complete primary structure of the small, active subunit of human carinxypeptidase N. Ear. J. Btochem J 75 603-607 flQR9L... 

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