Subunit composition

The Uj subunit is the main subunit of the calcium channel and contains the ion-conducting pore, the selectivity filter of the pore, and the sites for the inorganic and organic calcium channel blockers. Six different genes (classes A, B, C, D, E, and S) have been identified for the Oj subunit (see also Hofmann et al., 

They encode polypeptides of predicted molecular masses of 212 to 273 kDa, which are 41 to 70% 

Copyright 1996 by Academic Press, Inc. All rights of reproduction in any form reserved. 

FIGURE I Proposed subunit composition of the smooth muscle L-type calcium channel. This channel is a three-subunit oligomer (a, 3). The putative membrane configuration of individual subunits is taken 

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Apart from tliese mainstream metliods enabling one to gain a comprehensive and detailed stmctural picture of proteins, which may or may not be in tlieir native state, tliere is a wide variety of otlier metliods capable of yielding detailed infonnation on one particular stmctural aspect, or comprehensive but lower resolution infonnation while keeping tlie protein in its native environment. One of tlie earliest of such metliods, which has recently undergone a notable renaissance, is analytical ultracentrifugation [24], which can yield infonnation on molecular mass and hence subunit composition and their association/dissociation equilibria (via sedimentation equilibrium experiments), and on molecular shape (via sedimentation velocity experiments), albeit only at solution concentrations of at least a few tentlis of a gram per litre. 

NMD A receptors are selectively activated by A/-methyl-D-aspartate (NMD A) (182). NMD A receptor activation also requires glycine or other co-agonist occupation of an allosteric site. NMDAR-1, -2A, -2B, -2C, and -2D are the five NMD A receptor subunits known. Two forms of NMDAR-1 are generated by alternative splicing. NMDAR-1 proteins form homomeric ionotropic receptors in expression systems and may do so m situ in the CNS. Functional responses, however, are markedly augmented by co-expression of a NMDAR-2 and NMDAR-1 subunits. The kinetic and pharmacological properties of the NMD A receptor are influenced by the particular subunit composition. 

Other bioanalytical applications of systems in which the eluate of a first LC column is sampled in continuous and repetitive intervals and subjected to a second LC dimension are, for example, described by Wheatly et a/. (11) and Matsuoka et al. (12). Wheatly coupled gradient affinity LC with RPLC for the determination of the isoenzymatic- and subunit composition of glutathione 5-transferses in cytosol... 

In contrast to AMPA receptors, NMDA receptor channels display a prominent Ca2+ permeability, which is largely independent ofthe subunit composition. It has been shown by mutational analysis that the Ca2+ permeability of recombinant NMDA receptors is dependent on a residue at a position equivalent to the Q/R site of AMPA subunits. Both NR1 and NR2 subunits contain an asparagine (N) residue at this position. Replacing this N with an R within the NR1 subunit led to the formation of NMDA receptors with a strongly reduced Ca2+ permeability, whereas exchanging N for Q in the NR2 subunit had only a small effect,... 

Voltage-dependent Ca2+ Channels. Figure 2 Subunit composition of a HVA calcium channel. The selectivity filter of the channel is created by four glutamates (E). 

The exact subunit composition of the Low Voltage-activated (LVA) Ca2+ channels is unknown [3,6]. Three ai subunits, Cav3.x, have been identified which induce large T-type current after expression in Xenopus oocytes and in HEK cells in the absence of additional subunits. The T-type current can be affected by the a2S and the y6 subunit suggesting a maximal subunit composition of oq/o S./y. 

Figure 1 shows the phylogenetic relationship of the mitochondrial and bacterial Rieske proteins. Plant mitochondrial Rieske proteins form a separate cluster, whereas bacterial Rieske proteins are more closely related to Rieske proteins from fungi or mammals, although the subunit composition and organization of the bci complex is compa-... 

Almost simultaneously, Lindahl and co-workers proposed that Cluster C is the CO oxidation site based on EPR and ENDOR studies of the cyanide adduct of the enzyme (134). That proposal was based on the premise that CO and cyanide compete for the same binding site. Additionally, Xia and Lindahl have shown that, by mild SDS treatment, they can partially dissociate CODH/ACS, which is a tetra-meric enzyme with an subunit composition, into an isolated a subunit and an form (135). The form has the same level of CO oxidation activity as the native protein indicating that the a subunit is not involved in CO oxidation and that the /8 subunit must contain the clusters required for CO oxidation (135). In addition, CO2 alters the g values of the Credi form of the enzyme (136). 

This key enzyme of the dissimilatory sulfate reduction was isolated from all Desulfovibrio strains studied until now 135), and from some sulfur oxidizing bacteria and thermophilic Archaea 136, 137). The enzymes isolated from sulfate-reducing bacteria contain two [4Fe-4S] clusters and a flavin group (FAD) as demonstrated by visible, EPR, and Mossbauer spectroscopies. With a total molecular mass ranging from 150 to 220 kDa, APS reductases have a subunit composition of the type 012)32 or 02)3. The subunit molecular mass is approximately 70 and 20 kDa for the a and )3 subunits, respectively. Amino-acid sequence data suggest that both iron-sulfur clusters are located in the (3 subunit... 

Sulfite reductase catalyzes the six-electron reduction of sulfite to sulfide, m essential enzymatic reaction in the dissimilatory sulfate reduction process. Several different types of dissimilatory sulfite reductases were already isolated from sulfate reducers, namely desul-foviridin (148-150), desulforubidin (151, 152), P-582 (153, 154), and desulfofuscidin (155). In addition to these four enzymes, an assimila-tory-type sulfite reductase was also isolated from D. vulgaris. Although all these enzymes have significantly different subunit composition and amino acid sequences, it is interesting to note that, as will be discussed later, all of them share a unique type of cofactor. 

Desulforubidin was found in strains of the Desulfomicrobium genus and has been described as the sulfite reductase of this genus. The subunit composition and molecular mass are similar to what was observed for desulfoviridin. However, in desulforubidin all sirohydrochlorins are metalated as proved by Mossbauer spectroscopy (152). The as-isolated protein contains four [4Fe-4S] clusters two of them are exchange-coupled to two paramagnetic sirohemes. 

In two sulfate-reducing bacteria we found oxygen-tolerant formate dehydrogenases with different subunit composition. The formate dehydrogenase from D. desulfuricans is an af3y protein whereas the one from D. gigas is an afi protein. Both proteins contain two MGD cofactors but the protein from D. desulfuricans contains four heme c attached to the y subunit (16 kDa). 

D. gigas formate dehydrogenase seems to be quite different in terms of subunit composition. It does not contain a y subunit and no heme c was detected (225). Also, two MGD were identified, but surprisingly, the enzyme contains tungsten instead of molybdenum. Mossbauer and EPR studies confirmed the presence of two [4Fe-4S] + + clusters with similar properties to the ones found in D. desulfuricans FDH (247). 

The subunit composition of hemoglobin tetramers undergoes complex changes during development. The... 

Kohr G (2006) NMDA receptor function subunit composition versus spatial distribution. Cell... 

Given that the pharmacological and biophysical properties of recombinant GABAa receptors have been shown to depend critically on their subunit composition, much effort has been directed towards understanding the assembly of native receptors. This could provide a rational basis for the design of compounds able to interact with specific... 

Figure 11.10 Glycine receptor pharmacology and structure, (a) Amino acids that act as agonists at glycine receptors, and strychnine a competitive antagonist, (b) Subunit composition of foetal and adult glycine receptors in the spinal cord. The receptors are shown with a pentameric assembly but the a. and subunits are distinct from those that form GABAa receptors. Picrotoxin is also an effective glycine antagonist and in recombinant systems is selective for homomeric receptors...

A quantitative reconstitution approach was used to gain information as to the subunit composition of the -ATPase molecule [25]. Proteoliposomes prepared from asolectin and purified, radiolabeled ATPase molecules obtained by a freeze-thaw procedure similar to that of Dufour et al. [34] were shown to catalyze ATP hydrolysis-driven proton translocation, as indicated by the extensive quenching of aminochloromethoxyacridine fluorescence that occurs upon the addition of MgATP to the proteoliposomes, and the reversal of this quenching induced by... 

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