Tetrameric Complexes

Very recently the first All compound to be stable at room temperature, the tetrameric complex [ AlI(NEt3) 4], has been prepared and shown to feature a planar AI4 ring with Al-Al 265 pm, Al-I 265 pm and Al-N 207 pm. ... 

Kir channels may be homo- or hetero-tetrameric complexes, in some cases in tight association with beta-subunits (e.g., the KAtp channel). 

The prevalent receptor model for the excitatory amino acid is a tetrameric complex. As mentioned in the text, there is evidence that the channel conductance depends on the number of subunits that bind a ligand. Estimate the EC50 value and Hill coefficient for a dose-response curve assuming that the occupation at each subunit has a Kd value of 1 pi I, an % of 1, and that activation induces a transition to an active state independent of the state of the other subunits ... 

Triply bridging carbonates between three zinc centers have been identified in nine different X-ray structures deposited in the CSD 458,461,465-467 For example, a binuclear ft-OH zinc complex with a tetradentate /V-donor ligand absorbs atmospheric carbon dioxide to a triply bridged carbonate.468 Examples are also known where the metal atoms are in varying coordination environments. The complex cation [Zn3(bipyridine)6(/U3-C03)(H20)2]4+ contains one penta- and two hexacoordinate zinc centers.469 A tetrapodal compartmental ligand forms a tetrameric complex with zinc that contains the carbonate bridging between three of the four zinc centers.470... 

As to the stoichiometry of the H3-H4-DNA particle, two complexes were identified an H3-H4 tetramer and an H3-H4 octamer, each associated with about 140 base pairs of DNA. The complexing of 140 base pairs of DNA with H3 and H4 resulted in the formation of nucleosome-like particles, as observed by the EM, and reported to have an s20base pairs (Bina-Stein and Simpson, 1977 Bina-Stein, 1978). These results differ from those of Simon et al. (1978) who report that at least two complexes of H3 H4-DNA can be obtained upon reconstitution of H3, H4, and 150 bp DNA. In this experiment both an octamer and a tetramer of H3-H4 were found bound to 150 base pairs of DNA, having sM,w equal to 10.4 and 7.5 for the octamer and tetramer, respectively. The stoichiometry of the complexes obtained is dependent on the histone-to-DNA ratio. At low ratios of histone to DNA the predominant species contains an H3-H4 tetramer per 150 base pairs of DNA. At a histone-to-DNA ratio of 1 1 the octamer prevails. The nuclease and protease digestion experiments (Camerini-Otero et al., 1976 Sollner-Webb et al., 1976) were performed at a histone-to-DNA ratio of 0.5, conditions which for 140-base-pair DNA would lead primarily to a tetrameric complex. Therefore, it seems that a tetramer of H3 H4 is sufficient for the generation of nuclease-resistant fragments similar to those of complete nucleosomes. Upon addition of H2A and H2B to the tetrameric complex, nucleosomes are formed. Addition of H3-H4 to the tetrameric complex resulted in an octameric complex which is similar in compaction to nucleosomes. H3-H4 tetramers and octamers were similarly found complexed with about 140 base pairs of DNA upon reconstitution of H3-H4 with SV40 DNA. Both complexes were reported to be able to fold 140 base pairs of DNA (Thomas and Oudet, 1979). 

Figure 4 A proposed scheme for the structures of Mn associated with the S state transitions. In this scheme, 0 denotes either 0 " or OH" ligands. Each Mn ion in the tetrameric complex is proposed to also be coordinated to the protein via three O or N ligands, as indicated by EXAFS studies of Mn in the state (33,35), although the protein-derived ligands are not shown.

Figure 2.10 Structures of mammalian DNA methylases. (a) Ribbon representation of the structure of the tetrameric complex formed between the C-terminal domain of Dnmt3a2 (orange) and the C-terminal domain of Dnmt3L (green) (PDB code 2qrv). The AdoHcy molecules are colored cyan.

Thiopurines 22A-22C on reaction with [(tacnfCoCbJ, in presence of sodium hydroxide, undergo deprotonation to produce initially monomeric [(tacn)Co (22-N,S)Cl] which on subsequent deprotonation undergo self-assembly to form trimeric and/or tetrameric complexes (Scheme 1). Whereas 22A forms a mixture... 

The insulin receptor is composed of two heterodimers each heterodimer is composed of an a unit and a P unit. The a unit is extracellular and contains the insulin recognition and binding sites the p unit spans the cellular membrane and contains a tyrosine kinase. Although insulin can bind to a single ap dimer, it binds with higher affinity to the aPaP tetrameric complex. When insulin binds to an a unit, the tyrosine kinase associated with the corresponding p unit is stimulated. Following this, intracellular proteins such as IRS-1 and IRS-2 (IRS=insulin receptor substrate) are phosphorylated by the P subunit tyrosine kinase, and they in turn activate a network of phosphorylations within the receptor cell. 

The dimeric complex 74 reacts with phenylacetylene or ferrocenylacetylene to yield the tetrameric complexes 75a and 75b, respectively, according to equation 26. These complexes are stable in CDCI3 solution in the absence of air and can be characterized by H and NMR spectroscopies. The low solubility of 75a in unreactive organic solvents precludes detailed studies of the solution structure in reactive solvents it decomposes to a dimeric complex, 76, according to equation 27 3. j jjg association behavior of these complexes resembles that of analogous organolithium compounds - 303... 

The tryptophan biosynthetic pathway in E. coli. There are five enzymatically catalyzed reactions involved in tryptophan biosynthesis and five different polypeptides associated with these reactions. Polypeptides E and D normally make a tetrameric complex, which... 

In the presence of trace amounts of water, the tetrameric p,2-oxo complex (182) in 1,2-dimethoxyethane is transformed into a p, -oxo tetrameric complex (183 equation 254), characterized by an X-ray structure.574 In contrast, (182) 572,575 is inactive towards the oxidation of phenols. The reaction of N,N,N, AT -tetramethyl-l,3-propanediamine (TMP) with CuCl, C02 and dioxygen results in the quantitative formation of the /z-carbonato complex (184 equation 255).s76 This compound acts as an initiator for the oxidative coupling of phenols by 02. 6 Such jz-carbonato complexes, also prepared from the reaction of Cu(BPI)CO with 02 [BPI = 1,3 bis(2-(4-methyl-pyridyl)imino)isoindoline],577 are presumably involved as reactive intermediates in the oxidative carbonylation of methanol to dimethyl carbonate (see below).578 Upon reaction with methanol, the tetrameric complex (182 L = Py X = Cl) produces the bis(/z-methoxo) complex (185 equation 256), which has been characterized by an X-ray structure,579 and is reactive for the oxidatiye cleavage of pyrocatechol to muconic acid derivatives.580,581... 

Fig. 1. Domain structures of typical members of the kinesin superfamily. (A) Bar diagram of the kinesin heavy chain (KHC) of conventional kinesin (kinesin-1 family) as a typical representative of N-type motors (motor domain at the N-terminus, red) the cartoon model beneath the bar diagram shows the tetrameric complex of two heavy and two light chains. (B) M-type kinesin like MCAK of the kinesin-13 family. (C) C-type kinesin like Ned of the kinesin-14 family.

The amino-terminal region of the collagen IV molecule was isolated from proteolytic digests of basement membrane-rich tissue in the form of a cross-linked tetrameric complex, the so-called 7 S collagen. The al(IV)... 

Ecotin (eco) is a potent inhibitor of serine proteases that is derived from Escherichia coli. It was originally named for its ability to inhibit trypsin (E. coli trypsin inhibitor), but it is known to interact with and inhibit virtually all characterized tryp-sin-fold serine proteases. It is insensitive to the active site PI preference of the protease (the amino acid N-terminal to the cleaved or scissile bond ) and inhibits proteases with specificity towards basic, large hydrophobic, small aliphatic and acidic amino acids [2]. This remarkable breadth of inhibition classifies eco as a fold-specific inhibitor. It forms a unique tetrameric complex consisting of two protease molecules and two inhibitor molecules (the E2P2 complex), binding in a bi-dentate manner with two surface loop regions known as the primary and secondary sites (3) (Fig. 7.1). Eco itself is a 142 amino acid protein that forms a stable... 

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