Oligomeric structure dissociation

The oligomeric structure of the heterotrimeric G proteins increases the complexity and diversity of their signaling mechanisms. The intrinsic GTP cycle is direcdy associated with a cycle of subunit dissociation in which the oc subunit (which contains the nucleotide-binding site) can dissociate from a stable fiy dimer (Fig. 1). In the inactive GDP-bound state, high affinity between the oc and /fy subunits favors association. When GTP is bound, the affinity between subunits is lowered and dissociation is promoted. This allows both oc and fty subunits to act as coordinate or diverse regulators of downstream targets. Reassociation of the subunits inhibits downstream... 

The properties of a-D-galactosidase (mol. wt. 3.0 x 10 ) from Saccharomyces carlsbergensis have been shown to be similar to those of other extracellular enzymes in yeast. The carbohydrate content (57 %) consists of D-mannose (ca. 92%), D-glucose (7%), and 2-amino-2-deoxy-D-glucose (1%) 35% of its amino acid residues can be accounted for by L-threonine, L-serine, and l-aspartic acid. Although the enzyme remains active after treatment with sodium dodecyl sulphate, it is denatured by urea and guanidinium chloride. With urea or 60 °C heat denaturation, the enzyme dissociates into two types of subunits and was therefore considered to be the first reported external enzyme from yeast with an oligomeric structure. 

A cyanohydridoborato vanadium(II) complex, tra s-[V(NCBH3)2(thf)4], has been synthesized and structurally characterized (212).858 Additional oligomeric complexes have been prepared complexes that have been characterized include trinuclear vanadium complexes,859-862 dinuclear V-Fe complexes,863 and the first tetranuclear V11 species [V4(/x3-Cl)2(M-Cl)2(M-CF3C02)2(thf)6].864 The addition of tris(pyrazolyl)borate ligand to the tetranuclear species results in the dissociation of the complex to form a neutral dimeric species [V(tpb)-(/x-Cl)(/x-CF3CO2)2V(th03] (213) in which one of the vanadium centers is facially coordinated by tpb.864... 

The self-assembly of Pt(II) analogue lb with 2 was very slow due to the inactivity of the Pt(II)-pyridine bond. Thus, upon treatment of lb with 2, a kinetically distributed oligomeric mixture was initially formed. However, the mixture gradually turned into the thermodynamically most stable molecular square 3b after heating the solution for a few weeks at 100 °C. The use of bis-nitrate salt of 2, instead 2 itself, dramatically increased the reaction rate as well as the yield (79-81%). Similar improvements in reaction rate and yield were observed by the addition of NaNOj to the reaction. A significant difference in stability was found between 3a and 3b. The addition of la to 3a in D2O resulted in dissociation of 3a to give a mixture of 3a (ca. 50%) and two acyclic components which have la and 2 in 1 2 and 2 3 ratios. In a striking contrast, 3b remained intact even upon the addition of lb, as its structure had been locked by the inert Pt-Py bond. The square complex 3b was also found to show inclusion properties similar to 3a (Table 1). 

Fig. 33. Schematic representation of the effects of pressure on oligomeric proteins a) native dimeric protein with cavities/voids b) dissociation of the oligomer, hydration with electrostriction of polar/ionic groups, hydrophobic hydration of unpolar groups (-CR), release of void volume c) weakening of hydrophobic interactions provides pathways for water to penetrate into the interior of the protein, swelling of the core - molten-globule like state d) unfolding of subunits, disruption of the secondary/tertiary structure (hydration of residues not plotted here), loss of cavity volume within protein (adopted from ref. 139).

---