Subunits interchange

Hybridization studies between combinations of H and M subunits in vitro produce a binomial distribution of the five possible isozymes thus, for equal quantities of H and M subunits isozymes are formed in the proportion 1 4 6 4 1 (51,52). Subunit interchange can be induced by repeated freezing and thawing, dilution, or by mild denaturation. With increasing evolutionary distance the hybridization process becomes more difficult and the results more variable. It has been concluded (52) that the M chains from different species are more like each other than they are to the H chains of the same species. These conclusions are supported by chemical and immunological results (20,39,40,53,54). An antiserum against the M4 isozyme of one species will cross react with M4 isozyme from several species but not with its own H isozyme (36,54). The H isozyme is a much poorer immunogen than the M4 isozyme (55), and in some cases anti-H4 serum will cross react with M4 isozymes of other species (53,56,57). 

Fig. 10. The receptor—G-protein sequence. An activated receptor interacts with the trimeric GDP-ligated receptor to cause an interchange of GDP by GTP and dissociation into the activated Ga—GTP (left) and G y (right) subunits. These then interact with a variety of effectors. The purpose of the activated...

The terms polypeptide and protein are used interchangeably in discussing single polypeptide chains. The term protein broadly defines molecules composed of one or more polypeptide chains. Proteins having only one polypeptide chain are monomeric proteins. Proteins composed of more than one polypeptide chain are multimeric proteins. Multimeric proteins may contain only one kind of polypeptide, in which case they are homomultimeric, or they may be composed of several different kinds of polypeptide chains, in which instance they are heteromultimeric. Greek letters and subscripts are used to denote the polypeptide composition of multimeric proteins. Thus, an ag type protein is a dimer of identical polypeptide subunits, or a homodimer. Hemoglobin (Table 5.1) consists of four polypeptides of two different kinds it is an hetero-multimer. 

In addition to its role as the P-subunit of PHY, PDI acts independently by catalysing thiol/protein disulphide interchange. The role of PDI as the P-subunit in prolyl 4-hydroxylase is not related to its disulphide isomerase activity and experiments where the vertebrate PDI was mutated in both thioredoxin-like active domains had no effect on tetramer assembly (Vuori et al., 1992). PDI appears to function as a molecular chaperone, retaining the a-subunits in the correct catalytically active, non-aggregated form in the ER-lumen (John et al, 1993). Dissociation of the P-subunits results in insoluble aggregates of the a-subunits, analogous to a-subunits expressed in the absence of PDI. An additional function of PDI in the complex is to maintain the ER luminal location of the a-subunits, since deletion of the ER retention signal from PDI results in the secretion of the complex (Vuori et al., 1992). 

Figure 21.10 Cystamine may be used to make immunotoxin conjugates by a disulfide interchange reaction. Modification of antibody molecules using an EDC-mediated reaction creates a sulfhydryl-reactive derivative. A-chain toxin subunits containing a free thiol can be coupled to the cystamine-modified antibody to form disulfide crosslinks.

FIGURE 27-3 Neurotrophic cytokines and their receptors. Neurotrophic cytokines are related to IL6 and bind to cell surface receptor complexes that share a common structural organization. The four ligands interchangeably employ two distinct receptor subunits, leukemia inhibitory factor receptor 3 (LIF-Rpt) andgpl30, and some employ a ligand-specific a subunit. CNTF-R, ciliary neurotrophic factor CT-fR.cardiotrophin 1 receptor IL6-R, interleukin-6 receptor. 

Two of the three perpendicular symmetry axes are labeled p and q. A 180° rotation about axis q interchanges the positions of subunits A and C and also interchanges B and D. A similar rotation about p interchanges A with B, and C with D. (Source From T. Schirmer and P. R. Evans, Structural basis of the allosteric behaviour of phosphofructokinase, Nature 343 140, 1990.)... 

Phosphorylase exists in two interchangeable forms active phosphorylase a and a normally inactive phosphorylase b. Phosphorylase b is a dimer and is converted into phosphorylase a by phosphorylation of a single serine residue on each subunit by the enzyme phosphorylase kinase. The process can be reversed and phosphorylase inactivated by removal of the phosphate group by protein phosphatase I (Fig. 2a) (see Topic C5). 

K-casein also contains two Cys residues per monomer subunit and is thus capable of interacting with the whey proteins, e.g., mainly g-lactoglobulin, via the disulfide interchange mechanism at temperatures at or above 65°C. This latter phenomenon is believed to be important in providing colloidal stability to the milk casein micelle system, as well as to the whey proteins, in high temperature processed milk products. It has also been postulated that this latter interaction with g-lactoglobulin may alter the availability of K-casein in the micelle, and thus has a detrimental effect upon the cheese making properties of milk (4). 

Harteneck C, Wedel B, Koesling D, Malkewitz J, Bohme E, Schultz G. Molecular cloning and expression of a new alpha-subunit of soluble guanylyl cyclase. Interchangeability of the alpha-subunits of the enzyme. FEES Lett. 1991 292 217-222. Cary SP, Winger JA, Derbyshire ER, Marietta MA. Nitric oxide signaling no longer simply on or off. Trends Biochem. Sci. 2006 31 231-239. 

Interchangeability of translational components 5.1. Interchangeability of ribosomal subunits... 

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