A and P subunits

Waddle, J., and Baldwin, T. O. (1991). Individual a and p subunits of bacterial luciferase exhibit bioluminescence activity. Biochem. Biophys. Res. Commun. 178 1188-1193. 

Figure 9. Proposed cyclic mechanism for ATP synthesis by complex V involving all three catalytic sites of F,. In this scheme only the a and p subunits of F, are shown these are connected by a short stalk to F, in the inner membrane. Proton translocation through Fq driven by the proton motive force (AP) causes sequential conformational changes in each of the p-subunits and ATP synthesis as described in the text hexagons, high-affinity sites semicircles, low affinity sites parallelepipeds, intermediate-affinity sites (with no movement of F,).

Alpha and beta thalassemias are anemias that result from reduced production of a and P subunits of HbA, respectively. 

In this chapter we will review the recent investigations of the structure of both the a and P subunit, and the function of gastric H,K-ATPase. We will proceed from a brief overview of the tissue distribution to a successive discussion of structure, kinetics, transport properties, lipid dependency, solubilization and reconstitution, and inhibitors of H,K-ATPase that may label functionally important domains of the enzyme. 

The enzymes involved in the degradation of phenanthrene by Nocardioides sp. strain KP7 have been characterized. In this strain, the genes are chromosomal, and phdA and phdB encoding the a- and P-subunits of the dioxygenase, phdC and phdD that encode the ferredoxin, and ferredoxin reductase have been cloned and sequenced (Saito et al. 2000). The order of the genes... 

Co-expression of the human a- and p-subunits in the yeast Pichea pastoralis produces only trace amounts of active tetramer, with the majority being present in an unassembled form. Co-expression with human type III collagens, however, increases this assembly level tenfold. This indicates that collagen synthesis and the formation of an active prolyl 4-hydroxylase complex are mutually dependent processes (Vuorela et al, 1997). A similar observation has been noted for baculovirus encoded enzymes in insect cells (Lamberg et al, 1996). These findings support the hypothesis that this unusual control mechanism may be a common feature of collagen synthesis in all cell types. 

Soluble forms of guanylyl cyclase are activated by nitric oxide. These enzymes are homologous to the catalytic domains of the membrane-bound forms of GC. They are considered heterodimers because they appear to exist, under physiological conditions, as complexes of a and P subunits, each with Mr of 70-80 kDa. Both types of soluble GC contain three primary domains an amino-terminus heme domain responsible for binding nitric oxide (NO), a dimerization domain and a carboxy terminus catalytic domain. The aP heterodimer is required for enzyme activity [35]. This can be seen as similar to the situation for AC, which contains two catalytic entities within a single polypeptide chain (Fig. 21-5). 

There are multiple isoforms of the a and p subunits of soluble GC that exhibit distinct tissue and cellular distributions. Most of the isoforms are expressed in brain, including 0Ci 3 and and P3. The p2 subunit is found primarily in lung. This isoform also contains a consensus sequence at its carboxy terminus for isoprenylation or carboxymethyl-ation, which could serve to anchor the protein to the plasma membrane. It has been proposed that specific isoforms of the a and P subunits of GC may form heterodimers with distinct functional and regulatory properties, although this remains to be established with certainty. 

At larger NO/heme ratios (>2) the predominant reaction was nitrite plus nitrosylHb (Eq. (9)) which was equally distributed amongst the a and p subunits... 

Figure 17.8 Ribbon diagrams of the heterodimeric MoFe-protein (left) and homodimeric Fe-proteins of nitrogenase (right). The a- and P-subunits to the left of the MoFe-protein are shown in light and dark shading, respectively, while the metalloclusters are shown as dark space-filling models on the right side. The two subunits of the Fe-protein are shown in light and dark shading, with the 4Fe-4S cluster at the dimer interface as a space-filling model. (From Rees et al., 2005. Reproduced with permission of the Royal Society.)...

Sequier JM, Richards JG, Malherbe P, Price GW, Mathews S, et al. 1988. Mapping of brain areas containing RNA homologous to cDNAs encoding the a and P subunits of the rat GABAa gamma-aminobutyrate receptor. Proc Natl Acad Sci USA 85 7815-7819. 

The a- and P-subunits are structurally and evolutionarily related to each other and to myoglobin (Mb), the monomeric oxygen-binding protein in the muscles. 

The extent of upregulation varies with nAChR subtype and is typically much greater in cell lines than in native tissues after in vivo exposure to nicotine. The P2-containing nAChRs display the highest level of upregulation (Xiao and Kellar 2004), reflecting differences in the interface between adjacent a and p subunits, with respect to p2 versus p4 subunits (Sallette et al. 2004). Interestingly, inclusion of the... 

It was known that large organic cations such as the tetraethylammonium (TEA) ion and large inactivation peptides (part of the p subunit—T1 assembly) enter the transmembrane pore and the question arises as to how that would take place given the T1 domain s narrow central core. These researchers answer the question in reference 16 through their analysis of the structure and function at the cytoplasmic interface (the connection region between the a and P subunits). 

The ganglionic N-cholinoceptors apparently consist only of a and p subunits (a2P2). Some of the receptors for the transmitter y-aminobutyric acid (GABA) belong to this receptor family ... 

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