Chlorophyll-protein complexes

Recent advances in the studies on chlorophyll-protein complexes of higher plants are summarized in this article. Special emphasis is laid on the isolation, pigment composition and the absorption and fluorescence properties of the complexes.     

Corticosteroid-calcium complexes

Glucocorticoids and calcium ions are shown to interact to yield a complex with properties that are distinct from those of the reactants. Reaction of steroids with Ca2+ appears to require the dihydroxyacetone side chain, since other structures do not react. Evidence for complex formation are: increased aqueous solubility of cortisol when Ca2+ is added to an aqueous or a biphasic aqueous/chloroform (or ethyl acetate) system; increased rate of migration of cortisol during reversed-phase thin-layer chromatography and HPLC; chromatographic comigration of 45Ca2+ and 3H-labeled cortisol; coprecipitation of 45Ca2+-3H-cortisol complexes. After dissociation of the cortisol-calcium complex, the only steroid recovered was cortisol. By the above criteria, the properties of cortisol were not affected by Sr2+, Ba2+, or Mg2+. The cleavage patterns of cortisol in the mass spectrometer corresponded to that of 11 beta-hydroxyandrostenedione when Ca2+ was present, and to cortisol in its absence. We therefore postulate that the structure of the dihydroxyacetone side chain was transiently altered by Ca2+, resulting in a labile C17-C20 bond. These results support our earlier proposal that the chemical and physico-chemical properties of corticosteroids are modified by calcium ions.     

Stable lysozyme-cello-oligosaccharide complexes

Stable complexes that formed when [¹⁴C]cello-oligosaccharides and lysozyme were incubated under various conditions were isolated chromatographically and characterized. Complexation occurred over the pH range of 3–9 but was favored at high pH; the extent of complexation was inversely proportional to temperature. The stoichiometry of the complex was 1:1 sugar-protein, but no radiolabeled peptides could be isolated from a tryptic digest. The minimum requirements for association were native enzyme and a substrate composed of three or more (1→4)-β-D-glucopyranosyl residues. Acceptors and competitive inhibitors of lysozyme inhibited complexation and stimulated dissociation of the complex. Lysozyme did not hydrolyze the cello-oligosaccharides nor did it use them as D-glucosyl donors. Ultracentrifugation, molecular-sieve chromatography, and light-scattering studies indicated that the precipitated complexes were large, heterogeneous aggregates of protein and oligosaccharide which conform to the following equilibrium: n(protein+oligosaccharide)?(protein-oligosaccharide)_n. Polymerization is a cooperative phenomenon.     

Metal heterodiene complexes

In this article a survey is given of novel results in the field of the chemistry of α-diimine complexes of some metal atoms of the main group, transition metal and post transition metal area. It is shown that α-diimines of the type RNC(R′)(R″)CNR (= R-DAB) and 2-C₅H₄NC(R′)NR (= R-Pyca) are very versatile with regards to both their coordination chemistry and chemical reactivity when coordinated to a metal or metal cluster. The type of reaction occurring depends critically on the type of metal(s), the other ligands present on the metal(s), the type of coordination of the α-diimines, and particularly on the steric and electronic properties of R,R′ and R″ situated on the R-DAB and R-Pyca ligands.     

RNA-protein complexes.

RNA-binding proteins are an extremely diverse group of proteins, reflecting the diverse functional requirements of cellular RNAs. Whereas the number of structures of RNA-binding proteins or modules is increasing at a reasonable rate, that of protein-RNA complexes increments by only a few each year. The recently determined structure of a complex from the U2 small nuclear ribonucleoprotein particle shows the subtleties of RNA stem-loop recognition by ribonucleoprotein modules. A second structure provides the first direct information on double-stranded RNA recognition by the double-stranded RNA-binding module that occurs in a variety of functionally distinct proteins. Another two new complexes concern proteins interacting with tRNA. The first is methionyl-tRNAf(Met) transformylase, which has to compete with elongation factor Tu for charged initiator tRNAMet and does so by recognising specific features of the acceptor stem of tRNAf(Met). The second is prolyl-tRNA synthetase, complexed with its cognate tRNA, that has to specifically recognise the two guanines common to all tRNA anticodons specific for proline.     

Membrane protein complexes

A wide range of membrane protein structures have been published during the past two years. These have included proteins from both eucaryotic and heterologously overexpressed sources. Whereas some of these proteins were crystallised using conventional techniques, others employed the new methods of lipidic cubic phase crystallisation and antibody fragment co-crystallisation. These and other new approaches to the expression and crystallisation of membrane proteins are accelerating structural studies of membrane protein complexes.