Plants acceptor specificity

Dichlorophenoxy)butyric acid is converted in the presence of ATP into dichlorophenoxybutyryl coenzyme A. This acyl-CoA is converted by the electron acceptor flavine adenine dinucleotide (FAD) into dichlorophenoxycrotonyl-CoA. One carbon atom of the unsaturated bond is hydroxilated and dichlorophenoxy- -hydroxybutyric acid-CoA is formed. In certain plants possessing specific /3-oxidase enzyme systems, -ketobutyric acid-CoA is formed from this intermediate compound by the mediation of NAD and NADH in a reaction catalysed by -hydroxyacyl-CoA dehydrogenase. This compound is decomposed by hydrolysis into 2,4-D and acetyl-CoA. 

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

Fig. 10.3. Acceptor photobleaching analysis of interaction between barley MLO and calmodulin. Barley MLO is a plant-specific integral membrane protein that associates with the cytosolic calcium sensor protein Calmodulin...

The involvement of glycolipid and glycoprotein intermediates in the synthesis of polysaccharides from glycosyl-nucleotides in plants is considered to be a likely possibility. Such intermediates could act as specific primers, or acceptor substrates, for the formation of polysaccharides. Furthermore, subunits of complex heteropolysaccharides could be assembled on such intermediates, and later incorporated into polysaccharides, or directly cross-linked into the cell wall. Evidence of the involvement of such intermediates in the synthesis of polysaccharides in a number of organisms is presented in Sections XII,3,b and XII,3,c. 

The structures of polyprenyl diphosphate-linked intermediates of Salmonella O-specific-polysaccharide biosynthesis were confirmed by chemical synthesis of their analogs derived from the plant polyprenols ficaprenol and moraprenol (structurally related to bacterial polyprenol57) with the following study of their behavior as substrates of enzymic reactions. Synthetic polyprenyl a-D-galactopyranosyl diphosphate291,292 was found to serve as an effective acceptor for the transfer of L-rhamnosyl groups.293"295 Two synthetic, isomeric disaccharide derivatives,292 13 and296 14, were tested as acceptors for enzymic D-mannosyl transfer from GDP-Man, but only the former was found to be an efficient substrate.294... 

With the availability of labeled hormones of high specific activity and the application of the principles of affinity chromatography, researchers were able to isolate cellular proteins that bind to plant hormones in vitro. Such proteins have been referred to as receptor proteins, binding proteins, or acceptor proteins. Tacit in the concept of hormone receptor proteins is the stereo-specific interaction of the hormone and the receptor protein (19). The resulting hormone-protein complex participates in growth processes that depend on new or enhanced protein synthesis. Advances in molecular biology and related sciences have enabled many researchers to study the role of receptors in the control of nuclear functions or other activities and to determine the site of primary hormonal action. 

Phospholipases are very versatile enzymes which allow the transformation of inexpensive natural products into highly valuable compounds like specific structurally defined phospholipids, organic monophosphates or diphosphates and DAG with the natural absolute configuration. Of particular synthetic utility is PLD from bacterial sources which is able to effect the phosphoryl transfer in a water-containing biphasic system. PLD shows a wide substrate specificity for both the polar head and the alcohol acceptors as well as for the lipophilic part of the molecule. The enzyme behaves like a generic phosphodiesterase with broad substrate specificity and high transphosphatidylation ability. The molecular basis of this behavior should become clear by inspection of the three-dimensional structure and comparison with other phosphoric acid ester hydrolytic enzymes. The crystal structure of this enzyme has not been elucidated. The potential of the many different PLD from plants which show peculiar substrate specificity should allow one to expand the synthetic utility to the hydrolysis-synthesis of natural and unnatural phosphatidylinositols. 

---