Enzymes regulatory mechanisms

Description of domain structure of a protein Description of enzyme regulatory mechanism General description of function (s) of a protein Description of compound (s) that stimulate synthesis of a protein... 

The finding that DMAPP was rate limiting in the production of aCA was followed by studies (McGrath et al, 1977) on the diversion of this metabolite from polyisoprenoid to CA biosynthesis in P. cyclopium. The enzymes S and T from P. cyclopium were separated in order to study the control at the branch point. The enzyme isopentylpyrophosphate isomerase (EC 5.3.3.2) (I) was also included in the study, because it is responsible for the production of DMAPP from IPP. The early control of the diversion of DMAPP away from polyisoprenoids to secondary metabolites must, of course, be the appearance of cAATrp. Both transferases are not utilized to their maximum capacity, and the production of their end products seem to bear no relationship to their concentrations. It is apparent, therefore, that simple enzyme concentrations alone do not control the branch point. As cAATrp and DMAPP pools become rate limiting, other enzyme regulatory mechanisms must become more important. In this regard it is probably significant that... 

Such a pathway has both flow and direction. The enzymes catalyzing nonequilibrium reactions are usually present in low concentrations and are subject to a variety of regulatory mechanisms. However, many of the reactions in metabolic pathways cannot be classified as equilibrium or nonequilibrium but fall somewhere between the two extremes. 

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). 

The stability of phytoene desaturase and lycopene cyclase transcripts also influenced accumulation of carotenoids. Efforts in directed evolution of carotenogenic enzymes have also continued. Alternate approaches using systematic and combinatorial gene knockout targets have allowed for enhancement of carotenoid production in the absence of a priori assumptions of regulatory mechanisms. 

The concentration of catecholamines within nerve terminals remains relatively constant. Despite the marked fluctuations in the activity of catecholamine-containing neurons, efficient regulatory mechanisms modulate the rate of synthesis of catecholamines [ 11 ]. A long-term process affecting catecholamine synthesis involves alterations in the amounts of TH and DBH present in nerve terminals. When sympathetic neuronal activity is increased for a prolonged period of time, the amounts of mRNA coding for TH and DBH are increased in the neuronal perikarya. DDC does not appear to be modulated by this process. The newly synthesized enzyme molecules are then transported down the axon to the nerve terminals. 

HDC activity can be regulated by both hormonal and neuronal factors, most of which are poorly understood. Phosphorylation of the enzyme by PKA maybe an important regulatory mechanism. Several regulatory sites have recently been found in the promoter region of the gene. 

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