Enzymes, synthesis

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

Microorganisms exhibit nutritional preferences. The enzymes for common substrates such as glucose are usually constitutive, as are the enzymes for common or essential metabohc pathways. Furthermore, the synthesis of enzymes for attack on less common substrates such as lactose is repressed by the presence of appreciable amounts of common substrates or metabolites. This is logical for cells to consei ve their resources for enzyme synthesis as long as their usual substrates are readily available. If presented with mixed substrates, those that are in the main metabolic pathways are consumed first, while the other substrates are consumed later after the common substrates are depleted. This results in diauxic behavior. A diauxic growth cui ve exhibits an intermediate growth plateau while the enzymes needed for the uncommon substrates are synthesized (see Fig. 24-2). There may also be preferences for the less common substrates such that a mixture shows a sequence of each being exhausted before the start of metabolism of the next. 

Enzymic and Chemical Synthesis of the a-l 2-glucosidic Linkage Enzymic Synthesis, S. A. Barker, E. J. Bourne, P. M. Grant, andM. Stacey, Nature, 178(1956) 1221-1223. 

Studies of Aspergillus niger. Part XI. Enzymic Synthesis of a Pseudoaldobiuronic Acid, S. A. Barker, A. Gomez-Sanchez, and M. Stacey,/. Chem. Soc., (1959) 3264- 3269. 

On the other hand, as mentioned in the preceding subsection, a preparative-scale enzymic synthesis of 1-deoxy-D-r/ireo-pentulose can be achieved, according to Reaction 1, in the presence of an extract of B. pumilus. Obviously, this raises the question of the relevance of Eq. 1 to the production of the pentulose in microorganisms. Acetoin in Reaction 1 could be replaced by 3-hydroxy-3-methyl-2-butanone (then the by-product is acetone). More interestingly, it can be also replaced by pyruvate, then the pentulose is synthesized according to Reaction 3 ... 

AA biosynthesis. At an ever deeper level it is possible that high concentrations of the product AA can inhibit expression of the gene for formation of the RNA needed for enzyme synthesis. This form of suppression of gene action over many generations could lead to loss of the gene, causing the AA to become an essential amino acid. 

The absolute quantity of an enzyme reflects the net balance between enzyme synthesis and enzyme degradation, where 4 and represent the rate constants for the overall processes of synthesis and degradation, respectively. Changes in both the 4 and of specific enzymes occur in human subjects. 

Ghanges in the availability of substrates are responsible for most changes in metabolism either directly or indirectly acting via changes in hormone secretion. Three mechanisms are responsible for regulating the activity of enzymes in carbohydrate metabolism (1) changes in the rate of enzyme synthesis, (2) covalent modification by reversible phosphorylation, and (3) allosteric effects. 

Induction Repression of Key Enzyme Synthesis Requires Several Hours... 

Lipogenesis is regulated at the acetyl-CoA carboxylase step by allosteric modifiers, phosphorylation/de-phosphorylation, and induction and repression of enzyme synthesis. Citrate activates the enzyme, and long-chain acyl-CoA inhibits its activity. Insulin activates acetyl-CoA carboxylase whereas glucagon and epinephrine have opposite actions. 

The nature of the molecular defect Is unclear and presumably lies In the repression mechanism for the gene controlling formation of the enzyme protein. Exposure to any of the drugs listed In Table V results In further marked de-repressIon of enzyme synthesis and severe porphyria. 

DRUGS INDUCING MARKED DE-REPRESSION OF ENZYME SYNTHESIS... 

Rhodophyta, Rhodophyceae, Ceramales, Rhodomelaceae). Aust. J. Chem. 42 1591-1603. Collins, F. W., De Luca, V., Ibrahim, R. K., Voirin, B. and Jay, M. 1981. Polymethylated flavonols of Chrysosplenium americanum. I. Identification and enzymic synthesis. Zeit. Naturforsch. 36c 730-736. 

Mercenier A, J-P Simon, CV Wauven, D Haasm, V Stalon (1980) Regulation of enzyme synthesis in the arginine deiminase pathway of Pseudomonas aeruginosa. J Bacteriol 144 159-163. 

Katayama, Y. Fukuzumi, T. Enzymic synthesis of three lignin-related dimers by an improved peroxidase-hydrogen peroxide system. Mokuzai Gakkaishi 1978,24,664-667. 

Target tissue response Synthesis of new enzymes Modification of existing enzymes Synthesis of new enzymes Modification of existing enzymes... 

Hubl, U. and Stevenson, D.E. (2001) In vitro enzymic synthesis of mammalian liver xenobiotic metabolites catalyzed by ovine liver microsomal cytochrome P450. Enzyme and Microbial Technology, 29, 306-311. 

One of the recent fundamental advances in carbohydrate chemistry has been enzymic synthesis in vtiro this has now been realized with dextran8 and with levan.8 However, since phosphate sugars are not involved in the enzymic syntheses of these two bacterial polysaccharides, it is obvious that phosphorylation is just one process for the natural synthesis of polysaccharides. 

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