Enzymic synthesis, from modified precursors

If a substance is to be a NT it should be possible to demonstrate appropriate enzymes for its synthesis from a precursor at its site of action, although peptides are transported to their sites of location and action after synthesis in the axon or distal neuronal cell body. The specificity of any enzyme system must also be established, especially if they are to be modified to manipulate the levels of a particular NT, or used as markers for it. Thus choline acetyltransferase (ChAT) may be taken as indicative of ACh and glutamic acid decarboxylase (GAD) of GABA but some of the synthesising enzymes for the monoamines lack such specificity. 

VI. Enzymic Synthesis of Bacterial Polysaccharides from Modified Precursors... 

The polypeptide chains of all proteins are synthesized by the process described above. This mechanism gives rise to primary polypeptide chains, which are often further modified—for example, by cleavage into smaller peptides, by stmctural modification of selected amino acid residues, by splicing of the polypeptide chain, or by the formation of covalent bonds between polypeptide chains. Some of these secondary modifications are related to the correct folding of polypeptide chains and to the production of active enzymes or peptide hormones from inactive precursors (e.g., insulin from proinsulin). Also, the transport of proteins within the cell or the secretion of extracellular proteins is often linked to structural changes in polypeptide chains either during or after completion of synthesis. 

The total synthesis of sialosides by using the chemoenzymatic approach is as follows [74]. Sialic acid itself can be synthesized from ManNAc, mannose, or their derivatives by sialic acid aldolase enzyme through aldol condensation reaction. If ManNAc is chemically or enzymatically modified at C2, C4—C6 positions, sialic acid has structural modifications at C5, C7-C9 positions, respectively. The sialic acids are subsequently activated by a CMP-siahc acid synthetase to form a CMP-sialic acid, which is the donor used by sialyltransferases. Because CMP-sialic acid is tmstable, the CMP-Neu5Ac synthetase is valuable for the preparative enzymatic synthesis of sialosides. In the last steps, the CMP-sialic acid is transferred to galactose or GalNAc terminated glycosides by sialyltransferases to form structurally defined sialosides. Examples are that Chen and co-workers have recently developed a one-pot multienzyme system for the efficient synthesis of a-sialosides (Table 2) [12,76,79]. In this system, recombinant E. coli K-12 sialic acid aldolase catalyzed the synthesis of sialic acid precursors for... 

Group 1 includes, in addition to serotonin and acetylcholine, histamine" and the catecholamine transmitters dopamine and norepinephrine. Each of these compounds is synthesised from a circulating precursor. The synthesis of each is catalysed by an enzyme (e.g. tryptophan hydroxylase for serotonin choline acetyltransferase for acetylcholine) that has a relatively low affinity for its substrate, and which thus normally operates at less-than-maximal efficiency (i.e. because it virtually never is fully saturated with the tryptophan or choline). Group 2 includes the various peptides that have been found in brain neurons which are thought by many scientists to function as neurotransmitters, largely because of their presence in neurons, and their ability to modify ionic fluxes when applied to brain neurons. These compounds almost certainly are synthesised not by enzymes but by polyribosomes (i.e. strands of messenger RNA attached to ribosomes). The concentrations of amino acids needed to allow polysome-directed peptide synthesis to occur at maximal rates in brain apparently are quite low hence, it doesn t seem likely that... 

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