Choline representative esters

Simplified scheme of ACh hydrolysis at the active center of ACh. Rectangular area represents the active center of the enzyme with its anionic and esteratic sites. Top, the initial bonding of ACh at the active center. The broken line at left represents electrostatic forces. The broken line at right represents the initial interaction between the serine oxygen of the enzyme and the carbonyl carbon of ACh. The ester linkage is broken, choline is liberated, and an acetylated enzyme intermediate is formed (middle. Finally, the acetylated intermediate undergoes hydrolysis to free the enzyme and generate acetic acid (bottom). 

The structures of common lipids, (a) The structures of saturated and unsaturated fatty acids, represented here by stearic acid and oleic acid, (b) Three fatty acids covalently linked to glycerol by ester bonds form a triacylglycerol. (c) The general structure for a phospholipid consists of two fatty acids esterified to glycerol, which is linked through phosphate to a polar head group. The polar head group may be any one of several different compounds—for example, choline, serine, or ethanolamine. 

Cinnamic esters are also found as sugar esters, or as esters of a variety of other organic acids. For example, sinapoyl esters represent a class of UV-absorbing compounds in the family of the Brassicaceae. Examples include sinapoyl malate (1.19) present in leaves, and sinapoyl choline (1.20) present in roots (Ruegger and Chappie, 2001). 

The reaction of ChE s of different origin towards a homologous scries of choline esters permits a further classification within this group. This is exemplified in Fig. 1 for two representative types, the true ChE from the electric organ of Electrophorus electricus and the pseudo ChE from human plasma, both acting on acetylcholine as substrate. The curves show hydrolytic rates as function of pS = —log (substrate concentration) and demonstrate a fundamental difference between the two enzymes The true type exhibits a bell-shaped pactivity curve, indicative of autoinhibition at high substrate concentrations (18). The pseudo enzyme, on the other hand, possesses a tS-shaped curve i.e., the maximum rate is reached at and beyond an optimal substrate concentration. 

This type of modified lipopeptide cannot be deblocked under basic conditions because the labile palmitic acid thioester group would be preferentially hydrolyzed. The C-terminus of the peptide chain was selectively deprotected by removing the choline ester with choline esterase without affecting the palmitic acid thioester bond. The observed chemoselectivity here is exactly opposite to that found in nonenzymatic conversions. Some of the synthesized N-terminally deprotected lipopeptides, have been labeled with biotinand are expected to serve as anchors for a protein moiety in an artificial membrane. Butyrylcholine esterase mediated cleavage of the choline ester has been utilized l as the key step in the synthesis of S-palmitoylated peptides such as Myr-Gly-Cys(Plm)-Thr-Leu-Ser-Ala-OH, which represents the characteristic N-terminus of the a-subunit of human G o protein. 

The second example represents a large-scale human metabolomics study that was performed with LC/MS [54]. The aim of this study was to identify potential biomarkers from lipid profiles of some 600 human plasma samples. Lipids were extracted from plasma samples and subjected to LC/ESI-MS analysis. Several different classes of lipids, such as phosphatidylcholines, lysophosphatidyl-cholines, triglycerides, diglycerides, sphingomyelins, and cholesterol esters were the target of this study. To detect small differences in metabolic profiles, statistical methods were used to process this large set of data. Partial least-squares discriminant analysis of the data could locate potential biomarkers. 

Two types of enzyme that hydrolyze acetylcholine rapidly have been identified. One is represented by an enzyme present in the serum of many animals. This esterase attacks many other esters at faster rates than acetylcholine, and has been called nonspecific or pseudocholinesterase, in distinction to the so-called specific or true cholinesterase found in erythrocytes, nervous and electrical tissue. The latter hydrolyzes acetylcholine more rapidly than other choline esters. Extensive kinetic studies have been made with both types of enzyme. Studies with highly purified true cholinesterase have been particularly revealing in explorations of the chemical nature of enzymatic action. 

Besides the lichen substances discussed, a number of compounds have been isolated which simply represent general constituents of living cells, biosynthesis of which will not be taken up here. Sufficient to mention choline sulfate and ethanol sulfate esters, with which S-labeling studies have recently been carried out (Feige and Simonis, 1969). Finally, attention should be drawn to a report by Jackson and Keller (1970) on the formation of an unusual ferric oxide mineral through the action by a tropical lichen on basalt as an example of indirect biosynthesis. 

The enzymatic hydrolysis of lecithin to fatty acids, glycerol, phosphoric acid, and choline requires the participation of some four enzymes. The enzymes attacking the various bonds of a lecithin molecule are summarized in Table XIV. The bonds which are cleaved are referred to by number in the lecithin formula shown below the same numbers are used in Table XIV to indicate the respective enzymes involved. The cleavage of the fatty acid-glycerol bond, as represented by diagonal lines 1 and 2, is based on hydrolysis studies of simple esters. Diagonal lines 3 and 4 are only provisional, as presumably either a P—0 or C—O bond to phosphate could be cleaved. 

Fig. 1 Coarse-grained (CG) models of selected phospholipid molecules [41]. CG particles (transparent spheres) are superimposed onto the atranic structures of the chemical groups that they represent. NC choline, NH amine, PH and PHE phosphate, GL glycol, EST ester, CM aUcyl group, CT and CT2 terminal alkyl groups, CMD monounsaturated aUtyl group. The effective water particle (labeled W) has the same mass as three water molecules...

Lecithin is a phosphoglyceride (phosphatide) natural product isolated commercially from soybean oil and, in much lower quantity, from egg yolks. The terminology is somewhat confused for historical reasons (7). While the term lecithin is sometimes used synonymously with a-phosphatidyl choline, it more correctly denotes a crude mixture also containing P-phosphatidyl choline and phosphatidyl esters of other compounds, chiefly ethanolamine, inositol, and serine. (While phosphatidyl choline and phosphatidyl ethano-lamine are amphoteric, phosphatidyl inositol and phosphatidyl serine are anionic.) For soybean lecithin, R and R represent C16 saturated fatty acid and Cjg saturated and unsaturated... 

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