Serine Enzymes

Several strategies for the production of pure D- or L-amino acids rely on the use of enzymes. L-Serine (49) is synthesized by combining glycine (48) and formaldehyde in the presence of the enzyme serine hydroxymethyl transferase (66). 

Volume 244. Proteolytic Enzymes Serine and Cysteine Peptidases Edited by Alan J. Barrett... 

Fig. 3.6. Stereoelectronic control of the cleavage of the tetrahedral intermediate during hydrolysis of a peptide bond by a serine hydrolase. The thin lines represent the reactive groups of the enzyme (serine, imidazole ring of histidine) the thick lines represent the tetrahedral intermediate of the transition state. The full circles are O-atoms open circles are N-atoms. The dotted lines represent H-bonds the thick double arrow indicates an unfavorable dipole-dipole interaction [21]. A (R)-configured N-center B (S)-configured N-center.

Trifluoromethyl /1-thioalkyls and /1-amino alcohols are often good reversible inhibitors of esterases and proteases, respectively. Depending on the enzymes (serine or aspartyl enzymes), fluorinated alcohols are often less efficient inhibitors than the corresponding ketones, which act as analogues of the transition state (vide infra). Nevertheless, fluoroalcohols inhibit hydrolytic enzymes with high inhibition constants (Figure 7.25)." ... 

Generally amino acid conjugation is a detoxication reaction. However, amino acid conjugation with hydroxylamino groups (N-hydroxy) can lead to the formation of reactive nitrenium ions, as already discussed with sulfate conjugation and acetylation. For example, the conjugation of serine with N-hydroxy-4-aminoquinoline-l-oxide (Fig. 4.40 for structure) leads to such a reactive nitrenium ion. This requires the enzyme serine-tRNA synthetase. 

The enzyme serine hydroxymethyltransferase requires pyridoxal phosphate as cofactor. Propose a mechanism for the reaction catalyzed by this enzyme, in the direction of serine degradation (glycine production). (Hint See Figs 18-19 and 18-20b.)... 

Fig. 4.1 Mechanism of action of cyclosporine. Cyclosporine readily diffuses into the cytoplasm of the target cells where it binds to cyclophilins. The cyclosporine-cyclophilin complex stably associates with calcineurin and inhibits calcineurin activity. Calcineurin is a Ca2+-dependent enzyme— serine/threonine phosphatase— which after activation by Ca2+, dephosphorylates a cytosolic component of NFAT (NFATc, cytosolic factor of activated T cells). After dephosphorylation, NFATc migrates from the cytoplasm to the nucleus where it associates with NFATn and induces transcription of several cytokine genes including IL-2. Cyclosporine inhibits calcineurin activity after associating with cyclophilins, resulting in the inhibition of IL-2 production and other cytokines (see Color Insert)...

As well as complexing the substrate to the active site, many enzymes link covalently with the substrate, or a portion of it, to form an additional intermediate. Such intermediates occur in the action of enzymes as diverse as alkaline phosphatase (phosphoryl enzyme), serine and cysteine proteases (acyl enzymes), glycosidases (acylal enzymes) and aldolases. 

Continuing the folate cycle, THF reacts with serine to produce 5,10-methylenetetrahydrofolate, a reaction catalyzed by the vitamin B6-dependent enzyme serine/glycine hydrox-ymethyltransferase. 

Several pharmaceutical enzymes belong to the group of serine-histidine estero-proteolytic enzymes (serine proteases), which display their catalytic activity with the aid of an especially reactive serine residue, whoso p-hydroxyi group forms a covalent bond with the substrate molecule. This reaction takes place by cooperation with the imidazole base of histidine. The specificity of the enzymes is achieved by the characteristic strocture of their substrate-binding centers, which in these proteases are built according to the same principle. They consist of a hydrophobic slit formed by apolar aide chains of amino acids and a dissociated side chain-located carboxyl group of an aspartic add residue at the bottom. 

Despite the fact that cestodes appear to be capable of considerable lipid synthesis, the actual pathways and enzyme systems involved have been little studied. Sphingomyelin synthesis has been investigated in H. diminuta (37), where it probably involves the same five-step pathway thought to occur in vertebrates involving the enzymes serine palmitoyl-transferase,... 

C((x)-C(p) bond cleavage in a-amino acids is precedented by the PLP-containing enzyme serine hydroxymethyltransferase, interconverting serine with glycine and a formaldehyde equivalent, but in that case C-C bond cleavage is a retro aldol process This route is not obviously open to ACPC (9). 

A transferase that also has aldolase activity and has been used to prepare a number of chiral compounds is the enzyme serine hydroxymethyltransferase (SHMT) (EC 2.1.2.1). This enzyme, also known as threonine aldolase, catalyzes the physiological reaction of the interconversion of serine and glycine with pyridoxal phosphate (PLP) and tetrahydrofolate (FH4) as the shuttling cofactor of the C-1 unit. It also catalyzes a number of other reactions, some of which are independent of PLP and FH4 [72]. The SHMT-catalyzed aldolase reaction generates two stereocenters, which it does stereospecifically at the (/.-carbon, whereas it is less strict at the [l-carbon (Scheme 13). Nevertheless, this enzyme from porcine liver, Escherichia coU and Candida humicola (threonine aldolase) has been used to prepare a number of P-hydroxy-a-amino acids [73-76],... 

Serine and threonine dehydrases. Serine and threonine are not substrates in transamination reactions. Their amino groups are removed by the pyridoxal phosphate-requiring hepatic enzymes serine dehydratase and threonine dehydratase. The carbon skeleton products of these reactions are pyruvate and a-keto-butyrate, respectively. 

Thus, the terms reversible and irreversible as applied to the carbamoyl ester and organophos-phorate anti-ChE agents, respectively, reflect only quantitative differences in rates of decarbamoy-lation or dephosphorylation of the conjugated enzyme. Both chemical classes react covalently with the enzyme serine in essentially the same manner as does ACh. 

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