Serine constitution

Phosphovitin an egg yolk protein containing 10 % phosphate (M, 180,000). Serine constitutes 50% of the total amino acid content, and all the serine residues are phosphorylated. P. is synthesized in the liver of the laying hen rmd transported in the blood to the developing egg. 

Factor VII. This is a vitamin K-dependent serine protease that functions in the extrinsic coagulation pathway and catalyzes the activation of Factors IX and X. Factor VII is present constitutively in the surface membrane of pericytes and fibroblasts in the adventitia of blood vessels, vascular endothehum, and monocytes. It is a single-chain glycoprotein of approximately 50,000 daltons. 

P-Lactamases are enzymes that hydrolyze the P-lactam ring of P-lactamantibiotics (penicillins, cephalosporins, monobactams and carbapenems). They are the most common cause of P-lactam resistance. Most enzymes use a serine residue in the active site that attacks the P-lactam-amid carbonyl group. The covalently formed acylester is then hydrolyzed to reactivate the P-lacta-mase and liberates the inactivated antibiotic. Metallo P-lactamases use Zn(II) bound water for hydrolysis of the P-lactam bond. P-Lactamases constitute a heterogeneous group of enzymes with differences in molecular structures, in substrate preferences and in the genetic localizations of the encoding gene (Table 1). 

The mechanism for the lipase-catalyzed reaction of an acid derivative with a nucleophile (alcohol, amine, or thiol) is known as a serine hydrolase mechanism (Scheme 7.2). The active site of the enzyme is constituted by a catalytic triad (serine, aspartic, and histidine residues). The serine residue accepts the acyl group of the ester, leading to an acyl-enzyme activated intermediate. This acyl-enzyme intermediate reacts with the nucleophile, an amine or ammonia in this case, to yield the final amide product and leading to the free biocatalyst, which can enter again into the catalytic cycle. A histidine residue, activated by an aspartate side chain, is responsible for the proton transference necessary for the catalysis. Another important factor is that the oxyanion hole, formed by different residues, is able to stabilize the negatively charged oxygen present in both the transition state and the tetrahedral intermediate. 

This model clearly shows that the catalytic machinery involves a dyad of histidine and aspartate together with the oxyanion hole. Hence, it does not involve serine, which is the key amino acid in the hydrolytic activity of lipases, and, together with aspartate and histidine, constitutes the active site catalytic triad. This has been confirmed by constructing a mutant in which serine was replaced with alanine (Serl05Ala), and finding that it catalyzes the Michael additions even more efficiently than the wild-type enzyme (an example of induced catalytic promiscuity ) [105]. 

The 3-phosphoinositide-dependent protein kinase-1 (PDKl) is a 556-amino acid enzyme composed of three well-differentiated motifs an N-terminal domain, a constitutively activated serine/threonine kinase domain, and a Pleck-strin homology (PH) domain at its C-terminus [87-91]. The attractiveness of PDKl as a potential anticancer target is hnked to its ability to control the activity of a diverse set of AGC kinase members, in particular the three PKB isoforms [92], Full activation of PKB requires phosphorylation at two sites. 

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