Serine proteases inactivation

Definition of Ej and E2 eonformations of the a subunit of Na,K-ATPase involves identification of cleavage points in the protein as well as association of cleavage with different rates of inactivation of Na,K-ATPase and K-phosphatase activities [104,105]. In the Ei form of Na,K-ATPase the cleavage patterns of the two serine proteases are clearly distinct. Chymotrypsin cleaves at Leu (C3), Fig. 3A, and both Na,K-ATPase and K-phosphatase are inactivated in a monoexponential pattern [33,106]. Trypsin cleaves the E form rapidly at Lys ° (T2) and more slowly at Arg (T3) to produce the characteristie biphasic pattern of inactivation. Localization of these splits was determined by sequencing N-termini of fragments after isolation on high resolution gel filtration columns [107]. 

Cl, C2...C9 The 9 main components of complement Cl inhibitor A serine protease inhibitor which inactivates Clr/Cls Clq Complement fragment Iq ClqR Receptor for Clw facilitates attachment of immune complexes to mononuclear leucocytes and endothelium... 

The general kinetic outline for the inactivation of serine proteases by suicide... 

Bromomethyl-3,4-dibromo-3,4-dihydrocoumarin 1 (Fig. 11.4) and its chloro-methylated analogue 2b rapidly and progressively inactivate a-chymotrypsin and also the activities of a series of trypsin-like proteases. A benzyl substituent characteristic of good substrates of a-chymotrypsin was introduced at the 3-position to make inhibition more selective. This substituted dihydrocoumarin 3 irreversibly inhibited a-chymotrypsin and other proteases. These functionalized six-membered aromatic lactones, and their five- and seven-membered counterparts, 3//-benzofuran-2-ones 2a26 and 4,5-dihydro-3//-benzo[b]oxepin-2-ones 2c,27 were the first efficient suicide inhibitors of serine proteases. Their postulated mechanism of action is shown in Scheme 11.2. 

SCHEME 11.2 Postulated mechanism of inactivation of a serine protease by a functionalized dihydrocoumarin such as molecule 3 (i i = benzyl).28... 

SCHEME 11.3 Postulated mechanisms for the inhibition of serine proteases by coumarin derivatives. NuH nucleophile. Pathway a suicide-type inactivation (suicide substrate). Pathway b transient inactivation by formation of a stable acyl-enzyme (alternate substrate-inhibitor). 

DPP-4 is a serine protease that inactivates GLP-1. GLP-1 stimulates insulin secretion and suppresses glucagon release. The inhibition of DPP-4 prolongs the half-life of GLP-1 and brings about beneficial effects on glucose levels and glucose tolerance in type 2 diabetics. Backes et al. [64] report on the parallel optimization of enzyme binding affinity and inhibition, selectivity, ADME properties, and PK (Scheme 19). 

Figure 7. Two examples of irreversible inactivators that are not suicide substrates a) TPCK, a classic" affinity label of the serine protease chymotrypsin, b) ZFK-CH2-mesitoate, a quiescent" affinity label of the cysteine protease cathepsin B, and c) the kinetic scheme for both forms of affinity label-inactivation.

Note that penicillins and structurally related antibiotics are frequently deactivated by the action of bacterial -lactamase enzymes. These enzymes also contain a serine residue in the active site, and this is the nucleophile that attacks and cleaves the P-lactam ring (see Box 7.20). The P-lactam (amide) linkage is hydrolysed, and then the inactivated penicillin derivative is released from the enzyme by further hydrolysis of the ester linkage, restoring the functional enzyme. The mode of action of these enzymes thus closely resembles that of the serine proteases there is further discussion in Box 7.20. 

Circulahng plasmin is rapidly neutralized by aj-an-tiplasmin, a physiological serine protease inhibitor that forms an inert complex with plasmin. In contrast, hbrin-bound plasmin is resistant to inactivation by 2-an-tiplasmin. Under normal circumstances plasma t-PA is inactive because it is inhibited by PAI-1, while t-PA that is bound to hbrin is unaffected by PAI-1. In addition, plasma t-PA has a very rapid turnover in blood (half-life 5 to 8 minutes). For these reasons, hbrinolysis is normally restricted to the thrombus. 

Protein C. This vitamin K-dependent glycoprotein serine protease zymogen is produced in the liver. It is an anticoagulant with species specificity (19—21). Protein C is activated to Protein Ca by thrombomodulin, a protein that resides on the surface of endothelial cells, plus thrombin in the presence of calcium. In its active form, Protein Ca selectively inactivates, by proteolytic degradation, Factors V, Va, VIII, and Villa. In this reaction the efficiency of Protein Ca is enhanced by complex formation with free Protein S. In addition, Protein Ca activates tissue plasminogen activator, which promotes the conversion of plasminogen [9001-91-6] to plasmin [9001-90-5]. 

Activated Protein C (CJ [42617-41 -4] (19—21) is a naturally occurring serine protease that, in combination with free Protein S, degrades and inactivates Factors V, Va, VIII, and Villa. By degradation of these factors the blood becomes anticoagulated and thus Ca may be a useful therapeutic agent. 

Isocoumarins inactivate many serine proteases. For example, 7-amino-4-chloro-3-methoxyisocoumarin acylates serine 195 of elastases as follows.s... 

Penicillin and related antibiotics are inactivated by P-lactamases (Box 20-G), some of which resemble serine proteases in forming acyl enzymes with active site serine side chains.656 657 Others are zinc metallogen-zymes.658/659 Amidohydrolases such as asparaginase and glutaminase,660/661 deacetylases,662 and many other hydrolases can also be described as acyltransferases. 

The serine proteases act by forming and hydrolyzing an ester on a serine residue. This was initially established using the nerve gas diisopropyl fluorophosphate, which inactivates serine proteases as well as acetylcholinesterase. It is a very potent inhibitor (it essentially binds in a 1 1 stoichiometry and thus can be used to titrate the active sites) and is extremely toxic in even low amounts. Careful acid or enzymatic hydrolysis (see Section 9.3.6.) of the inactivated enzyme yielded O-phosphoserine, and the serine was identified as residue 195 in the sequence. Chy-motrypsin acts on the compound cinnamoylimidazole, producing an acyl intermediate called cinnamoyl-enzyme which hydrolyzes slowly. This fact was exploited in an active-site titration (see Section 9.2.5.). Cinnamoyl-CT features a spectrum similar to that of the model compound O-cinnamoylserine, on denaturation of the enzyme in urea the spectrum was identical to that of O-acetylserine. Serine proteases act on both esters and amides. 

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