Alkylation of heme

As mentioned earlier, alkylation of heme by artemisinin was first reported by Meshnick and coworkers after identification of heme-drug adducts by mass spectrometry, but no... 

Full nuclear magnetic resonance (NMR) assignments for artemisitene 27 were reported for the first time <2001J(P1)2421> and the NMR characterization of covalent adducts obtained by the alkylation of heme with artemisinin 9a and its derivatives has been described <2002ICA488, 2005AGE2060>. 

Robert, A. Coppel, Y Meunier, B. Alkylation of heme by the antimalarial drug artemisinin. Chem. Commurt, 2002, 5 414-A15. 

How can the alkylation of heme by artemisinin kill the parasite Plasmodium falciparum s histidine-rich protein (PfHRP-II) promotes the formation of hemozoin. Ibis protein contains repeats of the sequence His-His-Ala, together histidine and alanine making up 76% of the mature protein (27). HRP is able to bind approximately SO molecules of heme at pH 7 (28), and 17 at pH 4.8, thus acting as a scaffold for heme, important in the initiation of hemozoin chains (29). Recent studies suggest that the heme-artemisinin adducts (Figure 1) are able to bind to PfHRP-II widi a higher afBnify than heme itself, splace heme from PfHRP-II, and diat either low pH or chloroquine dissociates heme but not heme-artemisinin adducts from PfHRP-II (3Cf). 

They concluded that the parasiticidal action of trioxanes involved reductive cleavage of the peroxide bond by intracellular iron-sulphur redox centres (rather than heme) and subsequent alkylation of the redox centre. This type of redox centre is known to exist in many enzymes and Wu and coworkers proposed that structural differences between those in the parasite and those in mammalian systems could account for the high selective cytotoxicity of artemisinin. 

Asawamahasakda, W., Ittarat, I., Pu, Y.-M., Ziffer, H., and Meshnick, S.R. Reaction of antimalarial endoperoxides with specific parasite proteins, Antimicrob. Agents Chemother., 38, 1854, 1994 Yang, Y.-Z., Little, B., and Meshnick, R. Alkylation of proteins by artemisinin effects of heme, pH, and drug structure, Biochem. Pharmacol., 48, 569,1994. 

Bartnicki, E. W., N. O. Belser, and C. E. Castro, Oxidation of heme proteins by alkyl halides A probe for axial inner sphere redox capacity in solution and in whole cells , Biochemistry, 17, 5582-5586 (1978). 

Castro, C. E., and E. W. Bartnicki, Conformational isomerism and effective redox geometry in the oxidation of heme proteins by alkyl halides, cytochrome C, and cytochrome oxidase , Biochem., 14,498-503 (1975). 

Another type of inactivation, namely reversible inactivation, deserves mention because of its unusual nature and its potential to impact the interpretation of mechanism-based inactivation of CYP enzymes. In 1995, alkylbenzene and 1 -hexyne were reported to /V-alkylate, the heme moiety of chloroperoxidase in a P450-like reaction (52). These compounds inactivated this heme-containing enzyme in a manner that met the criteria for a mechanism-based inhibitor as defined by Silverman (44). However, the inactivated enzyme spontaneously lost... 

Dexter A, Hager L. Transient heme N-alkylation of chloroperoxidase by terminal alkenes and alkynes. J Am Chem Soc 1995 117 817-818. 

Chloramphenicol and secobarbital exhibit properties similar to those of tienilic acid, but they have not been studied in humans (11). Oxidative dechlorination of chloramphenicol with formation of reactive acyl chlorides appears to be an important metabolic pathway for irreversible inhibition of CYP. Chloramphenicol binds to CYP, and subsequent substrate hydroxylation and product release are not impaired. The inhibition of CYP oxidation and the inhibition of endogenous NADPH oxidase activity suggest that some modification of the CYP has taken place, which inhibits its ability to accept electrons from the CYP reductase (11). Secobarbital completely inactivates rat CYP2B1 functionally, with partial loss of the heme chromophore. Isolation of the N-alkylated secobarbital heme adduct and the modified CYP2B1 protein revealed that the metabolite partitioned between heme N-alkylation, CYP2B1 protein modification, and epoxidation. A small fraction of the prosthetic heme modifies the protein and contributes to the CYP2B1 inactivation (12). 

Ator MA, David SK, Ortiz de Montellano PR (1987) Structure and catalytic mechanism of horseradish peroxidase. Regiospecific meso alkylation of the prosthetic heme group by alkylhydrazines. J Biol Chem 262 14954-14960... 

One of the variables in the structures of the porphyrins present in heme proteins is the presence or absence of vinyl substituents on the periphery of the macrocycle. For example, b hemes have vinyl substituents whereas c hemes do not. Because of the sensitivity of such vinyl substituents during synthetic transformations, it has often been desirable to use octa-alkyl porphyrins in model studies of the spectroscopic properties of heme systems. The development of improved methods for the preparation of octa-alkyl porphyrins has likewise increased the availability of such porphyrins for model studies (20, 21). To assess the effect that replacement of the two vinyl substituents in protoporphyrin IX with alkyl (ethyl) groups has on the MCD properties of the heme system, an extensive and systematic study of the MCD properties of mesoheme IX-reconstituted myoglobin and horseradish peroxidase in comparison with the spectra of the native protoheme-bound proteins has been carried out (22). The structures of these two porphyrins are shown in Figure 3. 

N-substituted iron porphyrins form upon treatment of heme enzymes with many xenobiotics. The formation of these modified hemes is directly related to the mechanism of their enzymatic reactivity. N-alkyl porphyrins may be formed from organometallic iron porphyrin complexes, PFe-R (a-alkyl, o-aryl) or PFe = CR2 (carbene). They are also formed via a branching in the reaction path used in the epoxidation of alkenes. Biomimetic N-alkyl porphyrins are competent catalysts for the epoxidation of olefins, and it has been shown that iron N-alkylporphyrins can form highly oxidized species such as an iron(IV) ferryl, (N-R P)Fe v=0, and porphyrin ir-radicals at the iron(III) or iron(IV) level of metal oxidation. The N-alkylation reaction has been used as a low resolution probe of heme protein active site structure. Modified porphyrins may be used as synthetic catalysts and as models for nonheme and noniron metalloenzymes. 

Fig. 4. Proposed functions of the hydroxyfarnesylethyl group of heme A. (A) A possible electron transfer pathway formed by overlapping of rr-electron orbitals in the alkyl chain with that of the pyrrole. (B) A side-on coordination of the terminal double bond to Cub and a coordination of a deprotonated form of the double bond to Fe j. A proposed proton-pumping mechanism including the redox-coupled change in the two coordination states.

Fig. 3. Schematic representation of a possible conformation of the 2-alkyl group of heme A,...

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