Evidence for presence of an internal thiolester bond in third component of human complement.

Tack, Brian F.; Harrison, Richard A.; Janatova, Jarmila; Thomas, Matthew L.; Prahl, James W.

doi:10.1073/pnas.77.10.5764

Cited by 335 publications

(173 citation statements)

References 37 publications

(19 reference statements)

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“…If 3H-iodoacetic acid was used to label the released thiol group, both radioactive labels could be recovered in a single tryptic peptide. The two labeled residues were found in the sequence -Cys*-Gly-Glu-Glx*- (Tack et al, 1980), suggesting that there was a thioester between the thiol of the cysteine residue and the acyl group of the second glutamyl residue, forming a IS-member thiolactone ring (Fig. 3A).…”

Section: The Discovery Of the Internal Thioestermentioning

confidence: 99%

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

1997

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The covalent binding of complement components C3 and C4 is critical for their activities. This reaction is made possible by the presence of an internal thioester in the native protein. Upon activation, which involves a conformational change initiated by the cleavage of a single peptide bond, the thioester becomes available to react with molecules with nucleophilic groups. This description is probably sufficient to account for the binding of the C4A isotype of human C4 to amino nucleophiles. The binding of the C4B isotype, and most likely C3, to hydroxyl nucleophiles, however, involves a histidine residue, which attacks the thioester to form an intramolecular acyl-imidazole bond. The released thiolate anion then acts as a base to catalyze the binding of hydroxyl nucleophiles, including water, to the acyl function. This mechanism allows the complement proteins to bind to the hydroxyl groups of carbohydrates found on all biological surfaces, including the components of bacterial cell walls. In addition, the fast hydrolysis of the thioester provides a means to contain this very damaging reaction to the immediate proximity of the site of activation.

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Section: The Discovery Of the Internal Thioestermentioning

confidence: 99%

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

1997

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“…4 Biochemical assay for reactivity of C3 thiolester bond. Nucleophilic amines, such as CH3NH2, are inherently reactive with thiolester bonds; binding of the nucleophile is stoichiometric, with I mol of the nucleophile covalently bound for each mol of thiolester disrupted (20). Titration of thiolester reactivity was performed by a modification of the procedure of Tack et al (20) and Hostetter et al (21).…”

Section: Methodsmentioning

confidence: 99%

Biochemical Abnormalities of the Third Component of Complement in Neonates

Zach

Hostetter

1989

Pediatr Res

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ABSTRACT. The third component of complement, C3, is of central importance as an opsonin in the nonimmune host. Although gestational deficiencies in C3 levels are well recognized in neonates, defects in complement-mediated functions have not in every case correlated with low levels of complement proteins. Because opsonic functions of C3 are mediated through a reactive thiolester bond, we hypothesized that a biochemical dysfunction at this active site could explain the newborn's predisposition to infection, even with relatively normal C3 levels. We therefore examined the biochemical integrity of the C3 thiolester in an assay independent of all other complement proteins. As measured by ELISA, mean C3 levels from 44 neonates (24-43 wk) were significantly lower in infants < 30 wk gestational age (0.79 f 0.13 mg/mL) than in full-term newborns (1.19 +1 0.27 mg/ml, p < 0.05). Furthermore, biochemical reactivity of the thiolester bond, as measured by incorporation of the radiolabeled nucleophile, methylamine, correlated significantly with gestational age ( r = 0.45, p < 0.01). Functional C3 was defined as the product of thiolester reactivity and C3 level; 9/11 premature and 2/ 17 full-term infants had levels of functional C3 which were less than 50% of the adult norm. Structural analysis of neonatal C3 revealed the two-chain structure in all neonates; four neonates had an additional band at 205 kD which may represent an impairment in posttranslational processing of a precursor molecule. We conclude that defects in thiolester reactivity may constitute a newly identified mechanism for the newborn's susceptibility to infection. (Pediatr Res 26: 116-120, 1989) Abbreviations PAS, pooled adult serum CH3NH2, methylamine Sepsis is a leading cause of morbidity and mortality in neonates, with premature infants affected more frequently than fullterm newborns (I). This predisposition to infection is ascribable, at least in part, to age-related defects in host defense (2, 3). Primary among humoral defenses are the complement proteins, which serve as opsonins for common neonatal pathogens such as the group B Streptococcus or the K1 capsular serotype of Escherichia coli (4,5).

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“…The C3 moiety within this convertase is C3(H 2 O) formed on exposure and subsequent hydrolysis of the internal thioester, which is normally protected inside native C3 (14)(15)(16). C3(H 2 O) is "C3b-like"; it still contains C3a, but is conformationally similar to C3b.…”

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confidence: 99%

Properdin binding to complement activating surfaces depends on initial C3b deposition

Harboe

Johnson

Nymo

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Two functions have been assigned to properdin; stabilization of the alternative convertase, C3bBb, is well accepted, whereas the role of properdin as pattern recognition molecule is controversial. The presence of nonphysiological aggregates in purified properdin preparations and experimental models that do not allow discrimination between the initial binding of properdin and binding secondary to C3b deposition is a critical factor contributing to this controversy. In previous work, by inhibiting C3, we showed that properdin binding to zymosan and Escherichia coli is not a primary event, but rather is solely dependent on initial C3 deposition. In the present study, we found that properdin in human serum bound dose-dependently to solid-phase myeloperoxidase. This binding was dependent on C3 activation, as demonstrated by the lack of binding in human serum with the C3-inhibitor compstatin Cp40, in C3-depleted human serum, or when purified properdin is applied in buffer. Similarly, binding of properdin to the surface of human umbilical vein endothelial cells or Neisseria meningitidis after incubation with human serum was completely C3-dependent, as detected by flow cytometry. Properdin, which lacks the structural homology shared by other complement pattern recognition molecules and has its major function in stabilizing the C3bBb convertase, was found to bind both exogenous and endogenous molecular patterns in a completely C3-dependent manner. We therefore challenge the view of properdin as a pattern recognition molecule, and argue that the experimental conditions used to test this hypothesis should be carefully considered, with emphasis on controlling initial C3 activation under physiological conditions.

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Evidence for presence of an internal thiolester bond in third component of human complement.

Cited by 335 publications

References 37 publications

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

Biochemical Abnormalities of the Third Component of Complement in Neonates

Properdin binding to complement activating surfaces depends on initial C3b deposition

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