Activation of Complement Component C5

Abstract

Although the initiating complex of lectin pathway (called M1 in this study) generates C3/C5 convertases similar to those assembled by the initiating complex (C1) of the classical pathway, activation of complement component C5 via the lectin pathway has not been examined. In the present study kinetic analysis of lectin pathway C3/C5 convertases assembled on two surfaces (zymosan and sheep erythrocytes coated with mannan (E Man )) revealed that the convertases (ZymM1,C4b,C2a and E Man M1,C4b,C2a) exhibited a similar but weak affinity for the substrate, C5 indicated by a high K m (2.73-6.88 M). Very high affinity C5 convertases were generated when the low affinity C3/C5 convertases were allowed to deposit C3b by cleaving native C3. These C3b-containing convertases exhibited K m (0.0086 -0.0075 M) well below the normal concentration of C5 in blood (0.37 M). Although kinetic parameters, K m and k cat , of the lectin pathway C3/C5 convertases were similar to those reported for classical pathway C3/C5 convertases, studies on the ability of C4b to bind C2 indicated that every C4b deposited on zymosan or E Man was capable of forming a convertase. These findings differ from those reported for the classical pathway C3/C5 convertase, where only one of four C4b molecules deposited formed a convertase. The potential for four times more amplification via the lectin pathway than the classical pathway in the generation of C3/C5 convertases and production of pro-inflammatory products, such as C3a, C4a, and C5a, implies that activation of complement via the lectin pathway might be a more prominent contributor to the pathology of inflammatory reactions.The lectin pathway (1), a more recent discovery of complement, is activated when mannan-binding lectin (MBL), 2 a pattern-recognition molecule, binds to sugars present on the cell surface of microorganisms (2-8). In plasma, MBL circulates in complex with serine proteases called MBL-associated serine proteases (MASPs) (9 -11). We will refer to this complex as M1 in keeping with the complement nomenclature. Three types of MASPs are associated in the proenzyme form with MBL: MASP-1, MASP-2, and MASP-3. The exact function of MASP-1 and MASP-3 has yet to be determined, although MASP-1 has been shown to cleave C3 weakly (12, 13). MASP-2 has been shown to cleave C2 and C4 (12-14). Binding of MBL⅐MASP complexes to sugar residues present on the cell surfaces of microorganisms results in a conformational change of MBL (15, 16), which activates MASP-2. Activated MASP-2 cleaves C4 and C2 resulting in the formation of C3/C5 convertases (C4b,C2a) (17-19).C3/C5 convertases are enzymes that cleave C3 and C5. Products generated upon the cleavage of C3 and C5 (C3a, C3b, C5a, and C5b) have important biological activities (20). C3b, the larger cleavage product of C3, opsonizes bacteria, whereas C3a, the smaller product, is an anaphylatoxin with important functions such as chemotaxis of mast cells and contraction of smooth muscle cells (21-24). C5b, the larger cleavage product of C5, initiates formation ...

Reduced order modeling and analysis of the human complement system

“…The data used for model training was taken from the study of Morad et al [ 35 ] and is given in the Supporting Information ( S2 Table ). In this study, we assumed zymosan A differentially activated the lectin pathway, consistent with the study of Morad et al [ 35 ] and several previous studies [ 36 – 38 ]. However, the role of zymosan may be more complex, as it may activate all three complement pathways under certain conditions.…”

Reduced order modeling and analysis of the human complement system

“…1 Morad et al [34] and is given in Table T2. In this study, we assumed zymosan A 281 differentially activated the lectin pathway, consistent with the study of Morad et al [34] 282 and several previous studies [35][36][37]. However, the role of zymosan may be more 283 complex, as it may activate all three complement pathways under certain conditions.…”

The Carbohydrate-linked Phosphorylcholine of the Parasitic Nematode Product ES-62 Modulates Complement Activation

“…When the C4 is cleaved and C4b binds to a region of ES-62 or CRP or a neighboring attachment site, then the CRP-activated C1 complex is still highly mobile and would likely move before it can cleave C2. Variation in efficiency of generating C3 convertase has been observed before; for instance, lectin pathway activation is much more efficient than the classical pathway ( 41 , 42 ). The reason for this is the relative off-rate of cleaved C4b.…”