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S J Perkins

Publications and source records attributed to S J Perkins.

At least 19 recordsLinked to original sources
PubMed1992

Secondary structure changes stabilize the reactive-centre cleaved form of SERPINs. A study by 1H nuclear magnetic resonance and Fourier transform infrared spectroscopy.

Proteinase inhibitor members of the SERPIN superfamily are characterized by the presence of a proteolytically sensitive reactive-site loop. Cleavage within this region results in a conformational transition from an unstable "stressed" native protein to a more stable "relaxed" cleaved molecule. In order to identify the principal molecular aspects of this transition, 1H nuclear magnetic resonance (n.m.r.) and FT-IR spectroscopy were applied to the study of four SERPINs. 1H n.m.r. spectra of approximately 20 high-field ring-current-shifted methyl signals exhibited slightly different chemical shifts in the native and cleaved forms of alpha 1-antitrypsin (alpha 1-AT), alpha 1-antichymotrypsin (alpha 1-ACT) and C1 inhibitor (C1-INH), but not ovalbumin, between 20 degrees C and 90 degrees C. Ring current calculations based on crystal co-ordinates for cleaved alpha 1-AT and alpha 1-ACT and native ovalbumin showed that these signals originate from highly localized interactions between different buried residues corresponding to alpha-helix and beta-sheet segments of the SERPIN fold. The small shift changes correspond to small relative conformational side-chain rearrangements of about 0.01 nm to 0.05 nm in the protein hydrophobic core, i.e. the tertiary structure interactions in the two forms of the SERPIN fold are well-preserved, and changes in this appear unimportant for the stabilization found after reactive centre cleavage. Fourier transform infrared (FT-IR) spectroscopic studies of the amide I band showed that the native and cleaved forms of alpha 1-AT, alpha 1-ACT and C1-INH contain 28% to 36% alpha-helix and 38% to 44% beta-sheet. Second derivative FT-IR spectra using H2O and 2H2O buffers revealed very large differences in the amide I band between the native and cleaved forms of alpha 1-AT, alpha 1-ACT and C1-INH, but not for ovalbumin. The alpha-helix band was most sensitive to 1H-2H exchange, while the beta-sheet bands were not, and greater amounts of antiparallel beta-sheet were detected in the cleaved form. 1H n.m.r. showed that polypeptide amide 1H-2H exchange was greater in the native forms of alpha 1-AT, alpha 1-ACT and C1-INH than in their cleaved forms, whereas for ovalbumin it was unchanged. The FT-IR and 1H-2H exchange data show that alterations in the secondary structure are central to the stabilization of the cleaved SERPIN structure.(ABSTRACT TRUNCATED AT 400 WORDS)

Complement C1 Inactivator Proteins↗
PubMed1992

Neutron and X-ray solution-scattering studies of the ternary complex between proteoglycan-binding region, link protein and hyaluronan.

Proteoglycan aggregates of cartilage are stabilized by the formation of a ternary complex between the G1 domain at the N-terminus of the proteoglycan monomer (aggrecan), link protein and hyaluronan polysaccharide. Both the G1 domain and link protein contain similar three-domain structures formed from an immunoglobulin fold and two proteoglycan tandem repeats, the arrangement of which had been investigated by neutron and synchrotron X-ray scattering [Perkins, Nealis, Dunham, Hardingham & Muir (1991) Biochemistry 30, 10708-10716]. Here, solution scattering was used to investigate the ternary complexes formed between a proteolytic fragment of proteoglycan monomer containing G1 (termed binding region), link protein and hyaluronan oligosaccharides containing either 34 or 450 saccharide units (HA34 and HA450). The ternary complex with HA34 had a neutron radius of gyration, RG, at infinite contrast not exceeding 5.5 nm. The ternary complex with HA34 had an X-ray cross-sectional radius of gyration Rxs of 2.4 nm and a neutron Rxs at infinite contrast of 2.00 nm. Since both were similar or larger than the Rxs for binding region (X-rays, 2.04 nm; neutrons, 1.84 nm) and link protein (neutrons, 0.8 nm), analyses showed that the cross-sectional mean width of the ternary complex is greater than those in each of the free proteins, i.e. the two proteins associated side-by-side. Similar results were obtained with HA450 complexed with binding region and with both binding region and link protein. This structural model was verified by hydrodynamic simulations of the experimental sedimentation coefficient of 5.5 S, which showed that a compact ternary-complex structure was formed. Although scattering curve simulations using small spheres were limited for the ternary complex with HA34 because of its approximate RG value, the scattering data were compatible with the formation of a compact complex formed by side-by-side contacts between G1 and link protein.

Extracellular Matrix Proteins↗
PubMed1992

Molecular modeling of the domain structure of C9 of human complement by neutron and X-ray solution scattering.

C9 is the most abundant component of the membrane attack complex of the complement system of immune defense. This is a typical mosaic protein with thrombospondin (TSR) and low density lipoprotein receptor (LDLr) domains at its N-terminus and an epidermal growth factor-like (EGF) domain at its C-terminus. Between these lies a perforin-like sequence. In order to define the arrangement in solution of these four moieties in C9, high-flux neutron and synchrotron X-ray solution scattering studies were carried out. The neutron radius of gyration RG at infinite contrast is 3.33 nm, and its cross-sectional RG (RXS) is 1.66 nm. Similar values were obtained by synchrotron X-ray scattering after allowance for radiation effects. Stuhrmann analyses showed that the neutron radial inhomogeneity of scattering density alpha is 35 X 10(-5) from the RG data and 16 X 10(-5) from the RXS data. These values are typical for soluble glycoproteins and show no evidence for the existence of any large hydrophobic surface patches on free C9 that might form contacts with lipids. Indirect transformation of the neutron and X-ray scattering curves into real space showed that C9 had a maximum dimension estimated at 12 +/- 2 nm, and this suggests that the lengths of 7-8 nm deduced from previous electron microscopy studies in vacuo are underestimated. Molecular modeling of the C9 scattering curves utilized small spheres in the Debye equation, in which the analyses were constrained by the known volumes of the four moieties of C9 and the known sizes of the TSR and EGF-like domains. The most likely models for C9 suggest that these four regions of C9 are arranged in a V-shaped structure, with an angle of 10 degrees between the two arms, each of length 11.1 nm. This structure has a more hydrophobic character between the two arms. The scattering model is fully consistent with hydrodynamic sedimentation data on C9. Similar V-shaped hydrodynamic models could be developed for C6, C7, C8, and C9 of complement. Such a compact structure is atypical of other multidomain complement proteins so far studied by solution scattering and is fully compatible with mechanisms in which C9 is postulated, on activation, to undergo a drastic unfolding of its domain structure and to expose a more hydrophobic surface which can be embedded into lipid bilayers.

Complement C9↗
PubMed1991

Molecular modeling of the multidomain structures of the proteoglycan binding region and the link protein of cartilage by neutron and synchrotron X-ray scattering.

The interaction of proteoglycan monomers with hyaluronate in cartilage is mediated by a globular binding region at the N-terminus of the proteoglycan monomer; this interaction is stabilized by link protein. Sequences show that both the binding region (27% carbohydrate) and the link protein (6% carbohydrate) contain an immunoglobulin (Ig) fold domain and two proteoglycan tandem repeat (PTR) domains. Both proteins were investigated by neutron and synchrotron X-ray solution scattering, in which nonspecific aggregate formation was reduced by the use of citraconylation to modify surface lysine residues. The neutron and X-ray radius of gyration RG of native and citraconylated binding region is 5.1 nm, and the cross-sectional RG (RXS) is 1.9-2.0 nm. No neutron contrast dependence of the RG values was observed; however, a large contrast dependence was seen for the RXS values which is attributed to the high carbohydrate content of the binding region. The neutron RG for citraconylated link protein is 2.9 nm, its RXS is 0.8 nm, and these data are also independent of the neutron contrast. The scattering curves of binding region and link protein were modeled using small spheres. Both protein structures were defined initially by the representation of one domain by a crystal structure for a variable Ig fold and a fixed volume for the two PTR domains calculated from sequence data. The final models showed that the different dimensions and neutron contrast properties of binding region compared to link protein could be attributed to an extended glycosylated C-terminal peptide with extended carbohydrate structures in the binding region.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1991

Solution structure of human and mouse immunoglobulin M by synchrotron X-ray scattering and molecular graphics modelling. A possible mechanism for complement activation.

The pentameric 71-domain structure of human and mouse immunoglobulin M (IgM) was investigated by synchrotron X-ray solution scattering and molecular graphics modelling. The radii of gyration RG of human IgM Quaife and its Fc5, IgM-S, Fab'2 and Fab fragments were determined as 12.2 nm, 6.1 nm, 6.1 nm, 4.9 nm and 2.9 nm in that order. The RG values were similar for mouse IgM P8 and its Fab'2 and Fab fragments, despite the presence of an additional carbohydrate site. The IgM scattering curves, to a nominal resolution of 5 nm, were compared with molecular graphics models based on published crystallographic alpha-carbon co-ordinates for the Fab and Fc structures of IgG. Good curve fits for Fab were obtained based on the crystal structure of Fab from IgG. A good curve fit was obtained for Fab'2, if the two Fab arms were positioned close together at their contact with the C mu 2 domains. The addition of the Fc fragment close to the C mu 2 domains of this Fab'2 model, to give a planar structure, accounted for the scattering curve of IgM-S. The Fc5 fragment was best modelled by a ring of five Fc monomers, constrained by packing considerations and disulphide bridge formation. A position for the J chain between two C mu 4 domains rather than at the centre of Fc5 was preferred. The intact IgM structure was best modelled using a planar arrangement of these Fab'2 and Fc5 models, with the side-to-side displacement of the Fab'2 arms in the plane of the IgM structure. All these models were consistent with hydrodynamic simulations of sedimentation data. The solution structure of IgM can therefore be reproduced quantitatively in terms of crystallographic structures for the fragments of IgG. Putative Clq binding sites have been identified on the C mu 3 domain. These would become accessible for interaction with Clq when the Fab'2 arms move out of the plane of the Fc5 disc in IgM, that is, a steric mechanism exposing pre-existing Clq sites. Comparison with a solution structure for Clq by neutron scattering shows that two or more of the six globular Clq heads in the hexameric head-and-stalk structure are readily able to make contacts with the putative Clq sites in the C mu 3 domains of free IgM if if the Clq arm-axis angle in solution is reduced from 40 degrees-45 degrees to 28 degrees. This could be the trigger for Cl activation.

Amino Acid Sequence↗
PubMed1991

Neutron and X-ray scattering studies on the human complement protein properdin provide an analysis of the thrombospondin repeat.

Properdin is a regulatory glycoprotein of the alternative pathway of the complement system of immune defense. It is responsible for the stabilization of the C3 convertase complex formed between C3b and the Bb fragment of factor B. Neutron and X-ray solution scattering experiments were performed on the dimeric and trimeric forms of properdin. These have RG values of 9.1 and 10.7 nm, respectively. The scattering curves were compared with Debye sphere modeling simulations for properdin. Good agreements were obtained for models similar to published electron micrographs showing that the properdin trimer has a triangular structure with sides of 26 nm. Such a structure also accounted for sedimentation coefficient data on properdin. Primary structure analyses for mouse and human properdin have shown that this contains six homologous motifs known as the thrombospondin repeat (TSR), which is the second most abundant domain type found in the complement proteins. Sequences for these 12 TSRs were aligned with 19 others found in thrombospondin and the late complement components. Three distinct groups of TSRs were identified, namely, the TSRs found in thrombospondin and properdin, the TSRs mostly found at the N-terminus of the late complement components, and the TSRs found at the C-terminus of the late components. Averaged secondary structure predictions suggested that all three groups contain similar backbone structures with two amphipathic turn regions and one hydrophilic beta-strand region. The mean dimensions of the TSRs of properdin in solution were determined to be approximately 4 nm X 1.7 nm X 1.7 nm, showing that these are elongated in structure.

Amino Acid Sequence↗
PubMed1991

Molecular comparisons of the beta 2-microglobulin-binding site in class I major-histocompatibility-complex alpha-chains and proteins of related sequences.

beta 2-Microglobulin (beta 2m) binds non-covalently to the alpha 1, alpha 2 and alpha 3 domains of the alpha-chain of Class I major-histocompatibility-complex (MHC) molecules. On the basis of the crystal structures of human leucocyte antigens HLA-A2.1 and HLA-Aw68.1, we have used molecular-graphics analyses to define 44 contact points between 19 alpha-chain residues and 18 beta 2m residues. In 88 other alpha-chain sequences from the HLA-A, HLA-B, HLA-C, HLA-D, HLA-E, HLA-F and HLA-G locus products in man and the H-2, Qa and Tla loci in mouse, 37 contact sites were conserved to 90% or more, and in beta 2m sequences from seven other species 40% of contact sites were totally conserved. Four distinct regions form the contact points between the alpha-chain and beta 2m, one on each of the alpha 1 and alpha 2 domains and two on the alpha 3 domain. We have further studied the alpha-chain sequences of three non-MHC molecules, human CD1 and rat Fc receptor (FcRn), known to bind to beta 2m, and a third molecule, the putative product of the H301 (UL18) gene of human cytomegalovirus (CMV). CMV has been shown to bind beta 2m, and it has been postulated that the H301-gene product, which has sequence similarity to Class I HLA, is the protein responsible. These sequences exhibited much lower residue conservation with the MHC-linked group, although the alpha 3 domain was the most highly conserved, and gaps and insertions were required for optimal alignments with the 90 alpha-chain sequences. Of the 44 beta 2m-alpha-chain contacts defined for Class I HLA, 24 alpha-chain contact sites were conserved in CD1, 25 in FcRn and 17 in the H301-gene product. For CD1 and FcRn, the majority of the conserved beta 2m contacts were found in the alpha 2 domain and the major contact region in the alpha 3 domain. Together with the use of secondary-structure predictions, it was concluded that the binding of beta 2m in CD1 and FcRn was MHC-like at the alpha 3 domain, and probably also at the alpha 2 domain for FcRn, but non-MHC-like for the alpha 1 domain of both molecules and the alpha 2 domain of CD1. In the H301-gene product sequence, only the beta 2m contacts with the main region of the alpha 3 domain were noticeably conserved.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1991

Oligomeric domain structure of human complement factor H by X-ray and neutron solution scattering.

Factor H is a regulatory component of the complement system. It has a monomer Mr of 150,000. Primary structure analysis shows that the polypeptide is divided into 20 homologous regions, each 60 amino acid residues long. These are independently folding domains and are termed "short consensus repeats" (SCRs) or "complement control protein" (CCP) repeats. High-flux synchrotron X-ray and neutron scattering studies were performed in order to define its solution structure in conditions close to physiological. The Mr of factor H was determined as 250,000-320,000 to show that factor H is dimeric. This structure is maintained at concentrations between 1 and 11 mg/mL in the pH range 5-9. Zn2+ ions are an inhibitor of C3b cleavage by factor I, a reaction in which factor H acts as a cofactor. Additions of Zn2+ to factor H caused it to form oligomers containing 4-10 monomers. The radius of gyration RG of native factor H by X-rays or by neutrons in 0% or 100% 2H2O buffers is not measurable but is greater than 12.5 nm. Two cross-sectional radii of gyration RXS-1 and RXS-2 were determined as 3.0-3.1 and 1.8 nm, respectively. Analyses of the cross-sectional intensities show that factor H is composed of two distinct subunits. The RXS-1 corresponds to the cross-sectional properties of both subunits and exhibits an unusual radiation dependence on the X-ray flux. Since RXS-2 is close to the corresponding RXS of C4b binding protein (91% of which is formed from SCR/CCP domains), it is inferred that the SCR/CCP domains of factor H and C4b binding protein have similar solution structures. The use of hydrodynamic spheres to reproduce literature sedimentation coefficients of 5.5-5.6 S showed that these were compatible with a V-shaped arrangement of two rods (36 spheres each, length 87 +/- 5 nm) joined at an angle of 5 degrees. The use of a similar arrangement of 244 spheres arranged in two rods (length 77 nm) to fit the experimental X-ray and neutron scattering curves showed that the two rods are joined at an angle of 5 degrees. This model corresponds to an actual RG of 21-23 nm. The separation between each SCR/CCP in factor H is close to 4 nm. In the solution structure of factor H, the SCR/CCP domains are in a highly extended conformation.

Complement C3b↗
PubMed1990

Hydrodynamic data show that C1- inhibitor of complement forms compact complexes with C1-r and C1-s.

The C1- inhibitor of the complement cascade forms stoichiometric complexes with C1-r and C1-s and controls the activation of first component C1 of complement. Literature sedimentation coefficients s degrees 20,w for the complexes formed between C1- inhibitor, C1-r and C1-s were analysed using frictional ratios and the hydrodynamic sphere approach. A head-and-tail two-domain model for C1- inhibitor was combined with cylindrical hydrodynamic models for the six-domain structures of C1-r and C1-s. The hydrodynamic data show that the heavily glycosylated N-terminal domain of C1- inhibitor is positioned close to the two complement 'short consensus repeat' domains found in the centre of C1-r and C1-s.

Complement Activation↗
PubMed1990

Two-domain structure of the native and reactive centre cleaved forms of C1 inhibitor of human complement by neutron scattering.

The C1 inhibitor component of human complement is a member of the serpin superfamily, and controls C1 activation. Carbohydrate analyses showed that there are seven O-linked oligosaccharides in C1 inhibitor. Together with six N-linked complex-type oligosaccharides, the carbohydrate content is therefore 26% by weight and the molecular weight (Mr) is calculated as 71,100. Neutron scattering gives an Mr of 76,000 (+/- 4000) and a matchpoint of 41.8 to 42.3% 2H2O, in agreement with this carbohydrate and amino acid composition. Guinier plots to determine the radius of gyration RG were biphasic. Neutron contrast variation of C1 inhibitor in H2O-2H2O mixtures gave an overall radius of gyration RG at infinite contrast of 4.85 nm, from analyses at low Q, and a cross-sectional RG of 1.43 nm. The reactive centre cleaved form of C1 inhibitor has the same Mr and structure as the native molecule. The length of C1 inhibitor, 16 to 19 nm, is far greater than that of the putative serpin domain. This is attributed to an elongated structure for the carbohydrate-rich 113-residue N-terminal domain. The radial inhomogeneity of scattering density, alpha, is large at 59 x 10(-5) from the RG data and 28 x 10(-5) from the cross-sectional analysis, and this is accounted for by the high oligosaccharide content of C1 inhibitor. The scattering data were modelled using small spheres. A two-domain structure of length 18 nm based on two distinct scattering densities accounted for all the contrast variation data. One domain is based on the crystal structure of alpha 1 antitrypsin (7 nm x 3 nm x 3 nm). The other corresponds to an extended heavily glycosylated N-terminal domain of length 15 nm, whose long axis is close to the longest axis of the serpin domain. Calculation of the sedimentation coefficient s0(20),w for C1 inhibitor using the hydrodynamic sphere approach showed that a two-domain head-and-tail structure with an Mr of 71,000 and longest axis of 16 to 19 nm successfully reproduced the s0(20),w of 3.7 S. Possible roles of the N-terminal domain in the function of C1 inhibitor are discussed.

Borohydrides↗
PubMed1990

Structural comparisons of the native and reactive-centre-cleaved forms of alpha 1-antitrypsin by neutron- and X-ray-scattering in solution.

alpha 1-Antitrypsin is the best-characterized member of the serpin (serine-proteinase inhibitor) superfamily. Its solution structure was studied by high-flux neutron-scattering and synchrotron X-ray-scattering. Neutron data show that its absorption coefficient A1% 280,1cm is 5.4. The neutron radius of gyration RG at infinite contrast for native alpha 1-antitrypsin is 2.61 nm, characteristic of a moderately elongated structure, and its cross-sectional RG is 1.34 nm. The internal inhomogeneity of scattering densities within alpha 1-antitrypsin is high at 29 x 10(-5). The X-ray RG is 2.91 nm, in good agreement with the neutron RG of 2.82 nm in 1H2O. This RG is unchanged in reactive-centre-cleaved alpha 1-antitrypsin. These parameters are also unchanged at pH 8 in sodium/potassium phosphate buffers up to 0.6 M. The neutron and X-ray curves for native alpha 1-antitrypsin were compared with Debye simulation based on the crystal structure of reactive-centre-cleaved (papain) alpha 1-antitrypsin. After allowance for residues not visible in the crystallographic electron-density map, and rejoining the proteolysed site between Met-358 and Ser-359 by means of a relatively minor conformational re-arrangement, good agreement to a structural resolution of 4 nm is obtained with the neutron data in two contrasts and with the X-ray data. The structures of the native and cleaved forms of alpha 1-antitrypsin are thus similar within the resolution of solution scattering. This places an upper limit on the magnitude of the presumed conformational changes that occur in alpha 1-antitrypsin on reactive-centre cleavage, as indicated in earlier spectroscopic investigations of the Met-358-Ser-359 peptide-bond cleavage. Methods for scattering-curve simulations from crystal structures are critically assessed. The RG data lead to dimensions of 7.8 nm x 4.9 nm x 2.2 nm for native alpha 1-antitrypsin. The high internal inhomogeneity and the asymmetric shorter semi-axes of 4.9 nm and 2.2 nm suggest that the three oligosaccharide chains of alpha 1-antitrypsin are essentially freely extended into solvent in physiological conditions. This conclusion is also supported by the Debye simulations, and by modelling based on hydrodynamic parameters.

Binding Sites↗
PubMed1990

Conformational transition between native and reactive center cleaved forms of alpha 1-antitrypsin by Fourier transform infrared spectroscopy and small-angle neutron scattering.

alpha 1-Antitrypsin (alpha 1-AT) is the best-characterized member of the serpin superfamily of plasma proteins. Protease inhibitor members of this family undergo a characteristic reactive-center cleavage during expression of their inhibitory activity. The physical basis of this transition in alpha 1-AT from the stressed native conformation to the more stable reactive center cleaved (split) form was studied by Fourier transform infrared (FT-IR) spectroscopy and neutron scattering. The FT-IR spectra show that, while split alpha 1-AT has three intense well-resolved components associated with the presence of antiparallel beta-sheet and alpha-helix conformations, the amide I band of native alpha 1-AT has only one intense component, associated with the presence of beta-sheet structure. 1H-2H exchange within the polypeptide backbone, studied by FT-IR and NMR spectroscopy, shows that the native form undergoes greater exchange than the split form. Under the same conditions, neutron scattering shows no differences in the radius of gyration RG of the native and the split forms. In contrast, in high concentrations of phosphate approaching those used for crystallization, the native form (unlike the split form) undergoes dimerization. These data indicate that the conformational transition largely involves localized secondary and tertiary structure rearrangements. We propose that the energetically stressed native alpha 1-AT structure is the consequence of a significantly reduced number of hydrogen bonds in secondary structure components and that reactive-site cleavage between Met358 and Ser359 is the key for the development of the fully hydrogen bonded more stable serpin structure.

Fourier Analysis↗
PubMed1990

Molecular modeling of human complement component C4 and its fragments by X-ray and neutron solution scattering.

The solution structures of human complement component C4 and five derived fragments, C4u, C4(a + b), C4b, C4c, and C4d, were analyzed by synchrotron X-ray and neutron scattering. The X-ray radii of gyration RG for C4, C4u, and C4(a + b) in H2O buffers are similar at 5.23-5.28 nm, and likewise the cross-sectional radii of gyration RXS are similar at 2.48-2.52 nm. Molecular mass calculations using X-rays and neutrons show unexpectedly that C4c is dimeric; however, all the other forms are monomeric. C4c2 has an X-ray RG of 5.18 nm and an RXS of 2.89 nm. Neutron contrast variation gives RG values at infinite contrast of 4.87-4.93 nm for C4 and C4u, 4.79 nm for C4b, 4.94 nm for C4c2, and 2.69 nm for C4d. The RXS values at infinite contrast are 2.23-2.25 nm for C4 and C4u, 1.89 nm for C4b, and 2.62 nm for C4c2. These data show that a large conformational change occurs on going from C4 to C4b, but not on going from C4 to C4u, and this is attributed to the presence of the C4a moiety in C4u. Comparisons of the C4 and C4u scattering curves show that these are very similar out to a nominal resolution of 4 nm. Scattering-curve models were developed to account for the neutron scattering curves of C4, C4c2, and C4d in 2H2O buffers. The C4c monomer could be represented by a lamellar ellipsoid of size 8 nm x 2 nm x 18 nm. C4d was found to be 4 nm x 2 nm x 9 nm. The combination of these structures gave good accounts of the neutron data for C4, C4b, and C4c2 to resolutions of 5-6 nm. The C4 model was obtained by placing the long axis of C4d parallel to that of C4c such that the cross section is extended. C4b was best modeled by repositioning C4d relative to C4c such that this cross section becomes more compact. The C4 and C4b models are compared with possible structures for the C1 component of complement to show the importance of the surface accessibility of the protease domains and short consensus repeat domains in C1 for C4 activation.

Blood Donors↗
PubMed1990

Structural homologies of component C5 of human complement with components C3 and C4 by neutron scattering.

The complement component C5 is one of a family of structurally related plasma proteins that includes components C3 and C4. Activation of C5 is the initial step in the formation of the membrane attack complex of complement. Analysis of the solution structure of C5 and comparisons with similar analyses of the structures of C3 and C4 are reported here. Neutron solution scattering gave an Mr for C5 of 201,000, which demonstrates that C5 is monomeric in solution. The radius of gyration RG of C5 at infinite contrast is 4.87 nm and corresponds to an elongated structure. The longest length of C5 was determined to be at least 15-16 nm from three calculations on the basis of the RG, the scattering intensity at zero angle I(0), and the indirect transformation of the scattering curve into real space. Comparison of the RG and contrast variation data and indirect transformations of the scattering curves for C3, C4, and C5 show that these have very similar structures. Comparisons of the C5 scattering curve with Debye small-sphere models previously employed for C4 and C3 show that good curve fits could be obtained. Unlike previous studies that have suggested significant differences, these experiments indicate that, while C5 differs from C3 and C4 in its activation and inactivation pathways, significant structural homology exists between the native proteins, as might be predicted from their high (and similar) sequence homology.

Complement C3↗