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The conformation of two anti-poly(D-alanyl) antibodies was studied by small-angle X-ray scattering before and after interaction with hapten. With both samples a volume contraction could be observed upon interaction with the tetra-D-alanine hapten. The anti-poly@-alanyl) antibodies obtained by immunization with poly@-alanyl) diphtheria toxoid showed a decrease of the volume by 10% and a decrease of the radius of gyration by 7.7% when 90% of the binding sites were occupied by hapten. With anti-poly@-alanyl) human serum albumin antibodies, a smaller decrease of the volume of 3.2% and of the radius of gyration of 1.4% was found, when 62% of the binding sites were occupied. Since the other data determined for the antibodies, such as molecular weight, radii of gyration of the cross-section, and form T he interaction of the determinant group on the antigen with the combining site on the antibody represents a uniquely specific pattern of recognition on a molecular level. Antibodies are capable of inducing profound conformational changes in cross-reactive antigens, as exemplified by the conversion of a small synthetic periodic, not yet helical, polypeptide into an a-helical shape upon interaction with antibodies produced against a related ordered periodic polymer of a higher average molecular weight, which possessed under physiological conditions an a-helical structure (Schechter et id,, 1971 ). Similar observations were reported, concerning an arsanilated synthetic periodic polypeptide (ConwayJacobs er a/., 1970) and a peptide derived from myoglobin (Crumpton and Small, 1967). To the same category belongs the reaction of metmyoglobin with anti-apomyoglobin resulting in the release of heme (Crumpton, 1966), the activation of an enzymatically inactive mutant of 6-galactosidase (Rotman and Celada, 1968), stabilization of mutant catalase by complex formation with antibodies to normal catalase (Feinstein et a/., 1971), and the increase in RNase enzymatic activity upon adding anti-RNase antibodies to a mixture of S-protein and S-peptide, the two moieties obtained from RNase upon the cleavage of a single peptide bond by subtilisin (Cinader et al., 1971).It would be appealing to assume that, similarly to the induction of changes in antigen conformation by antibodies, transconformation might occur also within the antibody molecule as a result of its interaction with an antigen or a hapten.

CenA from Cellulomonas fimi is a beta-1,4-endoglucanase that binds tightly to cellulose. X-ray-scattering analyses show that the enzyme is tadpole-shaped: the previously identified catalytic and cellulose-binding domains comprise the head and tail respectively. It appears that this structural and functional organization is common to several cellulases from bacteria and fungi.

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The a and 0 2 subunits of tryptophan synthase were investigated by small-angle X-ray scattering. The molecular parameters are: radius of gyration, CI: 1.95 nm, pz : 3.01 nm; maximum particle diameter, CI: 5.8 nm, p 2 : 10.5 nm; and hydrated volume, CI : 60 nm3, p2 160 nm'. The shape of the CI subunit can best be described by a circular cylinder, slightly tapered at one end. An elongated elliptical cylinder with its cross section larger in the middle than at the ends was found to be a model equivalent in scattering to the p2 subunit.The CI& enzyme complex was found to have a radius of gyration of 4.01 nm, a maximum length of 13.5 nm, and a hydrated volume of 270 nm3. No satisfactory fit of the scattering data was obtainable by mere apposition of the models of the CI and p 2 subunits. Two cylinders overlapping laterally fit the experimental data considerably better, suggesting changes in the conformation of the subunits on forming the CI& complex.Tryptophan synthase from Escherichia coli is an CI& bienzyme complex, which catalyses the following reactions :Reaction (1) is catalyzed by the CI subunit and 100-fold more efficiently by the (x& complex. Reaction (2) is catalyzed by the fl2 subunit and 50-fold more efficiently by the complex. This mutual activation is thought to arise from heterologous subunit-subunit interactions during assembly of the complex. Moreover, the active sites appear to be juxtaposed at the intersubunit interface [l].The information on the structure and function of the complex and the component subunits has been reviewed recently [I]. Meanwhile the nucleotide sequences of the corresponding genes have been determined for E. roli [2], other enteric bacteria [3], and for Saccharomyces cerevisiae [4], showing that the amino acid sequences of the CI and / 3 polypeptide chains are highly conserved. Further, limited proteolysis has revealed that both the (x subunit ( M , 28727) and the p protomer (M, 42988) are composed of two autonomously folding structural domains that probably form the corresponding active sites at the interdomain interfaces [5 -91. Except for a preliminary crystallographic study of the a subunit [lo] and an estimate of the distance between the two active sites [Ill, little direct evidence on the tertiary and quaternary structure of tryptophan synthase and its subunits is available to date.In this paper we present the results of small-angle X-ray scattering studies on the CI and p2 subunits and the a& complex of tryptophan synthase. The data were fitted to various models equivalent in scattering and support a scheme for the assembly of the complex from its subunits. MATERIALS AND METHODS EnzymesTryptophan synthase (x-& complex was purified from the overproducing strain Escherichia coli trpR trpAEDl02/ F'trpAEDl02 [12]. The excess CI subunit produced by this strain was purified by affinity chromatography [13]. The p 2 subunit was prepared from the pure CI& complex by heat denaturation as described [12,14]. Protein aggregates were removed by gel chromatography on Sephacryl S200 (...