X-ray crystallographic studies of concanavalin A

Localized surface plasmon resonance biosensors

In this review, the most recent progress in the development of noble metal nano-optical sensors based on localized surface plasmon resonance (LSPR) spectroscopy is summarized. The sensing principle relies on the LSPR spectral shifts caused by the surrounding dielectric environmental change in a binding event. Nanosphere lithography, an inexpensive and simple nanofabrication technique, has been used to fabricate the nanoparticles as the LSPR sensing platforms. As an example of the biosensing applications, the LSPR detection for a biomarker of Alzheimer’s disease, amyloid-derived diffusable ligands, in human brain extract and cerebrospinal fluid samples is highlighted. Furthermore, the LSPR sensing method can be modified easily and used in a variety of applications. More specifically, a LSPR chip capable of multiplex sensing, a combined electrochemical and LSPR protocol and a fabrication method of solution-phase nanotriangles are presented here.

Structure of the concanavalin A-methyl alpha-D-mannopyranoside complex at 6-A resolution

The carbohydrate binding site of concanavalin A has been identified in crystals of the concanavalin A-methyl alpha-D-mannopyranoside complex and is 35 A from the iodophenol binding site (K. D. Hardman and C. F. Ainsworth (1973), Biochemistry 12,4442), which has been postulated to be adjacent to the carbohydrate-specific binding site (Edelman et al. (1972), Proc. Natl. Acad. Sci. U.S.A. 69, 2580). The crystals are orthorhombic in space group C222(1) and crystal denisty measurements indicate a protein mass of four monomers (molecular weight of 104 000) per asymmetric unit. However, the electron density map contains eight monomers/asymmetric unit, revealing lattice disorder. The electron density map with a nominal resolution of 6 A has been solved using three heavy-atom derivatives and the position and orientation of each monomer established. Atomic coordinates of the native protein which has previously been determined (K. D. Hardman (1973), Adv. Exp. Med. Biol. 40, 103) were transposed into this new space group and the gross conformations of the monomers, dimers, and tetramers were found to be very similar to the previous structure. However, some minor differences were apparent even at this resolution. After crystal growth, the methyl alpha-D-mannopyranoside was replaced by o-iodophenyl beta-D-glucopyranoside or methyl 2-iodoacetimido-2-deoxy-alpha-D-glucopyranoside in separate experiments, and difference electron density maps were calculated. The highest peaks for both iodinated sugar derivatives associated with each monomer agreed within a few angstroms of each other and were found near side chains Tyr-12 and -100 and Asp-16 and -208. This region is 10-14 A from the manganese, in good agreement with nuclear magnetic resonance (NMR) studies in solution (C. F. Brewer et al. (1973), Biochemistry 12, 4448) and with the site predicted from crosslinked 1222 crystal studies (K. D. Hardman (1973), Adv. Exp. Med. Biol. 40, 103).

Structure and function of concanavalin A

Lectins have been extensively used to analyze a variety of fundamental processes in cell biology. In conjuntion with our studies on the cell surface and mitosis, we have determined the amino acid sequence and three-dimensional struction of concanavalin A (Con A), the mitogenic lectin from the jack bean. Knowledge of the structure has been helpful in interpreting experiments on lymphocyte mitogenesis and the effects of Con A on cell surface receptor mobility. Con A subunits for molecular weight 25,500 are folded into dome-like structures of maximum dimensions 42 times 40 times 39 A. The domes are related by 222 symmetry to form roughly tetrahedral tetramers. Each subunit contains two large antiparallel pleated sheets, and subunits are joined to form dimers and tetramers by interactions involving one of these pleated sheets. We have examined the binding of a variety of carbohydrates to Con A and have obtained preliminary data which suggest that there are differences in the saccharide-binding behavior of Con A in solution and in the crystalline state. Dimeric chemical derivatives of Con A have been prepared and shown to have biological activities different from those of the native tetrameric protein. Under different conditions, native Con A exhibits two antagonistic activities on the lymphoid cell surface: the induction of cap formation by its own receptors and the inhibition of the mobility of a variety of receptors, including its own receptors. The dimeric derivative, succinyl-Con A, is just as effective a mitogen as the native lectin, but it lacks the ability to modulate cell surface receptor mobility. The data suggest that neither extensive immobilization of cell surface receptors nor cap formation is required for cell stimulation. Further studies on modulation of receptor translocation suggest that hypothesis that there exists a connecting network of colchicine-sensitive proteins that links receptors of different kinds and mediates their rearrangement. The degree of connectivity of this postulated network appears to be altered by changes in the state of attachment of various surface receptors to the network. Thus the network might provide the cell with a means of transmitting signals such as the stimulus for mitosis by lectins or antigens.

The covalent and three-dimensional structure of concanavalin A

The tentative amino-acid sequence and three-dimensional structure of the lectin concanavalin A have been determined. The amino-acid sequence, which was determined chemically, contains 238 residues. The sequences of three short stretches were assigned on the basis of x-ray crystallographic data. Interpretation of an electron density map at 2-A resolution indicates that the predominant structural element is extended polypeptide chain arranged in two anti-parallel pleated sheets or beta-structures. Residues not included in the beta-structures are arranged in regions of random coil. One of the pleated sheets contributes extensively to the interactions among the monomers to form both dimers and tetramers. The positions at which Mn(2+), Ca(2+), and saccharide are bound to the protein, and the point of cleavage for the formation of the naturally occurring fragments A(1) and A(2), have been tentatively assigned. Both metal-binding sites are at least 20-A removed from the position at which saccharides are bound. The saccharide-binding site is a deep pocket of approximately 6A x 7.5A x 18A, the inner portion of which is occupied by hydrophobic residues.