The crystal structure of turkey delta-crystallin, one of the principal soluble components of the avian lens, has been determined to a resolution of 2.5 Angstroms. It is a tetramer, molecular weight 200 kDa, with 222 symmetry. The subunit has a new fold composed of 3 mainly alpha-helical domains. One of the domains is a bundle of five long helices which forms a 20 helix bundle at the core of the tetramer.

Delta-crystallin is closely related to the enzyme argininosuccinate lyase (EC, with which it shares approximately 90% sequence identity, indicating that it is an example of a hijacked enzyme. It is also distantly related to the class II fumarases, aspartases, adenylosuccinases and 3-carboxy-cis,cis-muconate lactonising enzyme. Analysis of the structure reveals a putative active site cleft which is located on the boundary between 3 subunits of the tetramer.

This is the first 3D structure of a representative of this superfamily of enzymes which are central to metabolism.

REF: NATURE Structural Biology, vol.1, No.10, Pp724-733, October, 1994 A.Simpson, O.Bateman, H.Drieesen, P.Lindley, D.Moss, S.Mylvaganam, E.Narebor, and C.Slingsby,
`The structure of avian eye lens delta crystallin reveals a new fold for a superfamily of oligomeric enzymes'