The analysis of the HPPD structure and its superposition with the structures of DHBD and MPC highlight some important differences in the active sites of these enzymes. These comparisons also suggest that the pyruvate part of the HPPD substrate (4-hydroxyphenylpyruvate) and the O2 molecule would occupy the three free coordination sites of the catalytic iron atom. This substrate-enzyme model will aid the design of new inhibitors of the homogentisate biosynthesis reaction.

The three-dimensional solution structure of a nonspecific lipid transfer protein extracted from maize seeds determined by ' H NMR spectroscopy is described. This cationic protein consists of 93 amino acid residues. Its structure was determined from 1,091 NOE-derived distance restraints, including 929 interresidue connectivities and 197 dihedral restraints (4,$, x,) derived from NOES and 3J coupling constants. The global fold involving four helical fragments connected by three loops and a C-terminal tail without regular secondary structures is stabilized by four disulfide bridges. The most striking feature of this structure is the existence of an internal hydrophobic cavity running through the whole molecule. The global fold of this protein, very similar to that of a previously described lipid transfer protein extracted from wheat seeds (Gincel E et al., 1994, Eur J Biochem 226:413-422) constitutes a new architecture for a-class proteins. 'H NMR and fluorescence studies show that this protein forms well-defined complexes in aqueous solution with lysophosphatidylcholine. Dissociation constants, K d , of 1.9 f 0.6 x M and M were obtained with lyso-C,, and -Clz, respectively. A structure model for a lipidprotein complex is proposed in which the aliphatic chain of the phospholipid is inserted in the internal cavity and the polar head interacts with the charged side chains located at one end of this cavity. Our model for the lipidprotein complex is qualitatively very similar to the recently published crystal structure (Shin DH et al., 1995, Structure 3:189-199).