Crystal structure of a mammalian purple acid phosphatase.

Tartrate-resistant acid phosphatase (TRAP) is a mammalian di-iron- containing enzyme that belongs to the family of purple acid phosphatases (PAP). It is highly expressed in a limited number of tissues, predominantly in bone-resorbing osteoclasts and in macrophages of spleen. We have determined the crystal structure of rat TRAP in complex with a phosphate ion to 2.7 A resolution. The fold resembles that of the catalytic domain of kidney bean purple acid phosphatase (KBPAP), although the sequence similarity is limited to the active site residues. A surface loop near the active site is absent due to proteolysis, leaving the active-site easily accessible from the surrounding solvent. This, we believe, gives a structural explanation for the observed proteolytic activation of TRAP. The current structure was determined at a relatively high pH and without any external reducing agents. It is likely that it represents an oxidized and therefore catalytically inactive form of the enzyme.

Arrest of beta-amyloid fibril formation by a pentapeptide ligand.

Polymerization of amyloid beta-peptide (Abeta) into amyloid fibrils is a critical step in the pathogenesis of Alzheimer's disease. Here, we show that peptides incorporating a short Abeta fragment (KLVFF; Abeta16-20) can bind full-length Abeta and prevent its assembly into amyloid fibrils. Through alanine substitution, it was demonstrated that amino acids Lys16, Leu17, and Phe20 are critical for binding to Abeta and inhibition of Abeta fibril formation. A mutant Abeta molecule, in which these residues had been substituted, had a markedly reduced capability of forming amyloid fibrils. The present data suggest that residues Abeta16-20 serve as a binding sequence duringA beta polymerization and fibril formation. Moreover, the present KLVFF peptide may serve as a lead compound for the development of peptide and non-peptide agents aimed at inhibiting Abeta amyloidogenesis in vivo.