The crystal structure of a major secreted aspartic proteinase from Candida albicans in complexes with two inhibitors.
BACKGROUND
Infections caused by Candida albicans, a common fungal pathogen of humans, are increasing in incidence, necessitating development of new therapeutic drugs. Secreted aspartic proteinase (SAP) activity is considered an important virulence factor in these infections and might offer a suitable target for drug design. Amongst the various SAP isozymes, the SAP2 gene product is the major form expressed in a number of C. albicans strains.


RESULTS
The three-dimensional structures of SAP2 complexed with the tight-binding inhibitor A70450 (a synthetic hexapeptide analogue) and with the general aspartic proteinase inhibitor pepstatin A (a microbial natural product) have been determined to 2.1 A and 3.0 A resolution, respectively. Although the protein structure retains the main features of a typical aspartic proteinase, it also shows some significant differences, due mainly to several sequence insertions and deletions (as revealed by homology modelling), that alter the shape of the binding cleft. There is also considerable variation in the C-terminal structural domain.


CONCLUSIONS
The differences in side chains, and in the conformations adopted by the two inhibitors, particularly at their P4, P3 and P'2 positions (using standard notation for protease-inhibitor residues), allows the A70450 structure to complement, more accurately, that of the substrate-binding site of SAP2. Some differences in the binding clefts of other SAP isoenzymes may be deduced from the SAP2 structure.
Cutfield
S M
SM
Biochemistry Department, University of Otago, Dunedin, New Zealand.
Dodson
E J
EJ
Anderson
B F
BF
Moody
P C
PC
Marshall
C J
CJ
Sullivan
P A
PA
Cutfield
J F
JF
D000595:Amino Acid Sequence; D000935:Antifungal Agents / Q000737:chemistry* / Q000494:pharmacology; D016282:Aspartic Acid Endopeptidases / Q000037:antagonists & inhibitors / Q000737:chemistry*; D001665:Binding Sites; D002176:Candida albicans / Q000201:enzymology*; D018360:Crystallography, X-Ray; D015195:Drug Design; D004791:Enzyme Inhibitors / Q000737:chemistry* / Q000494:pharmacology; D005656:Fungal Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry*; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000737:chemistry*; D046911:Macromolecular Substances; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010436:Pepstatins / Q000737:chemistry* / Q000494:pharmacology; D010879:Piperazines / Q000737:chemistry* / Q000494:pharmacology; D011485:Protein Binding; D011487:Protein Conformation; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
D000935:Antifungal Agents; D004791:Enzyme Inhibitors; D005656:Fungal Proteins; D007527:Isoenzymes; D046911:Macromolecular Substances; D010436:Pepstatins; D010879:Piperazines; C014195:Streptomyces pepsin inhibitor; C083088:A 70450; D016282:Aspartic Acid Endopeptidases; C090753:SAP2 protein, Candida; C031375:pepstatin