Sunday, September 23, 2007

Japanese scientists describe crystal structures at the heart of antitumor compound synthesis

Japanese biochemists have brought the design of anticancer compounds a step closer in a study published recently in the Proceedings of the National Academy of Sciences USA1. Shingo Nagano, Yoshitsugu Shiro and their colleagues from the RIKEN SPring-8 Center, the University of Hyogo and Toyama Prefectural University have studied the biosynthesis of a natural product called staurosporine. This molecule is isolated from bacteria of the genus Streptomyces and is of interest because it exhibits antitumor activity. Staurosporine has been identified as a potent inhibitor of enzymes that regulate cell growth and death, known as protein kinases.

Staurosporine is a member of a family of compounds whose biosynthesis involves the formation of a base unit called an indolocarbazole core. Because of their potential as therapeutic agents for cancer and neurodegenerative diseases, indolocarbazole compounds have attracted scientists’ attention.

The formation of the indolocarbazole core in part involves binding a molecule of chromopyrrolic acid with an enzyme known as cytochrome P450 StaP. StaP is a member of the cytochrome P450 family of compounds which includes enzymes involved in steroid hormone biosynthesis, drug metabolism and many other physiologically important reactions.

Nagano and co-workers presents the first report of the precise arrangement of atoms during a key stage in the formation of the core structure. The configuration of atoms they describe is known as its crystal structure and is elucidated using the technology of x-ray crystallography, a method of determining the arrangement of atoms in a molecule by analyzing the reflection patterns of x-rays directed at it.

The study provides high resolution crystal structures of StaP in the presence and absence of the substrate chromopyrrolic acid (Fig. 1). The structure of the complex with the substrate StaP in place provides structural insights into the process of enzyme-substrate recognition and the molecular mechanism of indolocarbazole core formation.

This crystallographic study provides valuable insights into the process of staurosporine biosynthesis, the mechanism of indolocarbazole synthesis, and the diverse chemistry performed by cytochrome P450s.

“The ultimate goal of our project on indolocarbazole is structure-based design of enzymes that produce ‘unnatural’ indolocarbazole which have improved antitumor activity or new bioactivities,” says Nagano. Moving forward, in a study to be published shortly, the group has described the structure of an enzyme involved in another similar reaction. The ultimate aim is to synthesize a variety of indolocarbazole compounds that could have important therapeutic use in the fight against cancer.

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