Abstract
Ascorbate (AsA) is contained at high levels in photosynthetic organisms and serves as a scavenger of active oxygen species (AOS). Furthermore, it acts as an enzyme activator by maintaining prosthetic metal ions in the reduced form. Photosynthetic organisms are prone to oxidative stress because of the highly energetic reactions of photosynthesis and an abundant 0_2 supply. The imposition of biotic and abiotic stress conditions can give rise to excess concentrations of AOS, resulting in oxidative damage at the cellular level. Therefore, photosynthetic organisms have developed antioxidant defense systems. Ascorbate peroxidase (APX) isoenzymes are key enzymes for AOS-scavenging systems and are widely distributed in chloroplasts, microbodies, mitochondria, and cytosol of photosynthetic organisms. One of the most characteristic properties of APX isoenzymes is the instability in the absence of AsA. To determine the structure-function relation and to obtain the stereochemical information about the mechanisms of the rapid inactivation of APX isoenzymes, we carried out an X-ray crystallographic analysis for the recombinant stromal soluble APX (sAPX) of tobacco plants.
