Flavin monoamine oxidase family (L-amino acid oxidase)

General Activity
L-amino acid oxidases (LAAOs, EC 1.4.3.2) are enantioselective flavoenzymes that catalyze the stereospecific oxidative deamination of a wide range of L-amino acids to form α–keto acids, ammonia and hydrogen peroxide (H2O2).

Snake venom (sv)LAAOs are usually homodimeric FAD-(flavin adenine dinucleotide) or FMN-(flavin mononucleotide) binding glycoproteins. Flavins in LAAOs are responsible for the yellow color of snake venom and contribute to venom toxicity upon envenomation. In general, most svLAAOs activities are believed to be mediated by H2O2, however, the detailed mechanisms are still unclear.

svLAAOs have been found to have clear biological effects on platelet aggregation, edema formation, hemorrhage, cytotoxicity, and furthermore have anti-microbial, anti-viral, anti-HIV and tumor cell proliferation activities.

The reported effects of svLAAOs on platelets are controversial. There are reports of both activation and inhibition of platelet aggregation. Both effects are associated with the ability of svLAAOs to produce H2O2, since catalase attenuates their effect. The reason for inhibition of platelet aggregation might be the reduced binding of ADP by platelets exposed to H2O2. Activation of platelet aggregation is presumably caused by H2O2-induced amplification of thromboxan A2 synthesis and, consequently, platelet aggregation. Other studies imply that H2O2 production is not the only factor responsible for platelet activation. For example, in the case of NA-LAAO from Naja atra venom, it was shown that both H2O2 production and binding to platelet membrane proteins could be involved. It was suggested that the enzyme binds to the platelet membrane, enhancing the sensitivity of platelets to H2O2 and, at the same time, H2O2 released by the enzyme activates platelets by an unknown mechanism (Li et al., 2008).

Structure
svLAAOs are mostly found as non-covalently linked homodimeric complexes.