In enzymology, an L-amino acid oxidase (LAAO) (EC 1.4.3.2) is an enzyme that catalyzes the chemical reaction:
an L-amino acid + H2O + O2 ⇌ a 2-oxo acid + NH3 + H2O2 The enzyme was first described in 1944 by A. Zeller and A. Maritz. Not only are LAAOs quite variable in terms of molecular mass, they also vary widely regarding stability. In a similar vein, this enzyme performs in a myriad of biological activities including apoptosis-induction, edema-induction, hemorrhaging, and inhibition or induction of platelet aggregation. As suggested by the name of the family, LAAOs are flavoenzymes which function to catalyze the stereospecific oxidative deamination of an L-amino acid. The three substrates of the enzymatic reaction are an L-amino acid, water, and oxygen. The products are the corresponding α-keto acid (2-oxo acid), ammonia, and hydrogen peroxide. One example of the enzyme in action occurs with the conversion L-alanine into pyruvic acid (2-oxopropanoic acid):
Recent research has expanded understanding of snake venom LAAOs (sv-LAAOs), revealing their role in venom-induced tissue damage through oxidative stress and induction of cell death pathways such as autophagy, apoptosis, and necrosis. Structural studies have clarified their three-domain organization and active site variability, which underlie substrate specificity and functional diversity. In addition to their antimicrobial properties, sv-LAAOs exhibit selective cytotoxicity against tumor cells, making them a subject of growing interest for therapeutic development.
Abundance
Snake venom Although LAAOs are present in a variety of eukaryotic and prokaryotic organisms, snake venom is a particularly rich source of the enzyme and the LAAOs are proposed to supply toxic effects upon envenomation. LAAOs that have been purified from the venoms of various snake species have proven to be the best suitors for examining this novel family of enzymes. It has been determined in most cases concerning the snake families, such as Viperidae, Crotalidae, and Elapidae, that snake venom-LAAO (sv-LAAO) constitutes about 1%–9% of the total protein quantity.
Structure Most sv-LAAOs are reported as being homodimers with multiple subunits that have molecular weights around 50–70 kDa and the interaction between the subunits occurs via non-covalent interactions. Sv-LAAOs are present in the acidic, basic, and neutral forms of the protein. Studies that look at x-ray crystal structures have confirmed that sv-LAAOs are often found as functional dimers, with each dimer having three domains. The three domains are the substrate-binding site, FAD-binding site, and a helical domain. The substrate-binding site lies at the bottom of a funnel-shaped cavity approximately 25 Å deep, enabling substrate specificity among sv-LAAOs. Structural differences in active site topology likely account for species-dependent substrate preferences. Additionally, LAAOs are thermolabile, with cold inactivation and heat reactivation properties, necessitating specific storage and pre-activation conditions to preserve enzymatic activity. It has also been determined that the FAD prosthetic group becomes deeply entrenched in the enzyme structure, which allows for pervasive interactions with both neighboring atoms and conserved water molecules. Additionally, this flavin-containing prosthetic group has been classified as providing snake venom with its quintessential dark yellow coloration, which is shown in Figure 2. One unusual characteristic reported for sv-LAAOs regards the cold inactivation and heat reactivation properties of the protein. Thereby, most sv-LAAOs are considered to be thermolabile enzymes.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH−NH2 group of donors with oxygen as acceptor. The systematic name of this enzyme class is L-amino-acid:oxygen oxidoreductase (deaminating). This enzyme is also called ophio-amino-acid oxidase. As of late 2007, 11 structures have been solved for this class of enzymes, with PDB accession codes PDB: 1F8R, PDB: 1F8S, PDB: 1REO, PDB: 1TDK, PDB: 1TDN, PDB: 1TDO, PDB: 2IID, PDB: 2JAE, PDB: 2JB1, PDB: 2JB2, and PDB: 2JB3.
Biological function
Specificity The specific activities of sv-LAAOs with various L-amino acids have been explored. Many studies show that a number of sv-LAAOs exhibit a preference for hydrophobic L-amino acids as substrates. For example, results have indicated that most sv-LAAOs demonstrate relatively high specificities toward hydrophobic amino acids such as L-Met, L-Leu, and L-Ile in addition to aromatic amino acids such as L-Phe and L-Trp.
Activity This enzyme participates in 8 metabolic pathways: alanine and aspartate metabolism, methionine metabolism, valine, leucine and isoleucine degradation, tyrosine metabolism, phenylalanine metabolism, tryptophan metabolism, phenylalanine, tyrosine and tryptophan biosynthesis, and alkaloid biosynthesis. It employs one cofactor, flavin adenine dinucleotide (FAD). The enzyme binds to FAD in the first step of the catalytic process, thereby reducing FAD to FADH2. The FAD is regenerated from FADH2 by oxidation as a result of O2 being reduced to H2O2. The mechanism proceeds via oxidative deamination of the L-amino acid, which affords an imino acid intermediate. Following hydrolysis of the intermediate, the enzyme successfully affords the 2-oxo acid, as shown in Scheme 1.
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