Glutamate decarboxylase (GAD) catalyzes the decarboxylation of glutamic acid giving-aminobutyric acid (GABA) (figure 1). GAD uses pyridoxal phosphate (PLP) as co-factor and H+ as co-substrate. This enzyme is broadly present in the living organisms, from bacteria to human. GABA is one of the main neurotransmission inhibitors in nervous central system. In addition, there is increasing interest of GABA because there are evidences that it can lower blood pressure in patients with mild hypertension1 and other potential healthy beneficial effects2, although these mechanisms are not known yet. GABA becomes an attractive potential functional ingredient for food, but the chemically synthesized GABA is not accepted for uses in food3. GABA was found in some fermented food, for instance Kimchi4, and some strains of Lactobacillus brevis (L. brevis) were identified as ones of highly GABA producers3. In addition, L. brevis is recognized as putative probiotic due potential healthy effects in humans5. GAD from L. brevis and other lactic acid bacteria apparently play an important role in the low pH resistance since it uses protons in the GABA production. It was previously reported that the highest level of activity is reached when GAD is in hexameric form and lowest when is present as dimer6,7. Therefore, the oligomerization seems to play an important role in the GAD mechanism. In this work we have studied the effect of pH and salt concentration in the oligomerization of a recombinant GAD from L. brevis, using asymmetrical flow field-flow fractionation (AF4), dynamic light scattering (DLS) and molecular modeling methods.
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