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May 11, 2004Keywords:
ammonia assimilation glutamate dehydrogenase GDH Gracilaria sordida red
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Some Properties of Glutamate Dehydrogenase from the Marine Red Alga Gracilaria sordida (Harv.) W. Nelson
Abstract
Keywords: ammonia assimilation, glutamate dehydrogenase, GDH, Gracilaria sordida, red alga, enzyme activity
Glutamate dehydrogenases (GDH, EC 1.4.1.2-4) catalyse the entry of ammonium into the organic cycle via amination of a-ketoglutarate and its release via deamination of Lglutamate. NAD-GDH facilitates NAD-/NADP-dependent deamination while NADPH-GDH facilitates NADH-/NADPH-dependent amination reactions. In the study reported here, GDHs were extracted from the red alga Gracilaria sordida and purified 10- to 180-fold before examining their amination and deamination reaction properties. NAD-/NADP- and NADH-/ NADPH-dependent activities were the order of 11:1 and 1:1.8, respectively. The pH optima for amination and deamination were 8.2 and 8.4 under NADH- and NADPH- and 8.4 and 9 under NAD- and NADP- dependent activities, respectively. Whereas both NAD- and NADP-dependent deamination activities were activated by calcium ions (Ca2+), only NADPH-dependent amination was activated. The Km values (in mM) were 3.0–3.6 for ammonia, 2.0–3.3 for a-ketoglutarate, 0.00286 for NADH, 0.0033 for NADPH),1.7–2.1 for L-glutamate, 0.344 for NAD and 0.476 for NADP. It appears that the GDHs in G. sordida are dominated by NAD-dependent deamination enzyme and that in NADH-NADPH- dependent amination reactions, NADPH is more preferred. It is suggested that NAD-GDH\'s role in G. sordida could be as a catabolic shunt facilitating respiration. Anabolic functions could be assimilation of ammonia released during photorespiration and synthesis of N-rich transport compounds.
Western Indian Ocean Journal of Marine Sciences Vol.2(2) 2003: 179-186
Glutamate dehydrogenases (GDH, EC 1.4.1.2-4) catalyse the entry of ammonium into the organic cycle via amination of a-ketoglutarate and its release via deamination of Lglutamate. NAD-GDH facilitates NAD-/NADP-dependent deamination while NADPH-GDH facilitates NADH-/NADPH-dependent amination reactions. In the study reported here, GDHs were extracted from the red alga Gracilaria sordida and purified 10- to 180-fold before examining their amination and deamination reaction properties. NAD-/NADP- and NADH-/ NADPH-dependent activities were the order of 11:1 and 1:1.8, respectively. The pH optima for amination and deamination were 8.2 and 8.4 under NADH- and NADPH- and 8.4 and 9 under NAD- and NADP- dependent activities, respectively. Whereas both NAD- and NADP-dependent deamination activities were activated by calcium ions (Ca2+), only NADPH-dependent amination was activated. The Km values (in mM) were 3.0–3.6 for ammonia, 2.0–3.3 for a-ketoglutarate, 0.00286 for NADH, 0.0033 for NADPH),1.7–2.1 for L-glutamate, 0.344 for NAD and 0.476 for NADP. It appears that the GDHs in G. sordida are dominated by NAD-dependent deamination enzyme and that in NADH-NADPH- dependent amination reactions, NADPH is more preferred. It is suggested that NAD-GDH\'s role in G. sordida could be as a catabolic shunt facilitating respiration. Anabolic functions could be assimilation of ammonia released during photorespiration and synthesis of N-rich transport compounds.
Western Indian Ocean Journal of Marine Sciences Vol.2(2) 2003: 179-186
