Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)A new microbial transglutaminase (MTGase or MTG, EC 2.3.2.13) from a Streptomyces sp. strain isolated from Brazilian soil samples was characterized in crude and purified forms. The aim of this work is to provide relevant information about a new transglutaminase and to compare its characteristics with the well-known commercial transglutaminase from Ajinomoto Co. Inc. (Activa (R) TG-BP). The enzyme from Streptomyces sp., in both crude and pure forms, exhibited optimal activity in the 6.0-6.5 pH range and at 35-40 degrees C. The results for the commercial enzyme were the same. A second maximum of activity was observed at pH 10.0 with both the crude Streptomyces sp. enzyme and the commercial enzyme. This interesting fact has not been reported in the literature previously. The fact that this second maximum of activity does not appear on the purified form of the enzyme may suggest the presence of an isoenzyme on the crude extract. All of the enzymes tested were stable over the pH range from 4.5 to 8.0 and up to 45 degrees C. The decline in activity of the commercial transglutaminase above 45 C and pH 8.0 was more gradual. The activities of all the MTG samples were independent of Ca(+2) concentration, but they were elevated in the presence of K(+), Ba(2+), and Co(2+) and inhibited by Cu(2+) and Hg(2+), which suggests the presence of a thiol group in the MTG's active site. The purified enzyme presented a Km of 6.37 mM and a V(max) of 1.7 U/mL, while the crude enzyme demonstrated a K(m) of 6.52 mM and a V(max) of 1.35 U/mL.32308314Food Biochemistry Laboratory of UNICAMPMicrobial Resource Division (CPQBA/UNICAMP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)