Dias B, Weimer B
Western Dairy Center, Department of Nutrition and Food Sciences, Utah State University, Logan, Utah 84322-8700, USA.
Appl Environ Microbiol. 1998 Sep;64(9):3327-31. doi: 10.1128/AEM.64.9.3327-3331.1998.
L-Methionine gamma-lyase (EC 4.4.1.11) was purified to homogeneity from Brevibacterium linens BL2, a coryneform bacterium which has been used successfully as an adjunct bacterium to improve the flavor of Cheddar cheese. The enzyme catalyzes the alpha,gamma elimination of methionine to produce methanethiol, alpha-ketobutyrate, and ammonia. It is a pyridoxal phosphate-dependent enzyme, with a native molecular mass of approximately 170 kDa, consisting of four identical subunits of 43 kDa each. The purified enzyme had optimum activity at pH 7.5 and was stable at pHs ranging from 6.0 to 8.0 for 24 h. The pure enzyme had its highest activity at 25 degreesC but was active between 5 and 50 degreesC. Activity was inhibited by carbonyl reagents, completely inactivated by DL-propargylglycine, and unaffected by metal-chelating agents. The pure enzyme had catalytic properties similar to those of L-methionine gamma-lyase from Pseudomonas putida. Its Km for the catalysis of methionine was 6.12 mM, and its maximum rate of catalysis was 7.0 &mgr;mol min-1 mg-1. The enzyme was active under salt and pH conditions found in ripening Cheddar cheese but susceptible to degradation by intracellular proteases.
L-蛋氨酸γ-裂解酶（EC 4.4.1.11）从短乳杆菌BL2中纯化至同质，短乳杆菌是一种棒状细菌，已成功用作辅助细菌来改善切达干酪的风味。该酶催化蛋氨酸的α,γ消除反应，生成甲硫醇、α-酮丁酸和氨。它是一种依赖磷酸吡哆醛的酶，天然分子量约为170 kDa，由四个相同的43 kDa亚基组成。纯化后的酶在pH 7.5时具有最佳活性，在pH 6.0至8.0范围内24小时内稳定。纯酶在25℃时活性最高，但在5至50℃之间都有活性。活性受到羰基试剂的抑制，被DL-炔丙基甘氨酸完全灭活，且不受金属螯合剂的影响。纯酶具有与恶臭假单胞菌的L-蛋氨酸γ-裂解酶相似的催化特性。其催化蛋氨酸的Km为6.12 mM，最大催化速率为7.0 μmol min-1 mg-1。该酶在切达干酪成熟过程中的盐和pH条件下有活性，但易被细胞内蛋白酶降解。
