Monoacylglycerol lipases (MGLs) are present in all domains of life. However, reports on bacterial MGLs are still limited. Until now, reported bacterial MGLs are all thermophilic/mesophilic enzymes from warm terrestrial environments or deep-sea hydrothermal vent, and none of them originates from marine environments vastly subject to low temperature, high salts, and oligotrophy. Here, we characterized a novel MGL, GnMgl, from the marine cold-adapted and halophilic bacterium Glaciecola nitratireducens FR1064T. GnMgl shares quite low sequence similarities with characterized MGLs (lower than 31%). GnMgl and most of its bacterial homologs harbor a catalytic Ser residue located in the conserved C(A/S)HSMG motif rather than in the typical GxSxG motif reported on other MGLs, suggesting that GnMgl-like enzymes might be different from reported MGLs in catalysis. Phylogenetic analysis suggested that GnMgl and its bacterial homologs are clustered as a separate group in the monoglyceridelipase_lysophospholipase family of the Hydrolase_4 superfamily. Recombinant GnMgl has no lysophospholipase activity but could hydrolyze saturated (C12:0-C16:0) and unsaturated (C18:1 and C18:2) MGs and short-chain triacylglycerols, displaying distinct substrate selectivity from those of reported bacterial MGLs. The substrate preference of GnMgl, predicted to be a membrane protein, correlates to the most abundant fatty acids within the strain FR1064T, suggesting the role of GnMgl in the lipid catabolism in this marine bacterium. In addition, different from known bacterial MGLs that are all thermostable enzymes, GnMgl is a cold-adapted enzyme, with the maximum activity at 30°C and retaining 30% activity at 0°C. GnMgl is also a halotolerant enzyme with full activity in 3.5M NaCl. The cold-adapted and salt-tolerant characteristics of GnMgl may help its source strain FR1064T adapt to the cold and saline marine environment. Moreover, homologs to GnMgl are found to be abundant in various marine bacteria, implying their important physiological role in these marine bacteria. Our results on GnMgl shed light on marine MGLs.

中文翻译：

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在海洋细菌中广泛传播的一种新型的冷活性和耐卤素单酰基甘油脂肪酶的研究揭示了一组新的细菌单酰基甘油脂肪酶，它们含有异常的C（A / S）HSMG催化基元。

单酰基甘油脂肪酶（MGL）存在于生活的所有领域。但是，有关细菌MGL的报道仍然有限。到目前为止，已报道的细菌MGL都是来自温暖陆生环境或深海热液喷口的嗜热/中温酶，它们都不是源自遭受低温，高盐和寡营养的海洋环境。在这里，我们从海洋冷适应和嗜盐细菌硝化杆菌FR1064T中表征了一种新型MGL，GnMgl。GnMgl与特征性MGL的序列相似性非常低（低于31％）。GnMgl及其大多数细菌同源物在保守的C（A / S）HSMG基序中而不是其他MGL报告的典型GxSxG基序中带有催化的Ser残基，提示GnMgl样酶可能与报道的MGL催化作用不同。系统发育分析表明，GnMgl及其细菌同源物在Hydrolase_4超家族的单甘油脂酶_溶血磷脂酶家族中聚集为一个独立的基团。重组GnMgl没有溶血磷脂酶活性，但可以水解饱和（C12：0-C16：0）和不饱和（C18：1和C18：2）MG和短链三酰基甘油，与已报道的细菌MGL相比具有不同的底物选择性。GnMgl的底物偏好（预计是一种膜蛋白）与FR1064T菌株中最丰富的脂肪酸相关，表明GnMgl在该海洋细菌中的脂质分解代谢中的作用。此外，不同于已知的细菌MGL，它们都是热稳定酶，GnMgl是一种冷适应的酶，在30°C时具有最大活性，在0°C时仍保持30％的活性。GnMgl还是一种卤代酶，在3.5M NaCl中具有完全活性。GnMgl的耐寒和耐盐特性可能有助于其源菌株FR1064T适应寒冷和盐碱的海洋环境。此外，发现GnMgl的同源物在各种海洋细菌中都丰富，这暗示了它们在这些海洋细菌中的重要生理作用。我们在GnMgl上的研究结果为海洋MGL提供了启示。已发现GnMgl的同系物在各种海洋细菌中都丰富，这暗示了它们在这些海洋细菌中的重要生理作用。我们在GnMgl上的研究结果为海洋MGL提供了启示。已发现GnMgl的同系物在各种海洋细菌中都丰富，这暗示了它们在这些海洋细菌中的重要生理作用。我们在GnMgl上的研究结果为海洋MGL提供了启示。