Proteins are a substantial nitrogen source in soils provided that they can be hydrolyzed into bioavailable small peptides or amino acids. However, the strong associations between proteins and soil minerals restrict such proteolytic reactions. This study focused on how an extracellular fungal protease (Rhizopus sp.) hydrolyzed iron oxide-associated bovine serum albumin (BSA) and the factors that affected the proteolysis. We combined batch experiments with size-exclusion and reversed phase liquid chromatography and in situ infrared spectroscopic measurements to monitor the generation of proteolytic products in solution as well as the real-time changes of the adsorbed BSA during 24 h. Results showed that protease hydrolyzed the iron oxide-associated BSA directly at the surface without an initial desorption of BSA. Concurrently, the protease was adsorbed to vacant surface sites at the iron oxides, which significantly slowed down the rate of proteolysis. This inhibiting effect was counteracted by the presence of preadsorbed phosphate or by increasing the BSA coverage, which prevented protease adsorption. Fast initial rates of iron oxide-associated BSA proteolysis, comparable to proteolysis of BSA in solution, and very slow rates at prolonged proteolysis suggest a large variability in mineral-associated proteins as a nitrogen source in soils and that only a fraction of the protein is bioavailable.

中文翻译：

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氧化铁相关的牛血清白蛋白的蛋白水解。

蛋白质是土壤中的重要氮源，只要可以将其水解为可生物利用的小肽或氨基酸即可。但是，蛋白质和土壤矿物质之间的强关联限制了这种蛋白水解反应。这项研究的重点是细胞外真菌蛋白酶（Rhizopus sp。）如何水解氧化铁相关的牛血清白蛋白（BSA）以及影响蛋白水解的因素。我们将批量实验与尺寸排阻和反相液相色谱以及原位红外光谱测量相结合，以监控溶液中蛋白水解产物的产生以及24小时内吸附的BSA的实时变化。结果表明，蛋白酶直接在表面水解了与氧化铁相关的BSA，而没有BSA的初始解吸。同时，蛋白酶被吸附到氧化铁上空的表面部位，这大大减慢了蛋白水解的速度。这种抑制作用被预吸附的磷酸盐的存在或增加了BSA的覆盖率所抵消，这阻止了蛋白酶的吸附。与溶液中BSA的蛋白水解相当的与氧化铁相关的BSA蛋白水解的初始速率快，而在长时间的蛋白水解中非常慢的速率表明矿物相关蛋白作为土壤中的氮源存在很大的差异，并且只有一部分蛋白是生物利用度。