ABSTRACT
Glucooligosaccharide oxidase from
Acremonium strictum
was screened for potential applications in oligosaccharide acid production and carbohydrate detection. This protein is a unique covalent flavoenzyme which catalyzes the oxidation of a variety of carbohydrates with high selectivity for cello- and maltooligosaccharides. Kinetic measurements suggested that this enzyme possesses an open carbohydrate-binding groove, which is mainly composed of two glucosyl-binding subsites. The encoding gene was subsequently cloned, and one intron was detected in the genomic DNA. Large amounts of active enzymes were expressed in
Pichia pastoris
, with a yield of 300 mg per liter medium. The protein was predicted to share structural homology with plant cytokinin dehydrogenase and related flavoproteins that share a conserved flavin adenine dinucleotide (FAD)-binding domain. The closest sequence matches are those of plant berberine bridge enzyme-like proteins, particularly the characteristic flavinylation site. Unexpectedly, mutation of the putative FAD-attaching residue, H70, to alanine, serine, cysteine, and tyrosine did not abolish the covalent FAD linkage and had little effect on the
K
m
. Instead, the variants displayed
k
cat
values that were 50- to 600-fold lower, indicating that H70 is crucial for efficient redox catalysis, perhaps through modulation of the oxidative power of the flavin.
摘要
葡萄糖寡糖氧化酶
的葡萄糖寡糖氧化酶
中的葡萄糖低聚糖氧化酶进行了筛选,以确定其在低聚糖酸生产和碳水化合物检测中的潜在应用。该蛋白是一种独特的共价黄酮酶,可催化多种碳水化合物的氧化,对纤维寡糖和麦芽寡糖具有高选择性。动力学测量表明,这种酶具有一个开放的碳水化合物结合槽,主要由两个葡萄糖基结合亚位点组成。随后克隆了编码基因,并在基因组 DNA 中检测到一个内含子。大量活性酶在
Pichia pastoris
中表达了大量活性酶,每升培养基的产量为 300 毫克。据预测,该蛋白与植物细胞分裂素脱氢酶及相关黄素蛋白在结构上具有同源性,它们共享一个保守的黄素腺嘌呤二核苷酸(FAD)结合域。与之序列最接近的是植物小檗碱桥酶样蛋白,尤其是其特有的黄素化位点。出乎意料的是,将假定的 FAD 连接残基 H70 突变为丙氨酸、丝氨酸、半胱氨酸和酪氨酸并不能取消 FAD 的共价连接,而且对 FAD 的合成几乎没有影响。
K
m
.相反,显示的变量
k
cat
值低 50 到 600 倍,这表明 H70 对高效氧化还原催化作用至关重要,也许是通过调节黄素的氧化能力实现的。