抄録
最近, 納豆が健康食品として注目されているが, 「納豆の糸」の主成分はポリ-γ-グルタミン酸(PGA)であり, D-グルタミン酸を多く含んでいる. また, PGAには多彩な機能があり, その有用性にも注目が集まっている. しかしながら, 納豆菌のPGA生合成機構は長い間不明であった. 本稿では, PGAの色々な機能や生合成機構について紹介する.1. ポリ-γ-グルタミン酸の構造と機能 PGAはグルタミン酸のγ-カルボキシル基とα-アミノ基がイソペプチド結合で連なったポリマーである. 主として納豆菌(Bacillus subtilis)などのBacillus属細菌によって生産されるが, 好塩古細菌Natrialba aegyptiacaや腔腸動物であるヒドラもPGAを生産する. その分子質量は1万ぐらいのものから100万を越えるものもある(表1). 炭疽菌Bacillus anthraciの生産するPGAはD-グルタミン酸のみから構成されているが, 好アルカリ性細菌Bacillus haloduransやN. aegyptiaca, ヒドラL-グルタミン酸からなるPGAを生産する.
Poly-γ-glutamate (PGA) is an unusual polypeptide in which glutamate is polymerized via γ-amide linkages. PGA is produced by Bacillus strains, Natrialba aegyptiaca, and hydra. PGA, which is biodegradable and highly water-absorbent acidic polymer, is not attacked by proteases, evades mammalian immune defense mechanisms, binds Ca^<2+>, and serves as a cryoprotective material. PGA may physiologically function as an adaptation agent in various environments. Multifarious applications of PGA have been developed based on its function. PGA produced by Bacillus subtilis IFO 3336 contains a large amount of D-glutamate. D-Glutamate was formed from L-glutamate by glutamate racemase in this organism. Two kinds of glutamate racemase isozymes, Glr and YrpC, were found in B, subtilis IFO 3336, and Glr supplied D-glutamate for the PGA production. The pgsBCA genes encoding the membrane-associated PGA synthetase complex of B. subtilis IFO 3336 were isolated. The PgsBCA enzyme complex synthesized PGA in an amidoligation-like manner. The structure and function of the enzyme complex are also discussed.
