L 3.1 Khbi lugng phan tiir lectin
Cd kha nhídu cdng trinh khoa hoc da chira rang mdt sd phan tu lectin dugc cáu tao tir I mach polypeptit con da so cac dang lectin da dugc
nghien ciiu cd cau tao tir 2 mach polypeptit trd len. Mdi mach polypeptit tao thanh 1 tieu dan vi (Subunit). Cac tilu dan vi nay cd khdi lugng phan tir
gid'ng nhau hoac khac nhau (Strosberg et al, 1986) [106]. Nhd ap dung cac phuang phap hi6n dai nhu : sieu ly tam phan tich, nhilu xa tia Ranghen, quang ph6 huynh quang h^p phu v.v ... mdt sb dang lectin da dugc nghien ciiu kha ky v^ cau tnic khdng gian phan tụ Cau tnic khdng gian cua phan
tu lectin dugc nghien ciiu kha ky la lectin tir hat dau rua {Canavalia
ensiformis L.) edn goi la ConẠ
Cau tnic phan tir ConA dugc Bourillon dua ra nam 1973 khi ap dung phuang phap nhilu xa tia Ran ghen vdi do phan giai 2A°. Lectin nay cd dang hinh cau kich thudc 42 x 39 x 40 A° do 4 tilu dan vi tao thanh, mdi tilu dan vi cd khbi lugng phan tir gid'ng nhau bang 26 den 27 kDạ Mdi tilu dan vi cd 237 axit amin d pH 7, do đ Mr cua ConA la 108 kDạ Cac tieu dan vi nay lien ket vdi nhau bang cac lien ket khdng cdng hda tri va tao thanh phan tir gia tii dien [21,37).
Lectin tir m"am liia mi {Triticum vulgarae) ciing da dugc khao sat ky
ve cau taọ Phan tir nay gbm 2 tilu dan vi khdng dbi xiing. Mdi tilu dan vi
lai cd 4 ph'an khdng tuang dbng \t cau taọ Mdt phan gbm 42 gdc axit
amin, ba phan edn lai dugc hinh thanh tir 43 gbc axil amin. Bbn phan nay nhd 4 c^u disunfua (-S-S-) da tao thanh mdt cau hinh kha chac chan gbm mdt sb doan gap khuc. Tuy nhien su gap khiic nay la khdng tuan theo qui
luat.
Bat ky mdt dang lectin nao du cd cau tnic bac I hoac cd cau tnic khdng gian phiic tap deu chiia trung tam boat dong. Dd la trung tam lien ket hydratcacbon. Chinh trung tam nay quyet dinh boat đ lectin. Neu nhu d enzim, trung tam hoat đng cua chiing la cac gbc axit amin hoac phan phi protein (cac coenzim) thi d hUu het cac lectin trung tam boat đng cua chiing la do mdt sd gbc axit amin nhu tyrozin, xcrin, treonin, tripophan v.v
... cd kha nang lien ket manh vdi cac gdc dudng tao nen. Theo Decastle va
cdng su, lectin tir hat dau than đ {Phaseolus vulgaris), gdc axit amin
triptophan trong trung tam lien ket hydratcacbon giii vai trd quyet dinh hoat đng cua nd.
Nghien ciiu cau tao phan tir ConA, tac gia Agrawal (1967) eho biet : 2 gdc tirozin d vi tri 12 va 100 giii" vai trd quyet dinh trong trung tam hoat đng. Sir thay đi 2 gd'c nay lam giam boat do lectin ConA [21].
Cac dang lectin khac nhau, tat nhien cd thanh phan axit amin trong trung tam boat đng la khac nhaụ Cho den nay, van de trung tam boat đng cua lectin van edn phiic tap.
Theo Bourillon (1973) ngoai cac gbc axit amin tao nen trung tam boat đng, mdt sd ion kim loai ciing tham gia vao hoat đng lectin. Thudng gap nhat la cac ion Mg^"^, Ca"*^, Mn""". Den nay vai trd cua cac ion nay trong trung tam boat đng van chua dugc sang to, mac du ngudi ta ciing da biet chiing rat can thiet cho boat đng cua lectin.
Vay mdi phan tu lectin chiia bao nhieu trung tam hoat đng ? Cac nha khoa hoc da khang dinh, mdi phan tu lectin phai cd tir 2 trung tam boat đng trd l^n. Va do đ lectin la phan tir cd cau tnic da tri (polyvalent).
Nhd phuang phap nhilu xa tia Ranghen, ngudi ta da chiing minh rang d mdi tilu dan vi cua ConA cd mot trung tam lien ket hydratcacbon (trung tam boat đng) va 2 vi tri gan ca Ca""" va Mn""". Cac trung tam hoat đng nay deu each vi tri gan Ca""" la 7Á' va each vi tri gan Mn^"" la 6,8Â Giiia cac trung tam hoat đng cd mdt khoang each tuang đị Trong qua trinh boat đng, cac trung tam xich lai gan nhau, tuang tac vdi nhau tao nen diem tiep nhan hydratcacbon. Nhd diem tiep nhan nay, lectin da lien ket