链霉亲和素(streptavidin下称SA)是与亲和素(svidin下称AV)有相似生物学特性的一种蛋白质。

链霉亲和素分子由4条相同的肽链组成，其氨基酸组成中，甘氨酸和丙氨酸的含量较大，而且结合生物素的活性基团也是肽链中的色氨酸残基；链霉亲和素是一种稍偏酸性（pH6.0）的蛋白质，并且不带任何糖基。在蛋白水解酶作用下，链霉亲和素可在N端10～12和C端19-21间断裂，形成的核心链霉亲和素仍然保持完整的结合生物素的能力，所以可以提高融合率。

链霉亲和素(SA)是streptomyces avidinii菌的分泌物，其分子量及结台生物素的能力与鸡蛋清中的亲和素相似，等电点6.0，非特异性结合远比亲和素低。

一 、来源：AV 存在于鸡蛋清中，主要通过离子交换和分子筛方法提纯，从1L蛋清中可提取纯AV15~25mg。SA 是Streptomyces avicllrdi菌培养过程中的分泌产物，主要通过2一亚氨基生物索亲和层析法提纯，1L培养液中含蛋白1 0~60mg 。

二、分子量 ：一条完整的SA肽链中有159个氨基酸残基，分子量为16450，而AV 的一条肽链其含128个氨基酸残基，但分子量也达15600，两者十分接近，这是因为SA 的氨基酸中，甘、丙氨基酸含量较大，且又不含任何糖基，而AV 的每条肽链含4个甘露糖，3个氨基葡糖，含糖量达lO ，故分子量相对较大。

三、比消光系数：消光系数与蛋白中Tyr含量有关， 由于SA 中Tyr的含量比AV 多，所以SA的比消光系数也较AV 为高，两者分别为E 1mg/ml=3．40和E1mg/ml=1.57。在282nm 上这两种蛋白结合生物素后并不改变其消光系数，而在233nm上，它们结合生物素后郡便其消光系数值增高，现在一般都是利用蛋白的这种吸光特点来测定它们的活性。

Streptavidin, or SA, is a protein with similar biological properties to Avidin Svidin, or AV. The Streptavidin molecule is composed of four identical peptide chains, in which glycine and Alanine are more abundant in the amino acid composition, and the active group of biotin-binding is also the tryptophan residue in the peptide chain; Streptavidin is a slightly acidic (Ph 6.0) protein with no glycosyl groups. Under the action of proteolytic enzymes, Streptavidin can break between N Terminal 10 ~ 12 and C Terminal 19 ~ 21, and the formed core Streptavidin still retains the intact biotin binding capacity, so the fusion rate can be improved. Streptavidin is a secretory product of streptomyces AVIDINII, and its molecular weight and binding biotin ability are similar to those of Avidin in egg white. Its isoelectric point is 6.0, and its nonspecific binding is much lower than Avidin. Source: Av is present in egg white and purified by ion exchange and molecular sieve. AV15 ~ 25mg can be extracted from 1L egg white. Sa was isolated from Streptomyces Avicllrdi and purified by 2-imino affinity chromatography. The protein content of SA was 10 ~ 60 MG IN 1L culture medium. 2. Molecular Weight: A Complete SA peptide chain has 159 amino acid residues with a molecular weight of 16450, whereas a AV peptide chain has 128 amino acid residues with a molecular weight of 15600, which is very similar because SA Amino Acids The content of glycine and Alanine is high, and there is no sugar group. The molecular weight of Av is relatively high because it contains 4 mannose and 3 glucosamine. 3. Specific Extinction Coefficient: the Extinction Coefficient is related to the content of Tyr in protein. Because the content of Tyr in SA is more than that in Av, the specific extinction coefficient of Sa is also higher than that of Av, the two are e 1 mg / ml 3.40 and e 1 mg / ml 1.57 respectively. At 282 nm, the two proteins did not change their extinction coefficient when they combined with Biotin, but at 233 nm, their extinction coefficient increased It is now generally used to determine the protein activity of this light-absorbing characteristics.