1 XKIRSPIVSV LGHVDHGKTT LLDHIRGSAV ASREAGGITQ HIGATEIPXD
51 VIEGICGDFL KKFSIRETLP GLFFIDTPGH EAFTTLRKRG GALADLAILI
101 VDINEGFKPQ TQEALNILRX YRTPFVVAAN KIDRIHGWRV HEGRPFXETF
151 SKQDIQVQQK LDTKVYELVG KLHEEGFESE RFDRVTDFAS QVSIIPISAI
201 TGEGIPELLT XLXGLAQQYL REQLKIEEDS PARGTILEVK EETGLGXTID
251 AVIYDGILRK DDTIAXXTSK DVISTRIRSL LKPRPLEEXR ESRKKFQKVD
301 EVVAAAGIKI VAPGIDDVXA GSPLRVVTDP EKVREEILSE IEDIKIDTDE
351 AGVVVKADTL GSLEAVVKIL RDXYVPIKVA DIGDVSRRDV VNAGIALQED
401 RVYGAIIAFN VKVIPSAAQE LKNSDIKLFQ GNVIYRLXEE YEEWVRGIEE
451 EKKKKWXEAI IKPASIRLIP KLVFRQSKPA IGGVEVLTGV IRQGYPLXND
501 DGETVGTVES XQDKGENLKS ASRGQKVAXA IKDAVYGKTI HEGDTLYVDI
551 PENHYHILKE QLSGDLTDEE LDLXDKIAEI KRKKNPDWGX KAPF
这些中有19个x氨基酸,这个氨基酸X代表什么意思