是smiles,因为前后都有s,按英语讲,是最长的
答案有两个哦：
如果从实际上来说：
(1)答案是：是“色氨酸合成酶A蛋白质”， 一般的字典上是见不着的。
（一种含有267种氨基酸酶）的全称－－ 或许，你可以试试翻看医学专用词汇字典。
单词：
MethionylglutaminylarginyltyrosylglutamylserylleucylphenylalanylalanylglutaminylleucyllysylglutamylarginyllysylglutamylglycylalanylphenylalanylvalylprolyphenylalanYlvalythreonylleucylglycylaspartylprolylglycylisoleucylglutamylglutaminylsErylleucyllysylisoleucylaspartylthreonylleucylIsoleucylglutamylalanylglycylalanylasparthlalanylleucylglutamylleucylglycylisoleucylprolylphenylalanylseRylaspartylprolylleucylalanylaspartylglycylpRolylthreOnylisoleucylglutaminylasPfraginylalanylthreonylleucylarfinylalanylphenylalanylalanylalanylglycylvalythreonylprolylalanylglutaminylcysteinylphenylalanylglutamylmethionylleucylalanylleuOylisoleucylarginylglutaminyllysyhistidylprolylthreonylisoleucylprolylisoleucylglycylleucylmethionyltyrosylalanylasparaginylleucylvalylphenylalanylasparaginyllysyglycylisoleucylaspartylglutamylphenylalanylthrosylalanylglutaminylcysteinylglutamyllysylvalylglycylvalylaspartylserylvalylleucylvalylalnylaspartylvalylprolylvalylglUtaminylglutamylserylalanylprolylphenylalanylarginylglutaminylalanylalanylleucylarginylhistidylasparaginyvalylalanylprolylisoleucylprolylisoleucylphenylalanylisoleucylphenylalanylisoleucylcysteinylprolylprolylaspartylalanylaspartylaspartylaspartylleucylleucylarginylglutaminylisoleucylalanylseryltyrosylglycylarginylglycyltyrosylthreonyltyrOsylleucylleucylserylargin ylalanylglycylvalylthreonylglycylalanylglutamYlasparainylarginylalanylalanylleucylprolylleucylasparaginylhistidylleucylValylalanyllysylleucyllysylglutamyltyrosylasparaginylalanylalanylprolylprolylleucylglutaminylglgycylphenylalanylglycylisoleucylserylalanylprolylaspartylglutaminylvalyllysylalanylalanylisoleucylaspartylalanylglycylalanylalanylglycylalanylisoleucylserylglycylserylalanylisoleucylvalyllysylisoIeucylisoleucylglutamylglutaminylHistidylasparaginyliSoleucylglutamylprolylglutamyllysylmethionylleucylalanylalanylleucyllysylvalylphenylalanylcalylglutaminylprolylmethionlysylalanylalanylthreonylarginylserine
(2)第二种答案："Smiles"
原因：
It is the last lesson before the holidays. The students are very happy. Their English teacher is very happy, too. Their teacher plays some nice games with them. He sings some nice songs with them, and then he goes to the blackboard and writes SMILES on it.
"This is one of the longest words in English," he says to the class.
All the students laugh, and then one girl stands up and says, "Why is it one of the longest words in English?"
The teacher says nothing for a few seconds. Then he smiles and says, "Because there is a mile between the first letter and the last ."
翻译：
这是放假前的最后一节课。同学们都非常高兴，他们的英语老师也非常高兴。老师和他们一起做了好玩的游戏，还和他们一起唱好听的歌。后来，他走到黑板前，在上面写上了"SMILES"。
“这是英语单词中最长的一个，”他对全班同学说。
所有的同学们都笑了起来，后来，一个女孩儿站起来问：“为什么它是英语单词中最长的一个呢？”
老师没有说话，过了几秒钟，他笑笑说：“因为第一个字母和最后一个字母之间有一英里（mile）那么长。”
（希望以上内容有帮助，谢谢阅读）
参考资料：http://www.englishvod.net/Article/Class59/200409/2029.html||http://bbs.cn.yimg.com/user_img/200607/31/ba***********_1306073109327.gif
最长的是1，913个字母，
是“色氨酸合成酶A蛋白质”
Methionylglutaminylarginyltyrosylglutamylserylleucylphenylalanylalany
lglutaminylleucyllysylglutamylarginyllysylglutamylglycylalanylphenyla
lanylvalylprolyphenylalanYlvalythreonylleucylglycylaspartylprolylglyc
ylisoleucylglutamylglutaminylsErylleucyllysylisoleucylaspartylthreony
lleucylIsoleucylglutamylalanylglycylalanylasparthlalanylleucylglutamy
lleucylglycylisoleucylprolylphenylalanylseRylaspartylprolylleucylalan
ylaspartylglycylpRolylthreOnylisoleucylglutaminylasPfraginylalanylthr
eonylleucylarfinylalanylphenylalanylalanylalanylglycylvalythreonylpro
lylalanylglutaminylcysteinylphenylalanylglutamylmethionylleucylalanyl
leuOylisoleucylarginylglutaminyllysyhistidylprolylthreonylisoleucylpr
olylisoleucylglycylleucylmethionyltyrosylalanylasparaginylleucylvalyl
phenylalanylasparaginyllysyglycylisoleucylaspartylglutamylphenylalany
lthrosylalanylglutaminylcysteinylglutamyllysylvalylglycylvalylasparty
lserylvalylleucylvalylalnylaspartylvalylprolylvalylglUtaminylglutamyl
serylalanylprolylphenylalanylarginylglutaminylalanylalanylleucylarginy
lhistidylasparaginyvalylalanylprolylisoleucylprolylisoleucylphenylalan
ylisoleucylphenylalanylisoleucylcysteinylprolylprolylaspartylalanylasp
artylaspartylaspartylleucylleucylarginylglutaminylisoleucylalanylseryl
tyrosylglycylarginylglycyltyrosylthreonyltyrOsylleucylleucylserylargin
ylalanylglycylvalylthreonylglycylalanylglutamYlasparainylarginylalanyl
alanylleucylprolylleucylasparaginylhistidylleucylValylalanyllysylleucy
llysylglutamyltyrosylasparaginylalanylalanylprolylprolylleucylglutaminy
lglgycylphenylalanylglycylisoleucylserylalanylprolylaspartylglutaminylv
alyllysylalanylalanylisoleucylaspartylalanylglycylalanylalanylglycylala
nylisoleucylserylglycylserylalanylisoleucylvalyllysylisoIeucylisoleucyl
glutamylglutaminylHistidylasparaginyliSoleucylglutamylprolylglutamyllys
ylmethionylleucylalanylalanylleucyllysylvalylphenylalanylcalylglutaminy
lprolylmethionlysylalanylalanylthreonylarginylserine
参考资料：||
