己糖激酶家族包括己糖激酶I,II,III和IV,催化葡萄糖的磷酸化产生葡萄糖6-磷酸。己糖激酶IV,也被称为葡萄糖激酶,只有含有两个己糖激酶结构域的其他类型的己糖激酶的一半大小。尽管己糖激酶的研究取得了巨大的进展,葡萄糖激酶和其他己糖激酶之间的进化关系仍然不确定,导致脊椎动物己糖激酶出现的分子过程仍然存在争议。在这里,我们清楚地证明了在文昌鱼中存在单个己糖激酶样基因,Bjhk,显示组织特异性表达模式,在肝盲肠中表达最丰富,睾丸和卵巢。系统发育和同种学分析均表明BjHK是脊椎动物己糖激酶IV的原型,即葡萄糖激酶。我们还首次发现重组BjHK显示出类似脊椎动物己糖激酶I的功能性酶活性,II,III和IV。此外,在肝盲肠中检测到天然的葡萄糖激酶活性。最后,葡萄糖激酶活性在肝盲肠明显减少,而通过喂食却大大增加了。总之,这些表明Bjhk代表了葡萄糖激酶的原型,脊椎动物己糖激酶基因家族是通过基因复制进化而来的,肝盲肠在控制文昌鱼的葡萄糖稳态中起作用,支持肝盲肠是与肝脏同源的组织的观点。
Hexokinase family includes hexokinases I, II, III and IV, that catalyze the phosphorylation of glucose to produce glucose 6-phosphate. Hexokinase IV, also known as glucokinase, is only half size of the other types of hexokinases that contain two hexokinase domains. Despite the enormous progress in the study of hexokinases, the evolutionary relationship between glucokinase and other hexokinases is still uncertain, and the molecular processes leading to the emergence of hexokinases in vertebrates remain controversial. Here we clearly demonstrated the presence of a single hexokinase-like gene in the amphioxus Branchiostoma japonicum, Bjhk, which shows a tissue-specific expression pattern, with the most abundant expression in the hepatic caecum, testis and ovary. The phylogenetic and synteny analyses both reveal that BjHK is the archetype of vertebrate hexokinases IV, i.e. glucokinases. We also found for the first time that recombinant BjHK showed functional enzyme activity resembling vertebrate hexokinases I, II, III and IV. In addition, a native glucokinase activity was detected in the hepatic caecum. Finally, glucokinase activity in the hepatic caecum was markedly reduced by fasting, whereas it was considerably increased by feeding. Altogether, these suggest that Bjhk represents the archetype of glucokinases, from which vertebrate hexokinase gene family was evolved by gene duplication, and that the hepatic caecum plays a role in the control of glucose homeostasis in amphioxus, in favor of the notion that the hepatic caecum is a tissue homologous to liver.