Several monosaccharides constitute naturally occurring glycans, but it is uncertain whether they constitute a universal set like the alphabets of proteins and DNA. Based on the available experimental observations, it is hypothesized herein that the glycan alphabet is not universal. Data on the presence/absence of pathways for the biosynthesis of 55 monosaccharides in 12 939 completely sequenced archaeal and bacterial genomes are presented in support of this hypothesis. Pathways were identified by searching for homologues of biosynthesis pathway enzymes. Substantial variations were observed in the set of monosaccharides used by organisms belonging to the same phylum, genera and even species. Monosaccharides were grouped as common, less common and rare based on their prevalence in Archaea and Bacteria. It was observed that fewer enzymes are sufficient to biosynthesize monosaccharides in the common group. It appears that the common group originated before the formation of the three domains of life. In contrast, the rare group is confined to a few species in a few phyla, suggesting that these monosaccharides evolved much later. Fold conservation, as observed in aminotransferases and SDR (short-chain dehydrogenase reductase) superfamily members involved in monosaccharide biosynthesis, suggests neo- and sub-functionalization of genes led to the formation of the rare group monosaccharides. The non-universality of the glycan alphabet begets questions about the role of different monosaccharides in determining an organism’s fitness.

中文翻译：

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聚糖字母表并不普遍：一个假设

几种单糖构成天然存在的聚糖，但不确定它们是否构成像蛋白质和 DNA 字母表那样的通用集合。基于可用的实验观察，本文假设聚糖字母表不是通用的。在 12 939 个完全测序的古菌和细菌基因组中，提供了有关 55 种单糖生物合成途径存在/不存在的数据，以支持这一假设。通过寻找生物合成途径酶的同源物来鉴定途径。在属于同一门、属甚至物种的生物体使用的一组单糖中观察到了实质性的变化。单糖根据它们在古细菌和细菌中的流行程度分为常见、不太常见和稀有。据观察，在普通组中，较少的酶足以生物合成单糖。看来，共同群体起源于三个生命领域的形成之前。相比之下，稀有群体仅限于少数门中的少数物种，这表明这些单糖进化得晚得多。正如在参与单糖生物合成的氨基转移酶和 SDR（短链脱氢酶还原酶）超家族成员中观察到的那样，折叠保守表明基因的新功能化和亚功能化导致了稀有组单糖的形成。聚糖字母表的非通用性引发了关于不同单糖在决定生物体适应性方面的作用的问题。看来，共同群体起源于三个生命领域的形成之前。相比之下，稀有群体仅限于少数门中的少数物种，这表明这些单糖进化得晚得多。正如在参与单糖生物合成的氨基转移酶和 SDR（短链脱氢酶还原酶）超家族成员中观察到的那样，折叠保守表明基因的新功能化和亚功能化导致了稀有组单糖的形成。聚糖字母表的非通用性引发了关于不同单糖在决定生物体适应性方面的作用的问题。看来，共同群体起源于三个生命领域的形成之前。相比之下，稀有群体仅限于少数门中的少数物种，这表明这些单糖进化得更晚。正如在参与单糖生物合成的氨基转移酶和 SDR（短链脱氢酶还原酶）超家族成员中观察到的那样，折叠保守表明基因的新功能化和亚功能化导致了稀有组单糖的形成。聚糖字母表的非通用性引发了关于不同单糖在决定生物体适应性方面的作用的问题。正如在参与单糖生物合成的氨基转移酶和 SDR（短链脱氢酶还原酶）超家族成员中所观察到的那样，表明基因的新功能化和亚功能化导致了稀有单糖组的形成。聚糖字母表的非通用性引发了关于不同单糖在决定生物体适应性方面的作用的问题。正如在参与单糖生物合成的氨基转移酶和 SDR（短链脱氢酶还原酶）超家族成员中所观察到的那样，表明基因的新功能化和亚功能化导致了稀有单糖组的形成。聚糖字母表的非通用性引发了关于不同单糖在决定生物体适应性方面的作用的问题。