Every year during the first days of November, Mexico dresses up in bright colors, flowers and jolly candy skulls. Día de Muertos (Day of the Dead), as this festival is is called there, is a national holiday meant to honour those beloved ones that are not with us anymore. The whole festivity includes the decoration of the graves using flowers (specially the cempasúchitl); baking traditional sweets such as pan de muerto (bread of the dead) or calaveras dulces (sweet skulls); preparing at home the favourite dish of the people they are honouring; or simply making poems and jokes about the death of beloved, living ones.
If the idea of a merry festival made to remember the dead sound familiar is because it is. Just like Halloween, Día de Muertos was born with a mix of two cultures, in this case the mesoamerican traditions and the european culture introduced by the spaniards during the colonization of America. Halloween, however, had some huge advantages for its introduction all over the world. Costume parties are usually quite funny and, more importantly, the gargantuan american media industry is extremely efficient in selling us the idea. What is not to love about spooky costumes and sweets? Let’s just remember that, in that weird and wonderful place that is Mexico, they do the same in their own way.
As we saw last month, glycolisis is a nearly universal path for living beings and sugar can be used by practically any known organism for obtaining energy. But as it happens with Día de Muertos, just because everybody does the same does not means that they do it the same way. Recognized as an independent group from both Bacteria and Eukaryotes at the beginning of the 90’s, the Archaea started to slowly gain attention by virtue of their unusual biochemistry. The lipids in their membranes were weird, the composition of their ribosomes even weirder, and many strange oddities were discovered in these fascinating microbes without a clear pattern. During the next decades, Archaea raised in the academic worldview from primitive remnants of early life on Earth (Archaea means ancient) to an important and overlooked component of the biosphere, carrying a nearly unexplored biochemical diversity that separates them from the rest of the living beings. Even common components were metabolized in strange ways by these tiny beings, including the energetic cornerstone of known life: glucose.
Carbohydrate metabolism in Archaea is highly variable, and our knowledge is highly biased toward certain groups that can be grown in laboratory conditions. In this regard, Sulfolobus solfataricus has been used as a model species within Archaea. Isolated from volcanic wells all over the world, this archaeon grows optimally at 85ºC and a pH of 3.5. Is an aerobic member of Crenarchaeota that is able to use many different types of sugar as carbon source, which makes it an excellent model for studying carbohydrate metabolism in hyperthermophilic archaea. Working with these hotties is not an easy task (for starters, agar plates cannot be used at that temperature!), but they have at least some redeeming qualities. For example, due to their high stability, crystallization of its proteins is relatively easy, making Sulfolobus and other Archaea a big favourite for studying protein structure.
Going back to the carbohydrate metabolism, Sulfolobus, like many other archaea, lacks the canonical Embden-Meyerhoff-Parnas pathway that is so common in the rest of the domains of life. Instead, it uses two complementary pathways that are modifications of the traditional Entner-Doudoroff metabolism, the semiphosphorylated modification and the non phosphorylated. In both cases, glucose dehydrogenase is the first enzyme in the pathway. S. solfataricus possesses at least two characterized glucose dehydrogenases (GDH). As you can observe in our tree of the month, there are several related enzymes with predicted dehydrogenase activity over sugars. GDH-1 seems to be a broad spectrum dehydrogenase able to catalyze the oxidation of many 6 and even 5 carbon sugars. On the other hand, GDH-2 has a high specificity for glucose. Apparently, GDH-2 would operate in situations in which the supply of glucose is stable; while GDH-1 would act as an all-terrain dehydrogenase to metabolize any sugar before inducing the expression of a more specialized enzyme. The additional enzymes may form part of the pathway under certain conditions, such as in the gluconeogenic sense of the pathway or operating on other sugars.
Finally, given the diversity of metabolic oddities, modifications and novel enzymes that Archaea harbour in their most essential biochemical pathways, one can only imagine what metabolic mysteries must remain unexplored in these wonderful microorganisms.