Biology Reference
In-Depth Information
globins complicated by an intertwining polyphyletic history. We note that
this heritage is surpassed in complexity by other aquatic photosynthetic
microbes such as cryptophytes and chlorarachniophytes. These organisms
are endosymbionts arising from the assimilation of photosynthetic eukary-
otes by non-photosynthetic eukaryotes. Their phylogeny is particularly dif-
ficult to unravel (
Curtis et al., 2012
) and will not be considered here.
As is apparent from this section, oxygenic photosynthetic organisms are a
diverse lot. What is common in all of these species is the fact that highly spe-
cialized structures are actively extracting electrons from water and creating
molecular oxygen—all in close proximity to a host of competing metabolic
pathways within the cell. Given that globins have been shown (at least in
some cases) to help mitigate the production of—or damage from—reactive
molecules within a cell (
Hill et al., 1996; Scott et al., 2010
) and that some are
chloroplastic and induced by light (
Couture et al., 1994; Couture &
Guertin, 1996
), it is quite possible that globins have important and unique
roles within the regulation and efficiency of photosynthesis. Involvement in
other metabolic processes, such as assimilation of nitrogen from nitrogen
oxide sources, is also emerging and indicates related functions.
3.2. Evolution and phylogeny
Since Vinogradov and co-workers advanced their classification of globins
into three distinct families (
Vinogradov et al., 2005
), hundreds of additional
globin genes have been sequenced and their protein products categorized
accordingly (
Vinogradov et al., 2013
). As mentioned earlier, the FHbs
together with the single-domain globins (SDgbs) form the M (myoglobin-
like) family. The S family is comprised of proteins known as GCSs but has
recently grown to include Pgb and SSDgb. The third family is the T family,
made up of proteins known as the truncated globins. Whereas both the
M and S families of globins contain a canonical 3/3 protein structure, the
truncated globins possess the smaller 2/2 structure (
Wittenberg et al.,
2002
). Globins as an evolutionary group are quite ancient, having emerged
at the very beginning of life on this planet, more than 3 billion years ago
(
Vazquez-Limon et al., 2012
). This places their appearance well before
the development of multicellular organisms and even unicellular eukaryotes.
Because of the linkage of all the three families of globins, it is thought that
both the canonical 3/3 fold and the truncated 2/2 fold bear a single ancient
common ancestor (
Vinogradov et al., 2007
) although these two types of
domains separated from each other very early in evolution (
Vinogradov
