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molecules ( Davis & Pearlstein, 1979; Markovic, Proll, Bubenzer, & Scheer,
2007; Pr¨ ll, Wilhelm, Robert, & Scheer, 2006; Wright & Boxer, 1981 ), but
of course, myoglobin never encounters these pigments. Depending on their
localization, cyanobacterial and algal proteins may need to discriminate among
haem and its derivatives used in photosynthesis. The derivatives include linear
tetrapyrroles that are used by phycocyanins. In this regard, it is fitting to recall
that globins andphycocyanins are related ( Pastore&Lesk, 1990 ) and the issue of
cofactor recognition, in itself, is a topic worthy of exploration ( Aloy,
Ceulemans, Stark, & Russell, 2003 ). We have inspected the association of
Zn pheophorbide a methyl ester (as a solublemimic of chlorophyll) withGlbN
and have not found strong interactions ( Landfried, 2010 ). The possibility exists
that in some cases, the globins dobind a cofactor different fromthe haemgroup,
and we suggest that this should be kept inmind, in particular, when physiolog-
ical information on recombinant proteins and divergent or suspicious
sequences are studied in a non-photosynthetic host.
Since virtually all of the structural data discussed in Section 5 are based
upon recombinant protein prepared in a non-photosynthetic host ( E. coli ), a
control experiment may be as conceptually simple as verifying the nature of
the prosthetic group contained within the native form of these proteins. In
several cases, unique and highly selective antibodies already exist for these
proteins, and it may be possible to isolate purified proteins from whole-cell
extracts of actively growing cultures. Although the expression levels of glo-
bins is in almost all cases low, especially when compared with dominant pro-
tein complexes such as the cell's photosystems, purification of small amounts
of material may be achievable. This is particularly so, given recent trans-
criptomic results for several species, as we now know growth conditions
and media additives that stimulate the expression of several globin genes.
Instances such as those illustrated by Synechococcus 7002 and Synechocystis
6803 GlbNs are favourable because the haem group becomes covalently
attached to the protein matrix. A simple procedure relying on the peroxidase
activity of the haem group in the unfolded protein and coupled with chemi-
luminescence detection ( Dorward, 1993 ) would provide a rapid assessment.
In other cases, with small amounts of purified native globins, the most
productive analyses would involve delicate measurements such as chroma-
tography or mass spectrometry or both. These techniques can help us under-
stand the presence of cofactors within the purified proteins and potentially
identify any post-translational modifications that have occurred.
Such experiments may actually be fairly uneventful. The most probable
result would be finding haem bound to protein that is consistent with the
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