564
Alexandra Galetovi´ c C. and L. Dufossé
acids with different polarity characteristics as Phe an aromatic amino acid and Asn
that is a polar noncharged amino acid to neutral pH, respectively.
In relation to hydrogen bonds, the Arg33-Gln145 pair is present in phycocyanin
sequences of thermophiles but absent in other less stable proteins. Another amino
acid residue related to the stability of phycocyanins is Gln 111 located in the alpha
chain whose position is close to the phycocyanobilin. It is believed that the substitution of this amino acid can affect the thermostability since this type of amino acid
forms hydrogen bonds that stabilize the protein structure. In addition, Synechococcus
PCC6715 cyanobacteria, as in other thermostable phycocyanins, have Ala 43 and Asn
42 in the alpha and beta chains, respectively, which are important for the stability of
the protein because they can form hydrogen bonds (Liang et al. 2018).
The phycobilisomes of cyanobacteria and mesophilic algae lose color (bleaching)
above 60–65 °C, this could occur by the covalent modification of the chromophores
and not by modifications in the apoprotein because the absorption band of the chromophore disappears completely (Zhao and Brand 1989). On the other hand, Murthy
et al. (2004) showed that the temperature above 50 °C, affects the energy transfer
in the phycobilisome modifying the interaction between the PC chromophore of
Spirulina and its apoprotein due to disorganization of the PC structure.
Phycobilisomes are more stable when they are purified in solutions with high
ionic strength. In the alga Anacystis nidulans phycobilisomes purified in solutions
with high concentrations of phosphate tolerated temperatures exceeding 80 °C. This
factor should be considered in the purification processes of phycobiliproteins (Zhao
and Brand 1989).
22.3 Stability of Phycobiliproteins
Phycobiliproteins of cyanobacteria and mesophilic and thermophilic red algae have
a very similar secondary and tertiary protein structure. However, as discussed in the
previous paragraphs, the differences in stability may be due in part to the individual
interaction of some amino acids in apoproteins of these pigments. These interactions
can be affected by pH and temperature (Edwards et al. 1997).
22.3.1 Phycobiliproteins of Thermophilic Cyanobacteria
Studies show that the phycobiliproteins of thermophilic cyanobacteria are more stable
than those of mesophiles, in a wide range of temperature and pH. However, phycocyanin from the thermophilic cyanobacterium Phormidium laminosum does not have
a longer fluorescence lifetime than mesophilic phycocyanin, presumably because
the specific environment of tetrapyrrole more than other factors such as aggregation,
concentration, or thermophilic nature of the microorganism (Seibert and Connolly
1984).
Alexandra Galetovi´ c C. and L. Dufossé
acids with different polarity characteristics as Phe an aromatic amino acid and Asn
that is a polar noncharged amino acid to neutral pH, respectively.
In relation to hydrogen bonds, the Arg33-Gln145 pair is present in phycocyanin
sequences of thermophiles but absent in other less stable proteins. Another amino
acid residue related to the stability of phycocyanins is Gln 111 located in the alpha
chain whose position is close to the phycocyanobilin. It is believed that the substitution of this amino acid can affect the thermostability since this type of amino acid
forms hydrogen bonds that stabilize the protein structure. In addition, Synechococcus
PCC6715 cyanobacteria, as in other thermostable phycocyanins, have Ala 43 and Asn
42 in the alpha and beta chains, respectively, which are important for the stability of
the protein because they can form hydrogen bonds (Liang et al. 2018).
The phycobilisomes of cyanobacteria and mesophilic algae lose color (bleaching)
above 60–65 °C, this could occur by the covalent modification of the chromophores
and not by modifications in the apoprotein because the absorption band of the chromophore disappears completely (Zhao and Brand 1989). On the other hand, Murthy
et al. (2004) showed that the temperature above 50 °C, affects the energy transfer
in the phycobilisome modifying the interaction between the PC chromophore of
Spirulina and its apoprotein due to disorganization of the PC structure.
Phycobilisomes are more stable when they are purified in solutions with high
ionic strength. In the alga Anacystis nidulans phycobilisomes purified in solutions
with high concentrations of phosphate tolerated temperatures exceeding 80 °C. This
factor should be considered in the purification processes of phycobiliproteins (Zhao
and Brand 1989).
22.3 Stability of Phycobiliproteins
Phycobiliproteins of cyanobacteria and mesophilic and thermophilic red algae have
a very similar secondary and tertiary protein structure. However, as discussed in the
previous paragraphs, the differences in stability may be due in part to the individual
interaction of some amino acids in apoproteins of these pigments. These interactions
can be affected by pH and temperature (Edwards et al. 1997).
22.3.1 Phycobiliproteins of Thermophilic Cyanobacteria
Studies show that the phycobiliproteins of thermophilic cyanobacteria are more stable
than those of mesophiles, in a wide range of temperature and pH. However, phycocyanin from the thermophilic cyanobacterium Phormidium laminosum does not have
a longer fluorescence lifetime than mesophilic phycocyanin, presumably because
the specific environment of tetrapyrrole more than other factors such as aggregation,
concentration, or thermophilic nature of the microorganism (Seibert and Connolly
1984).