22 Phycobiliproteins as Food Additives
563
with an increase in temperature stability (Samsonoff and MacColl 2001; Pumas et al.
2011).
High molecular weight aggregates have been found in thermophilic cyanobacteria.
The thermophilic cyanobacterium Chroococcidiopsis sp. strain TS-821 isolated from
hot spring in Thailand grows at 50 °C, over a wide pH range. Its phycocyanin has a
molecular weight of 600 kDa, suggesting that it is added in the form of three-stacked
hexamer in a similar way phycoerythrin from Leptolyngbya sp. KC45 isolated from
hot spring in its native form is a 235 kDa hexamer (Hayashi et al. 1997; Pumas et al.
2011).
Thermophilic monomers are more stable than those of mesophiles. In the thermophilic cyanobacterium Synechococcus lividus the monomers are denaturated at
60–70 °C. The instability of the monomers at physiological temperatures (66–73 °C)
suggests that they should be added quickly to trimers to avoid denaturation (Edwards
et al. 1997).
Additionally, to the formation of high molecular weight aggregates, the stability of
phycobiliproteins of thermophiles has been linked to the presence of specific amino
acids at key points in the structure of the protein. In the phycocyanins of a thermophilic cyanobacterium (Synechococcus lividus) and another mesophilic cyanobacterium (Phormidium luridum) that grow at 52 °C and 25 °C, respectively, it was
observed that there are minimal differences in the amino acid composition. Phycocyanin has apolar amino acids which are important due to the hydrophobic interaction between their side chains and which could be responsible for their temperature
stability, rather than the hydrogen bonding interactions. Similarly in the PC of Synechococcus PCC6715 has been detected the substitution of conserved amino acids
that allows the stabilization of interactions between protein subunits when compared
to mesophilic proteins (Berks et al. 1963; Chen et al. 1994; Liang et al. 2018).
Likewise, the stability to the temperature and pH of C-phycocyanin of T. elongatus TA-1, a thermophilic cyanobacterium, may be due to the substitution of certain
amino acid residues at specific positions that allow ionic interactions in the phycobiliproteins. The difference in the evolutionarily conserved sequence of amino acids
found in the C-phycocyanin of thermophilic and mesophilic cyanobacteria lies in the
alpha chain, and a pattern of amino acid substitution in the beta chain has not been
found (Leu et al. 2013; Pumas et al. 2011; Liang et al. 2018).
The C-phycocyanin of Synechococcus PCC6715 has a sequence in amino acids
in the alpha chain with identity greater than 97% with respect to the alpha chain
of T. vulcanus and T. elongatus. The comparison in the amino acid sequence of
C-phycocyanins of highly thermostable cyanobacteria (Synechococcus PCC6715
and Thermosynechococcus sp.) and mild-thermostable (Cyanidium caldarium and
Galdiera sulphuraria) and mesophilic (Synechococcus elongatus PCC 7942, Spirulina, Leptolyngbya sp. N62DM) showed that there are differences in the sequence
of amino acid residues in the alpha chain, mainly in those amino acids that affect the
formation of salt bridges and hydrogen bridges (Liang et al. 2018).
Liang et al. (2018) showed that the salt bridge that stabilizes the structure in the
vicinity of phycobilin between Asp 28 and Arg33 alpha chain residues, is absent
in mesophilic and mild-thermophilic proteins, where the Asp is replaced by amino
563
with an increase in temperature stability (Samsonoff and MacColl 2001; Pumas et al.
2011).
High molecular weight aggregates have been found in thermophilic cyanobacteria.
The thermophilic cyanobacterium Chroococcidiopsis sp. strain TS-821 isolated from
hot spring in Thailand grows at 50 °C, over a wide pH range. Its phycocyanin has a
molecular weight of 600 kDa, suggesting that it is added in the form of three-stacked
hexamer in a similar way phycoerythrin from Leptolyngbya sp. KC45 isolated from
hot spring in its native form is a 235 kDa hexamer (Hayashi et al. 1997; Pumas et al.
2011).
Thermophilic monomers are more stable than those of mesophiles. In the thermophilic cyanobacterium Synechococcus lividus the monomers are denaturated at
60–70 °C. The instability of the monomers at physiological temperatures (66–73 °C)
suggests that they should be added quickly to trimers to avoid denaturation (Edwards
et al. 1997).
Additionally, to the formation of high molecular weight aggregates, the stability of
phycobiliproteins of thermophiles has been linked to the presence of specific amino
acids at key points in the structure of the protein. In the phycocyanins of a thermophilic cyanobacterium (Synechococcus lividus) and another mesophilic cyanobacterium (Phormidium luridum) that grow at 52 °C and 25 °C, respectively, it was
observed that there are minimal differences in the amino acid composition. Phycocyanin has apolar amino acids which are important due to the hydrophobic interaction between their side chains and which could be responsible for their temperature
stability, rather than the hydrogen bonding interactions. Similarly in the PC of Synechococcus PCC6715 has been detected the substitution of conserved amino acids
that allows the stabilization of interactions between protein subunits when compared
to mesophilic proteins (Berks et al. 1963; Chen et al. 1994; Liang et al. 2018).
Likewise, the stability to the temperature and pH of C-phycocyanin of T. elongatus TA-1, a thermophilic cyanobacterium, may be due to the substitution of certain
amino acid residues at specific positions that allow ionic interactions in the phycobiliproteins. The difference in the evolutionarily conserved sequence of amino acids
found in the C-phycocyanin of thermophilic and mesophilic cyanobacteria lies in the
alpha chain, and a pattern of amino acid substitution in the beta chain has not been
found (Leu et al. 2013; Pumas et al. 2011; Liang et al. 2018).
The C-phycocyanin of Synechococcus PCC6715 has a sequence in amino acids
in the alpha chain with identity greater than 97% with respect to the alpha chain
of T. vulcanus and T. elongatus. The comparison in the amino acid sequence of
C-phycocyanins of highly thermostable cyanobacteria (Synechococcus PCC6715
and Thermosynechococcus sp.) and mild-thermostable (Cyanidium caldarium and
Galdiera sulphuraria) and mesophilic (Synechococcus elongatus PCC 7942, Spirulina, Leptolyngbya sp. N62DM) showed that there are differences in the sequence
of amino acid residues in the alpha chain, mainly in those amino acids that affect the
formation of salt bridges and hydrogen bridges (Liang et al. 2018).
Liang et al. (2018) showed that the salt bridge that stabilizes the structure in the
vicinity of phycobilin between Asp 28 and Arg33 alpha chain residues, is absent
in mesophilic and mild-thermophilic proteins, where the Asp is replaced by amino