Role of Histidine 310 in Amydetes vivianii firefly luciferase pH and metal sensitivities and improvement of its color tuning properties.

Firefly luciferases emit yellow-green light and are pH-sensitive, changing the bioluminescence color to red in the presence of heavy metals, acidic pH and high temperatures. These pH and metal-sensitivities have been recently harnessed for intracellular pH indication and toxic metal biosensing. However, whereas the structure of the pH sensor and the metal binding site, which consists mainly of two salt bridges that close the active site (E311/R337 and H310/E354), has been identified, the specific role of residue H310 in pH and metal sensing is still under debate. The Amydetes vivianii firefly luciferase has one of the lowest pH sensitivities among the group of pH-sensitive firefly luciferases, displaying high bioluminescent activity and special spectral selectivity for cadmium and mercury, which makes it a promising analytical reagent. Using site-directed mutagenesis, we have investigated in detail the role of residue H310 on pH and metal sensitivity in this luciferase. Negatively charged residues at position 310 increase the pH sensitivity and metal sensitivity; H310G considerably increases the size of the cavity, severely impacting the activity, H310R closes the cavity, and H310F considerably decreases both pH and metal sensitivities. However, no substitution completely abolished pH and metal sensitivities. The results indicate that the presence of negatively charged and basic side chains at position 310 is important for pH sensitivity and metals coordination, but not essential, indicating that the remaining side chains of E311 and E354 may still coordinate some metals in this site. Furthermore, a metal binding site search predicted that H310 mutations decrease the affinity mainly for Zn, Ni and Hg but less for Cd, and revealed the possible existence of additional binding sites for Zn, Ni and Hg.

The orange light emitting luciferase from the rare Euryopa clarindae adult railroadworm (Coleoptera:Phengodidae): structural/functional and evolutionary relationship with green and red emitting luciferases.

Railroadworms luciferases emit the widest range of bioluminescence colors among beetles, ranging from green to red, being model enzymes to investigate the structure and bioluminescence colors relationships. Only three active railroadworms luciferases from the larval stage have been cloned and investigated: the Phrixothrix hirtus head lanterns red-emitting luciferase (PhRE); the Phrixothrix vivianii lateral lanterns green emitting luciferases (PvGR) and the Phengodes sp. dorsal lanterns yellow-green emitting luciferase (Ph). No active luciferase emitting in the yellow-orange region, however, has been cloned yet. Here we report the cloning and characterization of the orange emitting luciferase from the adult males of a rare Brazilian Cerrado railroadworm, Euryopa clarindae, and the transcriptional identification of two isozymes from the Amazon forest Mastinomorphus sp. railroadworm. The luciferase of E. clarindae has 548 residues, emits orange bioluminescence (600 nm), and displays intermediate kinetic values [KM(luciferin) = 50 µM, KM(ATP) ~ 170 µM] between those reported for green-emitting lateral lanterns and red emitting head lanterns luciferases. It displays 74-78% identity with the lateral lanterns luciferases of other railroadworms and 70% with the head lantern PhRE luciferase, and 96% with the larval Mastinomorphus sp. Mast-1, suggesting that this larva could be from the Euryopa genus. The phylogenetic analysis and kinetic/functional properties, place this orange-emitting enzyme as an intermediate form between the green-emitting lateral lanterns and red-emitting head lanterns luciferases. Major structural differences that could be associated with bioluminescence color determination are a relatively larger cavity size, and substitutions in the loops 223-235 and 311-316, especially N/C/T311, and their interactions which may help to close the bottom of LBS.