The Luciferases

There are three distinct classes of luciferases which bring about bioluminescence in fireflies, in certain aquatic species (both dioxygenases), and in bacteria ( mono-oxygenase) 140, 141). 

The enzymes shows no requirement for metal ions, and there are indications that the active site is extremely hydrophobic 142). In fact, the luciferyl adenylate (58) undergoes rapid, spontaneous oxygenation in organic solvents to give a hydroperoxide (60) which eventually decomposes decarboxylatively to the oxyluciferin (57), accompanied by luminescence 142). There is no need for oxygen activation and the enzyme 

A detailed mechanism has recently been advanced to explain how decarboxylation of (61) efficiently generates singlet-excited oxyluciferin (57) 146, 147). 

Just as for the firefly luciferyl derivative (58), these luciferins e.g. 62) undergo rapid oxygenation even in non-aqueous solvents decomposing thereafter with chemiluminescent decarboxylation 156, 157). Again, there is no evidence for the involvement of metal ion co-factors. This 

Similar behavior is observed with some of the flavoprotein monooxygenases, which also do not use a metal co-factor. The reduced flavin (68) has a structure which resembles that of luciferin (62) and reacts readily with molecular oxygen, through the intermediacy of its carbanion which forms charge-transfer intermediates leading to the hydroperoxide ion rather than to superoxide radical ion and pyrazyl radicals (158). Although the precise point of attachment of oxygen is controversial, the principle remains the same, namely the formation of a non-delocalized carbanion (69 or 70) (159—161). 

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Genes lux) encoding luciferase and the other proteias iavolved ia the bioluminescent reaction have been cloned. PuxA. and luxP genes code for the luciferase subunits, and the fatty acid reductase complex is coded by the luxCDE genes. 

Fig. 1.5 Fluorescence emission spectrum of the luciferase-oxyluciferin complex in the same solution as in Fig. 1.4 (solid line), compared with the luminescence spectrum of firefly luciferin measured in glycylglycine buffer, pH 7.6 (dotted line). The former curve from Gates and DeLuca, 1975 the latter from Selinger and McElroy, 1960, both with permission from Elsevier.

Orlova et al. (2003) theoretically studied the mechanism of the firefly bioluminescence reaction on the basis of the hybrid density functional theory. According to their conclusion, changes in the color of light emission by rotating the two rings on the 2-2 axis is unlikely, whereas the participation of the enol-forms of oxyluciferin in bioluminescence is plausible but not essential to explain the multicolor emission. They predicted that the color of the bioluminescence depends on the polarization of the oxyluciferin molecule (at its OH and O-termini) in the microenvironment of the luciferase active site the... 

The bioluminescence systems of Phengodidae (railroad worms) and Elateroidae (click beetles) are basically identical to that of Lampyridae (fireflies), requiring firefly luciferin, ATP, Mg2+ and a luciferase for light emission. However, there seem to be some differences. Viviani and Bechara (1995) reported that the spectra of the luminescence reactions measured with the luciferases of Brazilian fireflies (6 species) shift from the yellow-green range to the red range with lowering of the pH of the medium, like in the case of the Photinus pyralis luciferase (see Section 1.1.5), whereas the spectra... 

Extraction and purification of luciferin and luciferase (Viviani etal., 2002a) To isolate luciferin, the lanterns of the Australian A. flava were homogenized in hot 0.1 M citrate buffer, pH 5, and the mixture was heated to 95°C for 5 min. The mixture was acidified to pH 2.5-3.0 with HCl, and luciferin was extracted with ethyl acetate. Upon thin-layer chromatography (ethanol-ethyl acetate-water, 5 3 2 or 3 5 2), the active fraction of luciferin was fluorescent in purple (emission Lav 415 nm when excited at 290 nm). To isolate the luciferase, the cold-water extract prepared according to Wood (1993 see above) was chromatographed on a column of Sephacryl S-300. On the same... 

The biochemical mechanism of bacterial luminescence has been studied in detail and reviewed by several authors (Hastings and Nealson, 1977 Ziegler and Baldwin, 1981 Lee et al., 1991 Baldwin and Ziegler, 1992 Tu and Mager, 1995). Bacterial luciferase catalyzes the oxidation of a long-chain aldehyde and FMNH2 with molecular oxygen, thus the enzyme can be viewed as a mixed function oxidase. The main steps of the luciferase-catalyzed luminescence are shown in Fig. 2.1. Many details of this scheme have been experimentally confirmed. 

If the luciferase sample solution contains a flavin-reductase, luciferase activity can be measured by the addition of FMN and NADH, instead of FMNH2. In this case, the turnover of luciferase takes place repeatedly using the FMNH2 that is enzymatically generated thus, the luminescence reaction continues until aldehyde or NADH is exhausted. A crude luciferase extracted from luminous bacteria usually contains a flavin-reductase. 

Molecular weight. The molecular weight of C. hilgendorfii luciferase reported in the past varies considerably across a range of 50,000-80,000 (Chase and Langridge, 1960 Shimomura etal., 1961, 1969 Tsuji and Sowinski, 1961 Tsuji et al., 1974) it appears most likely to be 60,000-70,000. The luciferase is an acidic protein with an isoelectric point of 4.35 (Shimomura et al., 1961). The absorption spectrum of luciferase is that of a simple protein without any prosthetic group, showing a peak at 280 nm. Absorbance value at 280 nm of a 0.1% luciferase solution is approximately 0.96 (Shimomura etal., 1969). 

Luciferase turnover. The luciferase-catalyzed light-emitting reaction that forms oxyluciferin is fast, but the hydrolysis reaction of oxyluciferin into etioluciferin by luciferase is slow. The turnover rate (catalytic center activity) of luciferase was reported to be about 30/s for the luminescence reaction, and 0.03/s for the hydrolysis of oxyluciferin (Shimomura et al., 1969). 

Fig. 3.1.8 A diagram showing the reactions involved in the luciferase-catalyzed luminescence of Cypridina luciferin.

Johnson et al. (1962) measured the quantum yield of Cypridina luciferin in the luciferase-catalyzed reaction for the first time, using a photomultiplier calibrated with two kinds of standard lamps. The measurement gave a value of 0.28 0.04 at 4°C in 50 mM sodium phosphate buffer, pH 6.5, containing 0.3 M NaCl. The quantum yield... 

Dried shrimp was ground, defatted with benzene, and then extracted with cold water. The luciferase extracted was purified first by a batch adsorption onto DEAE cellulose (elution with 0.4 M NaCl), followed by gel filtration on a column of Sephadex G-150, anion-exchange chromatography on a column of DEAE-cellulose (gradient elution 0.05-0.5 M NaCl), and gel filtration on a column of Ultrogel AcA 34. The specific activity of the purified luciferase was 1.7 x 1015 photons s 1 mg-1, and the yield in terms of luciferase activity was about 28%. 

The bioluminescence reaction of Oplophorus is a typical luciferin-luciferase reaction that requires only three components luciferin (coelenterazine), luciferase and molecular oxygen. The luminescence spectrum shows a peak at about 454nm (Fig. 3.3.1). The luminescence is significantly affected by pH, salt concentration, and temperature. A certain level of ionic strength (salt) is necessary for the activity of the luciferase. In the case of NaCl, at least 0.05-0.1 M of the salt is needed for a moderate rate of light emission, and about 0.5 M for the maximum light intensity. 

Fig. 3.3.1 Luminescence spectrum of coelenterazine catalyzed by the luciferase of the decapod Oplophorus in 15 mM Tris-HCl, pH 8.3, containing 50 mM NaCl (solid line). For comparison, the luminescence catalyzed by the luciferase of the anthozoan sea pansy Renilla is shown with dashed line (in 25 mM Tris-HCl, pH 7.5, containing 0.1 M NaCl).

The product coelenteramide is not noticeably fluorescent in aqueous solutions, but is highly fluorescent in organic solvents and also when the compound is in the hydrophobic environment of a protein. When coelenterazine is luminesced in the presence of Oplophorus luciferase, the solution after luminescence (the spent solution) is not fluorescent, presumably due to the dissociation of coelenteramide from the luciferase that provided a hydrophobic environment at the time of light emission. An analogous situation exists in the bioluminescence system of Renilla (Hori et al., 1973). 

Campbell and Herring (1990) examined eight species of copepods, and found that all of them contain coelenterazine (the luciferin) and a luciferase. In the Euaugaptilus species, over 90% of the luciferase was found in their legs, and over 40% of the total coelenterazine was found in the bodies. 

According to Markova et al. (2004), the cDNA encoding the luciferase of Metridia longa was cloned and sequenced. The luciferase is a 219-amino acid protein with a molecular weight of 23,885. 

The particulate matter can be solubilized with 2-mercaptoethanol, giving a mixture of luciferase oligomers with molecular masses in multiples of approximately 20kDa. The luciferase activity is increased several times by the solubilization. The purification of the solubilized... 

Properties of the luciferases. According to Shimomura and Flood (1998) and Shimomura et al. (2001), all Periphylla luciferases L, A, B and C catalyze the oxidation of coelenterazine, resulting in the emission of blue light (Amax 465 nm). Luciferases B (40 kDa) and C (80 kDa) are apparently the dimer and tetramer, respectively, of luciferase A (20 kDa). The presence of a salt is essential for the activity of luciferase, and the optimum salt concentration is about 1M in the case of NaCl for all forms of luciferases. The luminescence intensity of luciferase L is maximum near 0°C, and decreases almost linearly with rising temperature, falling to zero intensity at 60°C the luminescence intensity profiles of luciferases A, B and C show their peaks at about 30°C (Fig. 4.5.3). The Michaelis constants estimated for luciferases A, B and C with coelenterazine are all about 0.2 xM, and that for luciferase L is 1.2 jiM. 

Fig. 4.5.5 Effect of pH on the luminescence of coelenterazine catalyzed by Periphylla luciferases A, B and C, and on the stability of the luciferases. The effect on light intensity (solid lines) was measured in 3 ml of 50 mM phosphate buffers, pH 4.1-7.25, and 50 mM Tris-HCl buffers, pH 7.1-9.7, all containing 1 M NaCl, 0.025% BSA, and 0.3 pM coelenterazine. To measure the stability (dotted lines), a luciferase sample (5 pi) was left standing for 30 min at room temperature in 0.1 ml of a buffer solution containing 1 M NaCl and 0.025% BSA and having a pH to be tested, and then luciferase activity in 10 pi of the solution was measured in 3 ml of 20 mM Tris-HCl, pH 7.8, containing 1M NaCl, 0.05% BSA, and 0.3 pM coelenterazine at 24°C. The amounts of luciferases used for measuring each point were luciferase A, 150 LU luciferases B and C, 170 LU. One LU = 5.5 x 108 quanta/s. From Shimomura etal., 2001.

Renilla luciferase. The luciferase of Renilla reniformis has been purified and characterized by Karkhanis and Cormier (1971) and Matthews et al. (1977a). The purified luciferase has a molecular weight of 35,000, and catalyzes the luminescence reaction of coelenterazine. The luciferase-catalyzed luminescence is optimum at pH 7.4, at a temperature of 32°C, and in the presence of 0.5 M salt (such as NaCl or KC1). The luciferase has a specific activity of 1.8 x 1015 photons s"1mg"1, and a turnover number of 111/min. The luminescence spectrum shows a maximum at 480 nm. The absorbance A28O of a 0.1% luciferase solution is 2.1. The luciferase has a tendency to self-aggregate, forming higher molecular weight species of lower luminescence activities. 

The cDNA encoding the luciferase of Renilla reniformis has been obtained and expressed in Escherichia coli (Lorenz et al., 1991). The cDNA contained an open reading frame encoding a 314-amino acid sequence. The recombinant Renilla luciferase obtained had a molecular weight of 34,000, and showed an emission maximum at 480 nm in the luminescence reaction of coelenterazine, in good agreement with the data of natural Renilla luciferase. 

All of the luciferases cause the emission of a bluish light when they catalyze the oxidation of coelenterazine. However, there are some marked differences between the decapod shrimp luciferases and the cnidarian luciferases (Matthews et al., 1977a,b). For example, the luminescence caused by the former (Amax about 452 nm) is bluer than that caused by the latter (7max 470-480 nm), and the optimum pH of the former, about 8.5, is significantly higher than that of the latter (Renilla, 7.4 Ptilosarcus, 7.0). The optimum temperature of the decapod shrimp luciferases (35°C) is higher than those of Ptilosarcus (23°C) and Renilla (32°C). 

Fig. 5.8 Influence of pH, temperature, NaCl concentration, and the concentration of coelenterazine on the light intensity of luminescence reaction catalyzed by the luciferases of Heterocarpus sibogae, Heterocarpus ensifer, Oplophorus gracilirostris, and Ptilosarcus gruneyi. Buffer solutions used 20 mM MOPS, pH 7.0, for Ptilosarcus luciferase and 20 mM Tris-HCl, pH 8.5, for all other luciferases, all with 0.2 M NaCl, 0.05% BSA, and 0.3 p,M coelenterazine, at 23°C, with appropriate modifications in each panel. Various pH values are set by acetate, MES, HEPES, TAPS, CHES, and CAPS buffers.

Inouye and Shimomura, 1997). With Ptilosarcus luciferase, the luminescence intensity of e-coelenterazine is also significantly higher than that of coelenterazine. With other coelenterazine luciferases, however, the luminescence intensity of e-coelenterazine is generally lower than that of coelenterazine for example, the luminescence intensities of e-coeienterazine measured with the luciferases of the decapod shrimps, the jellyfish Periphylla, and the copepod Pleuromamma, were 50%, 4%, and 0.8%, respectively, in comparison with that of coelenterazine. Thus, the luminescence of coelenterazine catalyzed by Pleuromamma luciferase is suppressed by the addition of e-coelenterazine. 

The New Zealand freshwater limpet Latia neritoides (Fig. 6.1.1) is the only known example of a freshwater luminous organism, with two possible exceptions certain species of luminous bacteria and the larvae of certain species of fireflies. The limpet inhabits shallow clear streams in the North Island of New Zealand, clinging to stones and rocks. Latia has a small oval-shaped shell (6-8 mm long), and secretes a luminous mucus that emits a greenish glow around the body only when disturbed the limpet does not show a spontaneous luminescence. The luminescence of Latia was first reported by Suter (1890) and further details including a positive luciferin-luciferase reaction were described by Bowden (1950). Both the luciferin and the luciferase have... 

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