246 



LIGHT AND LIFE 



This 100 per cent efficiency for light emission is indicative of the 

 high degree of protection afforded to the activated substrate molecule 

 by the enzyme. That the tight binding of the LHo-AMP or L-AMP 

 to the enzyme is an important factor in determining the quantum 

 yield is also suggested from temperature measurements. The quantum 

 yield has been determined at 3°C and at 27 °C, with no observed 

 change in efficiency over this temperature range. It is considered 

 significant that the fluorescence of luciferase (peak at 350 ra^i) over- 

 laps the broad absorption band of luciferin and oxyluciferin (peak 

 at approximately 395 m/i.) at neutral or alkaline pH where the quan- 

 tum yield is unity. It may be possible that the enzyme-substrate com- 

 plex forms a resonant molecule which is able to transfer energy 

 via tryptophan until it is trapped at an emitting site, in analogy 



Q 



PH 

 Fig. 26. Quantum yield (Q) of fiicny hioluinincstcnrc as a fiuiction of pH. 



