The structure of luciferase is heterodimeric; it consists of 2 subunits, alpha and beta.  Press here to check out the spacefilling model that shows how tightly the two individual components interact with each other noncovalently.   (6)  Note: the alpha subunit is shown in blue and the beta is depicted in red.      It has been hypothesized that the luciferase subunits work via an unknown synergetic mechanism; experiments have shown that the essential subunit for function is the alpha unit, since the active site is thought to be almost completely located on the alpha subunit; however, the beta unit is necessary for efficiency and quantum yield. (7)     The alpha and beta subunits are very similar in residue size, with the former polypeptide being composed of 355 amino acids (~40 kDa) and the latter being slightly smaller with 324 amino acids (~36 kDa). (8)  This cartoon model shows the similarity in size fairly well so push the little button, sit back and have a good chime.   Note: Same color scheme as above.      The tertiary structure of each subunit is folded in a (beta/alpha)8 motif, also known as the TIM barrel motif.  Furthermore, each subunit contributes to a common parallel helical bundle with their own alpha2 and alpha3 helices which then serves to relate the two subunits together.  The bundle is centered around a pseudo 2-fold axis. (9)  These motifs can be seen clearly in this model with the tertiary structure colored according to the specific structural component.       The location of the active site of luciferase is not known with 100% assurance; however, the research that has been performed seems to describe the plausible position with a good deal of supporting evidence.      As I stated before, the active site is believed to be located on the alpha subunit of the enzyme.  As is true with all other (beta/alpha)8 barrel enzymes, the active site is located at the terminal COOH end. (10)  Certain residues have been studied and have provided us with some insight into where the active site can be found.      First of all there is evidence that tryptophans   194 and 250 on the alpha subunit are important in the enzymatic activity of luciferase.  They are believed to interact with the isoallozazine ring of flavin, and thereby be just two of the residues that make of the active site. (11)  Here they are.       Cys 106 is also linked to the binding of flavin; a modification of this residue results in the diminished binding of flavin to the enzyme.  See it here with your very own eyes.   (12)     Related to this, is residue Asp 113, which also reduces the affinity for reduced flavin when it is manipulated.   (13)    Two more amino acid residues of interest is the His 45 residue of the alpha subunit and the Glu 88 residue of the beta subunit.  Not only do they form a hydrogen bond between subunits that is highly conserved in luciferase enzymes, but a mutation of the His 45 residue causes a significant decrease of enzymatic activity. (14) Press button...not yet...not yet...okay now...     The mutating of Ser 227 to an amino acid with a larger side chain has been observed to reduce aldehyde binding affinity without having any effect on flavin binding. (15)  This fact may predict the location for the long-chain aldehyde binding site.      Finally, here's a look at the combined locations of these substantial amino acids that appear to make up the active site of luciferase.

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