“Marley's face”, wrote Charles Dickens in his novella A Christmas Carol, “... had a dismal light about it, like a bad lobster in a dark cellar.”

Before the days of electric lighting, light-emitting seafood was probably much more frequently observed than it is today. In fact one of the founders of modern chemistry, Robert Boyle, studied it in a 1668 paper “Experiments concerning the relation between light and air in shining wood and fish”

But what might have caused Dickens’ decaying decapod to have a dismal light?

Several marine bacteria which live on the surface of other sea creatures are luminescent, but only glow when there is a high population density, probably the case on a bad lobster! They turn on the genes involved through a mechanism called quorum sensing. It’s likely that Dickens, and doubtless his readers, had seen fish and lobsters colonised by these bacteria and emitting their characteristic blue-green glow.

The light is produced by an enzyme called luciferase. Several different organisms produce light via luciferases, but they work in different ways. Bacterial luciferase takes a molecule of reduced flavin mononucleotide (FMNH₂) and a long chain aldehyde, together with oxygen, and catalyses the following reaction, giving off light in the process:

A long-chain aldehyde + FMNH₂ + O₂ = a long-chain fatty acid + FMN + H₂O + light.

The structure of bacterial luciferase was solved in 1996 (pdbe.org/1brl, pdbe.org/1luc) showing that it’s built from two very similar proteins, both containing a very well known protein fold for enzymes. This fold was first seen in the enzyme triosephosphate isomerase (TIM) and contains a barrel shaped central motif, so it was named the TIM barrel. As the central barrel motif of beta strands is only about 1.5 nm across, Dickens might call it a Tiny Tim Barrel!

Bacterial luciferase. Top: The structure of the luciferase enzyme,  Protein subunits are shown as ribbons and the FMN is shown as sticks. Bottom: The alpha and beta subunits superimposed. In both figures, the alpha (catalytic) subunit shown in green ribbon and the beta (non-catalytic) subunit shown in grey ribbon.  

While the two proteins that form the luciferase are very similar, (see fig above) only one of them, the alpha subunit, contains the active site of the light emitting reaction. However, the reaction requires both the alpha and beta subunits to be present. A further structure solved in 2009 (pdbe.org/3fgc) had one of the products, FMN, bound. It showed exactly where in the alpha subunit the ‘business’ end of the enzyme is located. It also revealed the subtle function of the beta subunit, which stabilises the active site of the alpha subunit.

Why bacteria, when they get together in large numbers, should expend energy to produce light isn’t fully understood, however we know of some examples of it being useful. Many species of bacteria are symbiotic with animals such as squid or angler fish which make use of the light producing bacteria to attract a meal, or to disguise themselves. In return for producing light, the bacteria are provided with a home and nutrients.

We at PDBe hope that only your fairy lights glow in the dark during this festive season… and not your dinner!