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June’s artwork represents the structure of the key enzyme in bioluminescence and brings light into our 2020 calendar.

Maybe for many, fireflies might remind us of a carefree childhood; warm summer nights and some magical flashes of light appearing in our backyards. Today, we know that adult fireflies emit light to attract a mate, using special organs located under their abdomens to send light signals. Males and females use species‐specific light signals as an interactive visual morse‐code that identifies the species and the sex of the signaller. They have developed a variety of ways of using the light to display, some have a steady glow and other species flash. Flashing signals differ in duration, timing, repetition and colour between species. The light may be yellow, green, or pale red, with wavelengths from 510 to 670 nm.

The emitted light is the product of a chemical reaction known as bioluminescence. This 2-step reaction involves luciferin (a small molecule) and luciferase (an enzyme which catalyses the oxidation of luciferin to oxyluciferin). It also requires the presence of ATP, oxygen and a metallic cation. Oxyluciferin is formed in an excited state and as it relaxes to the ground state emits a photon of light.

Two-step reaction: Starts with reaction of luciferin with ATP in the presence of luciferase and produces an enzyme-bound intermediate (luciferyl adenylate) (1). In the second step (2) the luciferyl adenylate is oxidized by molecular oxygen to form an excited state of oxyluciferin. The reaction emits light when oxyluciferin returns to the ground state.

Firefly luciferase is folded into 2 compact domains. The larger N-terminal domain consists of a β-barrel and two β-sheets surrounded by α-helices to form an αβαβα five-layered tertiary structure. The smaller C-terminal domain folds into α+β structure and is connected to the N-terminal domain by a flexible linker peptide creating a wide cleft between the two domains in which the active site is located. Studies showed that the conserved residues are located on the surface of the two domains facing each other across the cleft. But the cleft is far too big to surround the substrate and to allow the simultaneous interaction of the conserved surfaces. It seems more likely that the two domains come together during the bioluminescence reaction and enclose the substrate. This provides a suitable environment for the production of the light. Moreover, hydrophobic properties of active site allow the water molecules to be excluded from the active site to avoid intermolecular quenching of the excited state product.

The structure of firefly luciferase (PDB ID 2d1q) depicting the large N-terminal domain, the small C-terminal domain and the position of the active site

Firefly luciferase is not the only type of the luciferase that exists. The majority of studied luciferases have been found in animals, especially marine animals (jellyfish or sea pansy - PDB ID 2pse) but also in fungi, bacteria (PDB ID 1luc) and dinoflagellates (PDB ID 1vpr). Interestingly, the luciferase of different organisms varies in amino acid sequence and protein fold, leading to a suggestion that luciferase functionality has evolved several different times separately.

Because of its unique properties, luciferase has been used extensively in molecular and cell biology for visualisation of gene expression, post-translational modification, and protein-protein interactions. The applications of bioluminescence imaging have grown in the past decade and there is still a huge potential in using bioluminescence in biomedical research. Luciferase can still shed light on many biological processes.

About the artwork

The artwork was created by Elizabeth Brown, a 16 year old student from in Cambridge. An inverted pen drawing with chalk and acrylic paint, it represents the structure of luciferase and the conversion of chemical energy in the oxidation reaction into light.