On the origin and variation of colors in lobster carapace.

The chemical basis of the blue-black to pink-orange color change on cooking of lobster, due to thermal denaturation of an astaxanthin-protein complex, α-crustacyanin, in the lobster carapace, has so far been elusive. Here, we investigate the relaxation of the astaxanthin pigment from its bound enolate form to its neutral hydroxyketone form, as origin of the spectral shift, by analyzing the response of UV-vis spectra of a water-soluble 3-hydroxy-4-oxo-ß-ionone model of astaxanthin to increases in pH, and by performing extensive quantum chemical calculations over a wide range of chemical conditions. The enolization of astaxanthin is consistent with the X-ray diffraction data of ß-crustacyanin (PDB code: ) whose crystals possess the distinct blue color. We find that enolate formation is possible within the protein environment and associated with a large bathochromic shift, thus offering a cogent explanation for the blue-black color and the response to thermal denaturation and revealing the chemistry of astaxanthin upon complex formation.

Time-resolved and static-ensemble structural chemistry of hydroxymethylbilane synthase.

The enzyme hydroxymethylbilane synthase (HMBS, EC 4.3.1.8), 313 amino acid residues and MW 34 kDa, also known as porphobilinogen deaminase (PBGD), catalyses the stepwise polymerization of four molecules of porphobilinogen (PBG) to the linear tetrapyrrole 1-hydroxymethylbilane. Several crystallographic structures of HMBS have been previously determined, most recently including by time-resolved Laue protein crystallography of the Lys59Gln mutant form with reaction initiation undertaken by use of a flow cell carrying the substrate PBG. In this paper we review these structures and add new molecular graphics representations and analyses. Moreover we present a new structure refined at 1.66 A resolution using diffraction data recorded at cryo-temperature (100 K) in an attempt at trapping the polypeptide loop (residues 47 to 58) in the vicinity of the enzyme active site, missing in all previous structure determinations. This loop still has not appeared in the electron density maps, in spite of the advantage of cryo-temperature, but nevertheless the 1.66 A cryo-structure extends the ensemble of known HMBS structures. The cryomodel of protein, cofactor and 320 bound water molecules has been refined to a final R-factor and R-free of 0.198 and 0.247 respectively; the PDB deposition codes, coordinates and structure factors are 1GTK and R1GTKSF respectively. Finally a protein comparison study is presented of the Mycobacterium tuberculosis (MTb) HMBS, with the E. coli HMBS. This has been done as preparation for future structural studies on the MTb HMBS from this important disease bearing organism. The overall amino acid sequence identity is 41%. Most interestingly there is a two-residue reduction in length of the loop referred to above (Asp 50 and Gly 58 being missing in the MTb form). This gives the hope that this loop will be less flexible and thus might become visible to crystallographic analysis.