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Polyhydroxybutyrate (PHB) Synthases (PhaC): Toward understanding elongation granule formation and chain termination.
Author(s) -
Stubbe JoAnne,
Tian J.,
Lawrence A.,
Sinskey A. J.
Publication year - 2006
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.20.5.a888-a
Subject(s) - polyhydroxybutyrate , atp synthase , chemistry , biochemistry , granule (geology) , biophysics , stereochemistry , enzyme , biology , genetics , bacteria , paleontology
Polyhydroxybutyrate synthases catalyze the conversion of 3‐hydroxybutyryl‐CoA (HB‐CoA) to polyhydydroxybutyrate (PHB) in many procaryotes, when they find themselves in a nutrient limited environment and have available a carbon source that can be converted into HB‐CoA. Studies at [ 14 C]‐HB‐CoA to synthase ratios of 70,000:1 and 5:1 and analysis of products via SDS PAGE using autoradiography and antibodies to the synthase and PHB, have lead to a new model for priming and termination. In this model a nucleophile on the enzymes surface senses polymer size, perhaps via interaction with the phasin protein PhaP, and attacks the C‐terminus of the polymer at a residue 10 to 20 removed from the covalently bound active site cysteine. Transesterificaion then moves the polymer chain to a position on the synthase's surface where it can perhaps be hydrolyzed by a depolymerase. This process leaves the synthase with a 10‐20mer (HB) attached to the active site cysteine ready to generate additional chains. Three different mechanisms for granule formation have been considered: the micelle model in which the growing HB chains extending from the PhaC's surface form micelles, the budding model in which the PhaC binds to the inner plasma membrane and eventually with the help of PhaP, buds from the membrane to form a granule and the scaffold model. In the latter model as yet unidentified proteins would be essential for granule nucleation. Electron microscopy and fluorescence microscopy are being examined in an effort to distinguish between these models.