This review provides an overview of the structure, function, and catalytic mechanism of lacZ β‐galactosidase. The protein played a central role in Jacob and Monod's development of the operon model ...for the regulation of gene expression. Determination of the crystal structure made it possible to understand why deletion of certain residues toward the amino‐terminus not only caused the full enzyme tetramer to dissociate into dimers but also abolished activity. It was also possible to rationalize α‐complementation, in which addition to the inactive dimers of peptides containing the “missing” N‐terminal residues restored catalytic activity. The enzyme is well known to signal its presence by hydrolyzing X‐gal to produce a blue product. That this reaction takes place in crystals of the protein confirms that the X‐ray structure represents an active conformation. Individual tetramers of β‐galactosidase have been measured to catalyze 38,500 ± 900 reactions per minute. Extensive kinetic, biochemical, mutagenic, and crystallographic analyses have made it possible to develop a presumed mechanism of action. Substrate initially binds near the top of the active site but then moves deeper for reaction. The first catalytic step (called galactosylation) is a nucleophilic displacement by Glu537 to form a covalent bond with galactose. This is initiated by proton donation by Glu461. The second displacement (degalactosylation) by water or an acceptor is initiated by proton ion by Glu461. Both of these displacements occur via planar oxocarbenium ion‐like transition states. The acceptor reaction with glucose is important for the formation of allolactose, the natural inducer of the lac operon.
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Lessons from the lysozyme of phage T4 Baase, Walter A.; Liu, Lijun; Tronrud, Dale E. ...
Protein science,
April 2010, Letnik:
19, Številka:
4
Journal Article
Recenzirano
Odprti dostop
An overview is presented of some of the major insights that have come from studies of the structure, stability, and folding of T4 phage lysozyme. A major purpose of this review is to provide the ...reader with a complete tabulation of all of the variants that have been characterized, including melting temperatures, crystallographic data, Protein Data Bank access codes, and references to the original literature. The greatest increase in melting temperature (Tm) for any point mutant is 5.1°C for the mutant Ser 117 → Val. This is achieved in part not only by hydrophobic stabilization but also by eliminating an unusually short hydrogen bond of 2.48 Å that apparently has an unfavorable van der Waals contact. Increases in Tm of more than 3–4°C for point mutants are rare, whereas several different types of destabilizing substitutions decrease Tm by 20°C or thereabouts. The energetic cost of cavity creation and its relation to the hydrophobic effect, derived from early studies of “large‐to‐small” mutants in the core of T4 lysozyme, has recently been strongly supported by related studies of the intrinsic membrane protein bacteriorhodopsin. The L99A cavity in the C‐terminal domain of the protein, which readily binds benzene and many other ligands, has been the subject of extensive study. Crystallographic evidence, together with recent NMR analysis, suggest that these ligands are admitted by a conformational change involving Helix F and its neighbors. A total of 43 nonisomorphous crystal forms of different monomeric lysozyme mutants were obtained plus three more for synthetically‐engineered dimers. Among the 43 space groups, P212121 and P21 were observed most frequently, consistent with the prediction of Wukovitz and Yeates.
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