1NS4

Crystal structure of galactose mutarotase from Lactococcus lactis mutant E304Q complexed with glucose

Summary for 1NS4

• Entry DOI: 10.2210/pdb1ns4/pdb
• Related: 1NS0 1NS2 1NS7 1NS8 1NSM 1NSR 1NSS 1NSU 1NSV 1NSX 1NSZ
• Descriptor: GALACTOSE MUTAROTASE, beta-D-glucopyranose, SODIUM ION, ... (4 entities in total)
• Functional Keywords: mutarotase, epimerase, galactose metabolism, isomerase
• Biological source: Lactococcus lactis
• Total number of polymer chains: 2
• Total formula weight: 77627.25

Authors

Holden, H.M.,Thoden, J.B. (deposition date: 2003-01-27, release date: 2003-02-11, Last modification date: 2023-08-16)

Primary citation

Thoden, J.B.,Kim, J.,Raushel, F.M.,Holden, H.M.
The Catalytic Mechanism of Galactose Mutarotase
Protein Sci., 12:1051-1059, 2003

PubMed Abstract: Galactose mutarotase catalyzes the first step in normal galactose metabolism by catalyzing the conversion of beta-D-galactose to alpha-D-galactose. The structure of the enzyme from Lactococcus lactis was recently solved in this laboratory and shown to be topologically similar to domain 5 of beta-galactosidase. From this initial X-ray analysis, four amino acid residues were demonstrated to be intimately involved in sugar binding to the protein: His 96, His 170, Asp 243, and Glu 304. Here we present a combined X-ray crystallographic and kinetic analysis designed to examine the role of these residues in the reaction mechanism of the enzyme. For this investigation, the following site-directed mutant proteins were prepared: H96N, H170N, D243N, D243A, E304Q, and E304A. All of the structures of these proteins, complexed with either glucose or galactose, were solved to a nominal resolution of 1.95 A or better, and their kinetic parameters were measured against D-galactose, D-glucose, L-arabinose, or D-xylose. From these studies, it can be concluded that Glu 304 and His 170 are critical for catalysis and that His 96 and Asp 243 are important for proper substrate positioning within the active site. Specifically, Glu 304 serves as the active site base to initiate the reaction by removing the proton from the C-1 hydroxyl group of the sugar substrate and His 170 functions as the active site acid to protonate the C-5 ring oxygen.

PubMed: 12717027
DOI: 10.1110/ps.0243203

Experimental method

X-RAY DIFFRACTION (1.85 Å)