The structure of a GH149 β‐(1 → 3) glucan phosphorylase reveals a new surface oligosaccharide binding site and additional domains that are absent in the disaccharide‐specific GH94 glucose‐β‐(1 → 3)‐glucose (laminaribiose) phosphorylase

Abstract Glycoside phosphorylases (GPs) with specificity for β‐(1 → 3)‐gluco‐oligosaccharides are potential candidate biocatalysts for oligosaccharide synthesis. GPs with this linkage specificity are found in two families thus far—glycoside hydrolase family 94 (GH94) and the recently discovered glycoside hydrolase family 149 (GH149). Previously, we reported a crystallographic study of a GH94 laminaribiose phosphorylase with specificity for disaccharides, providing insight into the enzyme's ability to recognize its' sugar substrate/product. In contrast to GH94, characterized GH149 enzymes were shown to have more flexible chain length specificity, with preference for substrate/product with higher degree of polymerization. In order to advance understanding of the specificity of GH149 enzymes, we herein solved X‐ray crystallographic structures of GH149 enzyme Pro_7066 in the absence of substrate and in complex with laminarihexaose (G6). The overall domain organization of Pro_7066 is very similar to that of GH94 family enzymes. However, two additional domains flanking its catalytic domain were found only in the GH149 enzyme. Unexpectedly, the G6 complex structure revealed an oligosaccharide surface binding site remote from the catalytic site, which, we suggest, may be associated with substrate targeting. As such, this study reports the first structure of a GH149 phosphorylase enzyme acting on β‐(1 → 3)‐gluco‐oligosaccharides and identifies structural elements that may be involved in defining the specificity of the GH149 enzymes.

The table below summarises the geometric issues observed across the polymeric chains and their t to the electron density. The red, orange, yellow and green segments on the lower bar indicate the fraction of residues that contain outliers for >=3, 2, 1 and 0 types of geometric quality criteria. A grey segment represents the fraction of residues that are not modelled. The numeric value for each fraction is indicated below the corresponding segment, with a dot representing fractions <=5% The upper red bar (where present) indicates the fraction of residues that have poor t to the electron density. The numeric value is given above the bar.

Mol Chain Length
Quality of chain In the tables below, the ZeroOcc column contains the number of atoms modelled with zero occupancy, the AltConf column contains the number of residues with at least one atom in alternate conformation and the Trace column contains the number of residues modelled with at most 2 atoms.
Mol Chain Residues Atoms ZeroOcc AltConf Trace 3 Residue-property plots i O These plots are drawn for all protein, RNA and DNA chains in the entry. The rst graphic for a chain summarises the proportions of the various outlier classes displayed in the second graphic. The second graphic shows the sequence view annotated by issues in geometryand electron density. Residues are color-coded according to the number of geometric quality criteria for which they contain at least one outlier: green = 0, yellow = 1, orange = 2 and red = 3 or more. A red dot above a residue indicates a poor t to the electron density (RSRZ > 2). Stretches of 2 or more consecutive residues without any outlier are shown as a green connector. Residues present in the sample, but not in the model, are shown in grey.
• Molecule 1: Bacterial beta-1,3-oligosaccharide phosphorylase Chain A:   MET  GLY  SER  SER  HIS  HIS  HIS  HIS  HIS  HIS  SER  SER  GLY  LEU  VAL  PRO  ALA  GLY  SER  MET  SER  GLN  SER  PRO  ASN  THR  LEU  ALA  ASN  GLU  GLU  THR  THR  SER  ILE  ASP  LYS  S19  V49  G92  Y101   •   Y126  Y129  I135  F136  E137  R148  R161  L177  I181   T197  E208  I223  P230  S231  A239  L244  L250  V251  A277  Y278  F279  L280  N281  T282  I297  H304  T313  R334  R353  L362  R437  R438  R470  Q474  D488  H492  Y506  E519  GLU  ASP  ASP  P523  W524  S525  I536  P573   R598  K614  S615  H618  P638  E639  Y670  R673  R738  F739  Q740  D780  K781  N794  Y846  S911  Q912  V920  F947  W958  E983  Y993  K1035  C1059  L1060  L1061  R1062  L1085  E1089  I1102  N1116  R1139  N1144  R1156 • Full wwPDB X-ray Structure Validation Report 6HQ6 The Z score for a bond length (or angle) is the number of standard deviations the observed value is removed from the expected value. A bond length (or angle) with |Z| > 5 is considered an outlier worth inspection. RMSZ is the root-mean-square of all Z scores of the bond lengths (or angles). Chiral center outliers are detected by calculating the chiral volume of a chiral center and verifying if the center is modelled as a planar moiety or with the opposite hand.A planarity outlier is detected by checking planarity of atoms in a peptide group, atoms in a mainchain group or atoms of a sidechain that are expected to be planar.
Mol Chain #Chirality outliers #Planarity outliers There are no bond length outliers.
All (3)  The all-atom clashscore is dened as the number of clashes found per 1000 atoms (including hydrogen atoms). The all-atom clashscore for this structure is 2.
All (79)  In the following table, the Percentiles column shows the percent Ramachandran outliers of the chain as a percentile score with respect to all X-ray entries followed by that with respect to entries of similar resolution.
The Analysed column shows the number of residues for which the backbone conformation was analysed, and the total number of residues.
Mol Chain Analysed Favoured Allowed Outliers Percentiles In the following table, the Percentiles column shows the percent sidechain outliers of the chain as a percentile score with respect to all X-ray entries followed by that with respect to entries of similar resolution.
The Analysed column shows the number of residues for which the sidechain conformation was analysed, and the total number of residues.  All (2)