ALTERNATIVE BINDING MODES FOR CHLORAMPHENICOL AND 1-SUBSTITUTED CHLORAMPHENICOL ANALOGUES REVEALED BY SITE-DIRECTED MUTAGENESIS AND X-RAY CRYSTALLOGRAPHY OF CHLORAMPHENICOL ACETYLTRANSFERASE

THERE IS A WIDE BETA-BULGE INVOLVING RESIDUES LYS 177, TYR 178, AND LEU 187.

RESIDUES 157 - 162 FORM AN EXTENSION TO THE SIX STRANDED BETA-SHEET OF AN ADJACENT SUBUNIT OF THE TRIMER, RESULTING IN A SEVEN-STRANDED SHEET WHICH SPANS THE SUBUNIT INTERFACE.

Refined Crystal Structure of Type III Chloramphenicol Acetyltransferase at 1.75 Angstroms Resolution

Crystal Structure of the Asp-199-Asn Mutant of Chloramphenicol Acetyltransferase to 2.35 Angstroms Resolution. Structural Consequences of Disruption of a Buried Salt-Bridge.

Evidence for Transition-State Stabilization by Serine-148 in the Catalytic Mechanism of Chloramphenicol Acetyltransferase

Substitutions in the Active Site of Chloramphenicol Acetyltransferase. Role of a Conserved Aspartate

Structure of Chloramphenicol Acetyltransferase at 1.75-Angstroms Resolution

Crystallization of a Type III Chloramphenicol Acetyl Transferase

TYPE III CHLORAMPHENICOL ACETYLTRANSFERASE

MNYTKFDVKNWVRREHFEFYRHRLPCGFSLTSKIDITTLKKSLDDSAYKFYPVMIYLIAQAVNQFDELRMAIKDDELIVW
DSVDPQFTVFHQETETFSALSCPYSSDIDQFMVNYLSVMERYKSDTKLFPQGVTPENHLNISALPWVNFDSFNFNVANFT
DYFAPIITMAKYQQEGDRLLLPLSVQVHHAVCDGFHVARFINRLQELCNSKLK

SHEET
SHEET 1 ACTUALLY HAS SEVEN STRANDS. RESIDUES
157 - 162 FORM AN EXTENSION TO THE SIX STRANDED BETA-
SHEET OF AN ADJACENT SUBUNIT OF THE TRIMER, RESULTING
IN A SEVEN-STRANDED SHEET WHICH SPANS THE SUBUNIT
INTERFACE.

THE FINAL STRAND, SER 157 - VAL 162, IS IN AN ADJACENT
(THREE-FOLD RELATED) SUBUNIT OF THE TRIMER. N ASN 159
IS HYDROGEN BONDED TO O SEH 34.

THERE IS A WIDE BETA-BULGE INVOLVING RESIDUES LYS 177,
TYR 178, AND LEU 187.

THE MUTATED RESIDUE FORMS PART OF THE CHLORAMPHENICOL BINDING POCKET. THE CONFORMATION OF THE ENZYME IS ESSENTIALLY IDENTICAL TO THE WILD TYPE. HOWEVER, CHLORAMPHENICOL BINDS IN A DIFFERENT ORIENTATION TO THAT OBSERVED IN THE WILD TYPE BINARY COMPLEX, WITH SHIFTS OF UP TO 2 ANGSTROMS IN ATOMIC POSITIONS. THE OBSERVED MODE OF BINDING IN THE CRYSTAL STRUCTURE IS ALMOST CERTAINLY NON-PRODUCTIVE. IN SPITE OF THIS OBSERVATION, THE MUTANT ENZYME IS STILL ACTIVE WITH A 60 FOLD DROP IN CATALYTIC ACTIVITY.

SOLVENT ATOMS (COBALT OR WATER) LYING ON CRYSTALLOGRAPHIC SYMMETRY AXES HAVE BEEN GIVEN OCCUPANCIES EQUAL TO 1.0/(ROTATIONAL SYMMETRY OF THAT AXIS). ATOMS OF THE SUBSTRATE CHLORAMPHENICOL HAVE BEEN ASSIGNED AN OCCUPANCY OF 0.66 BECAUSE OF UNCERTAINTY ABOUT WHETHER THE SITE IS FULLY OCCUPIED. THE LOW VALUES OF THE REFINED TEMPERATURE FACTORS SUGGEST THAT THE OCCUPANCY IS SOMEWHAT HIGHER THAN 0.66 AND IS PROBABLY CLOSE TO 1.0. IT SHOULD BE NOTED THAT THE TEMPERATURE FACTORS QUOTED IN THE JRNL REFERENCE RELATE TO AN INPUT OCCUPANCY OF 0.66, NOT AN OCCUPANCY OF 1.0.

THE COBALT IONS PLAY A CRUCIAL ROLE IN STABILIZING THE CRYSTAL LATTICE.

THE NUMBERING SCHEME ADOPTED IS BASED ON THE ALIGNMENT OF A NUMBER OF CAT SEQUENCES. FOR THE TYPE III ENZYME WHOSE COORDINATES ARE PRESENTED IN THIS ENTRY, MET 6 IS THE N-TERMINAL RESIDUE AND THERE IS NO RESIDUE NUMBER 79.

ATOMS OF SIDECHAINS SHOWING VERY WEAK (LESS THAN 0.2 $E/A**3) OR UNINTERPRETABLE ELECTRON DENSITY HAVE BEEN OMITTED FROM THE MODEL.

CAT IS A TRIMER OF IDENTICAL SUBUNITS EACH CONTAINING 213
AMINO ACIDS (SUBUNIT MOLECULAR WEIGHT 25000). THE
THREE-FOLD AXIS OF THE TRIMER IS COINCIDENT WITH THE
CRYSTALLOGRAPHIC THREE-FOLD AXIS. THE COORDINATES
PRESENTED IN THIS ENTRY ARE FOR ONE SUBUNIT OF THE TRIMER.
THE COORDINATES FOR THE OTHER TWO SUBUNITS CAN BE DERIVED
BY APPLYING ROTATIONS OF 120 DEGREES AND 240 DEGREES ABOUT
THE Z AXIS.

TO GENERATE THESE SYMMETRY RELATED SUBUNITS APPLY THE
FOLLOWING MATRICES TO THE COORDINATES GIVEN BELOW:

1. -0.50000 -0.86603 0.00000
0.86603 -0.50000 0.00000
0.00000 0.00000 1.00000

2. -0.50000 0.86603 0.00000
-0.86603 -0.50000 0.00000
0.00000 0.00000 1.00000

MNYTKFDVKNWVRREHFEFYRHRLPCGFSLTSKIDITTLKKSLDDSAYKFYPVMIYLIAQAVNQFDELRMAIKDDELIVW
DSVDPQFTVFHQETETFSALSCPYSSDIDQFMVNYLSVMERYKSDTKLFPQGVTPENHLNISALPWVNFDSFNLNVANFT
DYFAPIITMAKYQQEGDRLLLPLSVQVHHAVCDGFHVARFINRLQELCNSKLK