Sandbox Reserved 1051

This Sandbox is Reserved from 02/09/2015, through 05/31/2016 for use in the course "CH462: Biochemistry 2" taught by Geoffrey C. Hoops at the Butler University. This reservation includes Sandbox Reserved 1051 through Sandbox Reserved 1080.

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Antigen 85C from Mycobacterium tuberculosis

Introduction

Antigen 85C is one of three distinct protein components of the Ag85 complex in the cell wall of M. tuberculosis. This serine hydrolase catalyzes the transfer of mycoyl groups, unique components of the cell wall of mycobacteria. Several three dimensional structures of Ag85C have been solved, including the wild type enzyme as well as active site variants due to site-directed mutagenesis and covalent modification.

spinqualitylabelsall models Antigen 85C in Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D Background The antigen 85 (ag85) complex in Mycobacterium tuberculosis, which is responsible for causing the disease Tuberculosis, is composed of three intracellular membrane proteins: Ag85A, B, and C. The Ag85 complex is a major component of the cell wall, with each protein catalyzing the transfer of important cell wall constituents into the membrane. The cell wall of Mycobacterium tuberculosis is composed of three primary molecules: peptidoglycans, arbinogalactans, and mycolic acids. Ag85C is of particular interest due to its transfer of mycolic acids, which is one of the major components in determining cell wall integrity. The mycolic acids are responsible for forming the outermost layer of the cell wall. Mycolic acids have a long fatty acid chain and exhibit extreme hydrophobicity, which effectively creates a hydrophobic envelope surrounding the bacterium. The hydrophobic envelope created by they mycolic acids creates a barrier against small hydrophilic molecules, such as Tuberculosis antibiotics. By targeting this mycoloyltransferase activity, inhibition of Ag85C offers potential for cell wall disruption and subsequent antibiotic targeting for normally drug-resistant Mycotaberia tuberculosis. General Structure Figure 1: Octylthioglucoside, a substrate analog, shown in the binding pocket of Ag85C Antigen 85C was crystallized in its dimeric form. The secondary structure shown in the monomeric form is composed of helices, shown in pink, with one interwoven beta sheet, shown in yellow. The confrontation of a central β-sheet bordered by α–helices creates an a α/β hydrolase fold in Ag85C, and this tertiary conformation is highly conserved across enzymes that function in this capacity. The substrate binding pocket of Ag85C is composed of two separate but equally important components; there is carbohydrate binding pocket for the trehalose, and there is a fatty acid binding pocket for the mycolic acid. As a result, trehalose monomycolate can effectively bind to the Ag85C binding pocket. This binding pocket is shown in the left image with a substrate mimic. Enzymatic Activity Figure 2: Relation of the catalytic triad to the octylthioglucoside analog in Ag85C Mutagenesis studies have confirmed the Ag85C functions through a Glu-His-Ser catalytic triad, similar to that of chymotrypsin. By modifying each of the catalytic residues separately testing the enzyme’s relative activity, it has been shown that mutation of any one of these residues dramatically reduces activity. The S124 alcohol’s nucleophilicity is inductively strengthened through H260 and E224, which allows the S124 residue to catalyze a reaction that involves trehalose 6, 6’-dimycolate. The formation of the functional catalytic triad relies on upon Van der Waals interaction between C209 and the peptide bond between L232 and T231. This interaction results in a kinked conformation of the α9 helix, which promotes that activity of the catalytic triad. As a result, Ag85C, a mycolyl transferase, can facilitate the modification of trehalose monomycolates to trehalose dimycolates, which are then transported to the bacterial cell wall. This reaction is shown in Figure 3 below. Figure 3: General reaction catalyzed by Antigen 85C Methods of Inhibition Figure 4: Cys209 stabilizing kinked formation of alpha-9 helix in the native Ag85C enzyme Due to the importance of Ag85C enzymatic activity in maintaining the integrity of the Mycobacteria tuberculosis cell wall though mycolic acid modifications, the Ag85C enzyme represents a potentially effective avenue for inhibiting cell growth. The conformational sensitivity of the active site residues, H260, E228, and S124, relies entirely upon Van der Waals interaction between C209 and L232-T 23, which can be seen in both the scene and Figure 4. The C209 facilitated interaction causes the α9 helix to acquire a kinked conformation that promotes optimal interaction distances between catalytic residues. As a result, C209 has been a specific target residue for Ag85C inhibition. Figure 5: Ebselen inhibition relaxing the alpha-9 helix in Ag85C-Ebselen Ag85C can be inhibited by ebselen, which covalently bounds to the sulfur in C209. Ebselen is a thiol-modifying agent that serves as an electrophile for a C209 nucleophilic attack that results in sulfur-selenium bond formation. Co-crystallization of ebselen with Ag85C provides an explanation for the mechanism of ebselen-based inhibition. The addition of ebselen increases the distance between C209 and L232-T231, which effectively disrupts the interaction that holds the α9 helix in the active conformation. The disruption of this interaction causes the α9 helix to relax. Furthermore, the bulk of ebselen creates steric hindrance with the α9 helix residues, which can be seen in Figure 5. The pink helix represents the native enzyme, and the tan helix represents Ag85C covalently bound to ebselen, which is shown in green. Relaxation of the α9 helix due to ebselen removes E228 and H260, which now interacts with S148, from the active site. The absence of these residues destroys the charge relay mechanism, and as a result, the nucleophilicity of the S124 alcohol is not longer strengthened, which decreases serine hydrolytic activity. Inhibitors Additional thiol-modifying agents, p-chloromercuribenzoic acid and iodoacetamide, were crystalized with Ag85C. The structures show that each of these thiol-reactive inhibitors covalently bound to C209 and caused a relaxation of the α9 helix in a similar fashion to ebselen. Figure 6: Known thiol-reactive inhibitors of Ag85C