Mycobacterium
tuberculosis has a
complex life cycle transitioning between active and dormant growth
states depending on environmental conditions. LipN (Rv2970c) is a
conserved mycobacterial serine hydrolase with regulated catalytic
activity at the interface between active and dormant growth conditions.
LipN also catalyzes the xenobiotic degradation of a tertiary ester
substrate and contains multiple conserved motifs connected with the
ability to catalyze the hydrolysis of difficult tertiary ester substrates.
Herein, we expanded a library of fluorogenic ester substrates to include
more tertiary and constrained esters and screened 33 fluorogenic substrates
for activation by LipN, identifying its unique substrate signature.
LipN preferred short, unbranched ester substrates, but had its second
highest activity against a heteroaromatic five-membered oxazole ester.
Oxazole esters are present in multiple mycobacterial serine hydrolase
inhibitors but have not been tested widely as ester substrates. Combined
structural modeling, kinetic measurements, and substitutional analysis
of LipN showcased a fairly rigid binding pocket preorganized for catalysis
of short ester substrates. Substitution of diverse amino acids across
the binding pocket significantly impacted the folded stability and
catalytic activity of LipN with two conserved motifs (HGGGW and GDSAG)
playing interconnected, multidimensional roles in regulating its substrate
specificity. Together this detailed substrate specificity profile
of LipN illustrates the complex interplay between structure and function
in mycobacterial hormone-sensitive lipase homologues and indicates
oxazole esters as promising inhibitor and substrate scaffolds for
mycobacterial hydrolases.