Abstract
Enzymes are continually evolving in response to environmental pressures. In order to increase enzyme fitness, amino acid substitutions can occur leading to a changing function or an increased stability. These evolutionary drivers determine the activity of an enzyme and its success in future generations in response to changing conditions such as environmental stressors or to improve physiological function allowing continual persistence of the enzyme. With recent warning reports on antibiotic resistance and multidrug resistant bacterial infections, understanding the evolution of β-lactamase enzymes, which are a large contributor to antibiotic resistance, is increasingly important. Here, we investigated a variant of the SHV β-lactamase identified from a clinical isolate of Escherichia coli in 2011 (SHV-129, G238S-E240K-R275L-N276D) to identify the first instance of a global suppressor substitution in the SHV β-lactamase family. We have used this enzyme to show that several evolutionary principles are conserved in different class A β-lactamases, such as active site mutations reducing stability and requiring compensating suppressor substitutions in order to ensure evolutionary persistence of a given β-lactamase. However, the pathway taken by a given β-lactamase in order to reach its evolutionary peak under a given set of conditions is likely different. We also provide further evidence for a conserved stabilizing substitution among class A β-lactamases, the back to consensus M182T substitution. In addition to expanding the spectrum of β-lactamase activity to include the hydrolysis of cefepime, the amino acid substitutions found in SHV-129 provide the enzyme with an excess of stability, which expands the evolutionary landscape of this enzyme and may result in further evolution to potentially include resistance to carbapenems or β-lactamase inhibitors.