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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

The chemical structure consequences of beta-lactams

Problem

Penicillin is perhaps one of the most famous antibiotic drugs. The core structure of penicillin and its derivatives is centered around a structure known as a beta-lactam, which is a 4 membered cyclic amide. (Figure 1) A beta-lactam is able to covalently interact and inhibit Penicillin Binding Proteins (PBPs) within bacteria. PBPs are responsible for cross-linking cell wall structures within bacteria, and this crosslinking strengthens the wall. Upon beta-lactams binding to PBPs; however, cell wall synthesis is inhibited which causes bacterial lysing.
The ring itself has very interesting properties, as its unusual square shape holds each bond at a 90° angle, resulting in great molecular strain. This strain is what gives beta-lactams their effectiveness at binding; a similar lactam ring structure with additional atoms in the ring is known to be not as reactive.Some bacteria have means of resisting beta lactam antibiotics. One such resistance mechanism is a bacterial enzyme called beta-lactamase, which hydrolyzes the lactam ring, preventing its binding to the PBP.
Figure 1. A general penicillin structure with beta-lactam ring in red.
The beta-lactam ring reacts with an active site serine side chain (CH2OH) on PBPs. What kind of chemical reaction would best describe this interaction with the strained ring?
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