The structural consequences of beta-lactams
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The structural consequences of beta-lactams
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[post_date] => 2025-01-14 06:35:40
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
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.
Figure 1. A general penicillin structure with beta-lactam ring in red
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.
[post_title] => The structural consequences of beta-lactams
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[question] => 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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[answer] => 4
[description] => Reason for the Correct Answer:
Decarboxylation would result in the removal of a carboxyl group.
Anhydride formation would require the formation of an ester between two carbonyl groups.
Nucleophilic substitution properly describes the nucleophilic attack of an -OH group onto a carbonyl to create an ester.
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[answers] => Array
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[each_answer] => A.Anhydride formation
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[each_answer] => B.Decarboxylation
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[each_answer] => C.Transesterification
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[each_answer] => D.Nucleophilic substitution
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => Which of the members of the ring would be considered sp2 hybridized as seen in Figure 1?
[value] => Array
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[answer] => 2
[description] => Reason for the Correct Answer:
Due to the fact that the nitrogen exists within a lactam ring, it is reduced, making it geometrically an sp3 hybrid.
An sp3 hybridization occurs when a C atom is attached to 4 groups.
An sp2 hybridization occurs when a C atom is attached to 3 groups, therefore the carbonyl carbon is the sp2 hybrid.
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[each_answer] => A.The β-carbon
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[each_answer] => B.The carbonyl carbon
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[each_answer] => C.The α-carbon
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[each_answer] => D.The nitrogen
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[quiz_unique_key] => 2377279144
[question] => Which of the following could not be an effective means by which bacteria resist beta-lactams?
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[answer] => 4
[description] => Reason for the Correct Answer:
An outer cell membrane overlaying the cell wall describes a gram negative bacteria, which are naturally more resistant to beta-lactam drugs.
Up-regulating genes expressing beta-lactamases would result in the hydrolyzation of the beta-lactam, rendering it inert.
Due to the permanent binding of beta-lactams to PBPs, competitive inhibition of a different protein would not affect already bound beta-lactams.
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[each_answer] => A.Up-regulating genes to express beta-lactamases
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[each_answer] => B.An outer cell membrane to prevent drug access to the cell wall
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[each_answer] => C.Mutations in PBP protein structure to prevent beta-lactam binding
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[each_answer] => D.Making proteins to compete with the beta-lactams already bound to PBPs
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[3] => Array
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[quiz_unique_key] => 2261298308
[question] => With the growing presence of multidrug resistant bacteria, antibiotic resistance is a major modern health concern. Which of the following is the evolutionary explanation of the rise of antibiotic resistance?
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[answer] => 3
[description] => Reason for the Correct Answer:
Sterilizing techniques by definition kill all bacteria, therefore could not influence bacterial evolution.
Resistance mechanisms being turned on would not be an evolutionary explanation of drug resistance.
Evolution depends upon selection of random mutations which then decide which traits become more or less present in a population. Antibiotics act as that selective mechanism to select for resistance.
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[answers] => Array
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[0] => Array
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[each_answer] => A.Increased global travel exposes humans to previously unencountered, naturally resistant bacteria
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[each_answer] => B.Innate resistance mechanisms present in bacteria being increasingly ‘turned on’ upon low level (non-toxic) exposure to drugs
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[each_answer] => C.Random mutations providing resistance are ‘selected for’ due to the presence of antibiotics
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[each_answer] => D.Increased use of sterilizing techniques such as high temperature machines like autoclaves result in increased resistance
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[quiz_unique_key] => 83407773
[question] => If a beta-lactam irreversibly binds to a PBP, what kind of enzyme interaction would this be categorized as?
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[answer] => 2
[description] => Reason for the Correct Answer:
Uncompetitive inhibition takes places upon the binding of an inhibitor to an enzyme-substrate complex.
Feedback inhibition is a process of a product of a pathway self-inhibiting steps earlier on in the pathway.
Noncompetitive antagonism is either due to allosteric binding of an inhibitor to an enzyme, or permanent active site binding.
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[each_answer] => A.Uncompetitive inhibition
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[each_answer] => B.Noncompetitive antagonist
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[each_answer] => C.Feedback inhibition
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[each_answer] => D.Competitive antagonist
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