Penicillin G

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Drug Name

Penicillin G, also known as benzylpenicillin, is a β-lactam antibiotic. It is used to treat infections caused by gram-positive cocci, particularly streptococcal infections. Due to poor oral absorption, this form of penicillin is usually administered via injection. It is also used when rapid and high penicillin levels are needed to treat severe infections. Penicillin G has in vitro activity against both gram-positive and gram-negative aerobic and anaerobic bacteria. The bactericidal activity of penicillin G results from the inhibition of cell wall synthesis and is mediated through penicillin G binding to penicillin-binding proteins (PBPs).

Table 1. Basic profile of penicillin G.

Description Intravenously or intramuscularly administered, narrow-spectrum antibacterial drug
Target(s) Penicillin-binding proteins (PBPs)
Generic Benzylpenicillin
Commercial Name Bicillin, Bicillin L-A, Pfizerpen
Combination Drug(s) N/A
Other Synonyms Benzylpenicillin, Benzylpenicillinic acid, Benzyl benicillin
IUPAC Name (2S,5R,6R)-3,3-dimethyl-7-oxo-6-(2-phenylethanoylamino)-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid
Ligand Code in PDB PNN (closed form), PNM (open form)
PDB Structure 3UDI (structure of PBP1a in complex with penicillin G)
ATC code J01CE01
RotateHydrogensLabels
Figure 1. 2D and 3D structures of Penicillin (PDB ligand code: PNN).

Antibiotic Chemistry

As a penicillin antibiotic, penicillin G (Figure 2) consists of a β-lactam ring (shown in pink) fused to a five-membered thiazolidine ring (shown in orange).

Figure 2. 2D structure of penicillin G showing the functional moieties responsible for antibacterial activity. Structure created using ChemDraw.
Figure 2. 2D structure of penicillin G showing the functional moieties responsible for antibacterial activity. Structure created using ChemDraw.

Drug Information

Table 2. Chemical and physical properties (DrugBank).

Chemical Formula C16H18N2O4S
Molecular Weight 334.39 g/mol
Calculated Predicted Partition Coefficient: cLogP 1.08
Calculated Predicted Aqueous Solubility: cLogS -3.1
Solubility (in water) 0.285 mg/mL
Predicted Topological Polar Surface Area (TPSA) 86.71 Å2

Drug Target

Penicillin G disrupts cell wall biosynthesis in bacteria by inhibiting enzymes known as penicillin-binding proteins (PBPs). The name of this class of enzymes originates from the ability of penicillin and other β-lactam antibiotics to bind to these proteins. Peptidoglycan, a polymer consisting of amino acids (peptido-) and sugars (-glycan), is the major component of the bacterial cell wall, and its mesh-like structure provides the cell wall with structure and rigidity. Some PBP enzymes catalyze the final steps of the peptidoglycan synthesis pathway, which include polymerizing glycan strands and cross-linking adjacent chains to form the characteristic mesh structure of peptidoglycan. Other PBPs regulate peptidoglycan recycling and cell wall remodeling.

Inhibition of the PBPs responsible for cross-linking results in a severely weakened cell wall, which then causes bacterial cell lysis and death. However, inhibition of those PBPs involved only in peptidoglycan remodeling is non-lethal to the bacteria.

Learn more about PBPs.

Drug-Target Complex

The target of penicillin G, PBP1a of Acinetobacter baumannii, is made of three domains, two of which are connected by a linker:
* N-terminal transglycosylase (TG) domain
* C-terminal transpeptidase (TP) domain
* OB domain

In AbPBP1a, the N-terminal transglycosylase (TG) domain is connected through a β-rich linker to a C-terminal transpeptidase (TP) domain. The linker is made of a six-stranded β-sheet and two perpendicular α-helices. The β-sheet is made of two strands from the TG domain, two from the interdomain region, and two from the TP domain.

Although full characterization of the OB domain in AbPBP1a awaits completion, it has three structurally conserved loops with ligand-binding properties. AbPBP1a is the first example of a multimodular PBP containing the OB-fold domain.

The active site, including the nucleophilic Ser434, is located in the TP domain. The antibiotic reacts with the Ser434 nucleophile and forms an acyl-enzyme covalent complex (Figure 1). As the β-lactam blocks the Ser434 residue, the natural peptide substrate can no longer access the active site of PBP1a. Thus, aztreonam inactivates the enzyme and prevents transpeptidation.

Figure 3. Ribbon representation of PBP1a bound to penicillin G (color-coded atomic stick figure: C-spring green, N-blue, O-red, S-yellow). The inset shows the covalent linkage that forms between penicillin G and Ser434 (color-coded atomic stick figure: C-gold, O-red). (PDB ID: 3UDI; Han et al., 2011).
Figure 3. Ribbon representation of PBP1a bound to penicillin G (color-coded atomic stick figure: C-spring green, N-blue, O-red, S-yellow). The inset shows the covalent linkage that forms between penicillin G and Ser434 (color-coded atomic stick figure: C-gold, O-red). (PDB ID: 3UDI; Han et al., 2011).

Penicillin G forms hydrogen bonds with Thr672, Lys669, and Thr670. Lys437 is positioned to interact with Asn489. The amide bond of Pen G is wedged between the Thr672 backbone carbonyl and the Asn489 side chain. The side chain of Tyr707 that normally blocks the benzyl binding site in the apo structure is displaced by approximately 6.1 Å (see Figure 4a) toward the solvent-exposed surface when the enzyme is complexed with penicillin G (see Figure 4b). This allows the gem-dimethyl group of penicillin G to form van der Waals interaction with the benzyl group (Hans et al., 2011).

Figure 4. (a) Ribbon representation of the active site in the apoenzyme structure of PBP1a (PDB ID: 3udf; Han et al., 2011). (b) Ribbon representation of the PBP1a active site after penicillin G binding. (PDB ID: 3UDI; Han et al., 2011). Color-coded atomic stick figure: C-khaki, O-red.
Figure 4. (a) Ribbon representation of the active site in the apoenzyme structure of PBP1a (PDB ID: 3udf; Han et al., 2011). (b) Ribbon representation of the PBP1a active site after penicillin G binding. (PDB ID: 3UDI; Han et al., 2011). Color-coded atomic stick figure: C-khaki, O-red.

Pharmacologic Properties and Safety

Penicillin G may cause serious and even fatal electrolyte disturbances when given intravenously in large doses.

Table 3. Pharmacokinetics: ADMET of penicillin G

Features Comment(s) Source
Bioavailability (%) <1% FDA
IC50 (µg/mL) 0.020 µM (for binding to PBP1a in S. pneumoniae), 0.028 µM (for binding to PBP1b in S. pneumoniae), 0.0059 µM (for binding to PBP2x in S. pneumoniae), 0.038 µM (for binding to PBP2a in S. pneumoniae), 0.13 µM (for binding to PBP2b in S. pneumoniae), 0.0024 µM (for binding to PBP3 in S. pneumoniae) (Kocaoglu, 2015)
Ki (µM) 12 µM (for E. coli), 8 µM (for K. citrophila) (Alvaro, 1999)
Half-life 0.4 to 0.9 hours FDA
Duration of Action N/A N/A
Absorption Site Binds to serum proteins (45-68%), mainly albumin DrugBank
Transporter(s) Renal organic cation transporter DrugBank
Metabolism About 16-30% of an intramuscular dose is metabolized to penicilloic acid, an inactive metabolite. A small percentage of the drug is hydroxylated into active metabolites, which are excreted via urine DrugBank
Excretion ~ 58 to 85% recovered in urine; renal clearance is extremely rapid. Nonrenal clearance is mainly via the liver FDA
AMES Test (Carcinogenic Effect) Non AMES toxic DrugBank
hERG Safety Test (Cardiac Effect) Weak inhibitor DrugBank
Liver Toxicity Liver injury is rare, and most reports of liver injury come from patients with liver disease or those who are receiving hepatotoxic agents. Usually, the cause of liver injury with ampicillin use is hypersensitivity or allergy. LiverTox

Drug Interactions and Side Effects

Table 4. Drug interactions and side effects of penicillin G.

Features Comment(s) Source
Total Number of Drug Interactions 31 drugs Drugs.com
Major Drug Interaction(s) bcg (Tice BCG, Tice BCG Vaccine) cholera vaccine, live methotrexate typhoid vaccine, live Drugs.com
Alcohol/Food Interaction(s) N/A N/A
Disease Interaction(s) Clostridioides difficile-associated diarrhea (major); Asthma/allergies (moderate) Drugs.com
On-target Side Effects Pain, swelling, bruising, or irritation around the IV needle Drugs.com
Off-target Side Effects Nausea, vomiting, mild diarrhea, black or hairy tongue Drugs.com
CYP Interactions None Drugs.com

Links

Table 5: Links to learn more about Penicillin G.

Comprehensive Antibiotic Resistance Database (CARD) ARO:3000632
DrugBank DB01053
Drugs.com https://www.drugs.com/mtm/penicillin-g-sodium.html
FDA - Penicillin G https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/065068s013lbl.pdf
LiverTox: National Institutes of Health (NIH) https://www.ncbi.nlm.nih.gov/books/NBK547993/
PubChem CID 5904

Learn about penicillin G resistance.

References

Alvaro, G., Fernandez-Lafuente, R., Blanco, R. M., Guisán, J. M. (1991) Stabilizing effect of penicillin G sulfoxide, a competitive inhibitor of penicillin G acylase: its practical applications. Enzyme Microb Technol. 13, 210-4. https://doi.org/10.1016/0141-0229(91)90130-3

Han, S., Caspers, N., Zaniewski, R.P., Lacey, B.M., Tomaras, A.P., Feng, X., Geoghegan, K.F., Shanmugasundaram, V. (2011) Distinctive attributes of β-lactam target proteins in Acinetobacter baumannii relevant to development of new antibiotics. J Am Chem Soc. 133, 20536-45. https://doi.org/10.1021/ja208835z

Jia, B., Raphenya, A. R., Alcock, B., Waglechner, N., Guo, P., Tsang, K. K., Lago, B. A., Dave, B. M., Pereira, S., Sharma, A. N., Doshi, S., Courtot, M., Lo, R., Williams, L. E., Frye, J. G., Elsayegh, T., Sardar, D. Westman, E. L., Pawlowski, A. C., Johnson, T. A., Brinkman, F. S., Wright, G. D., and McArthur, A. G. (2017) CARD 2017: expansion and model-centric curation of the Comprehensive Antibiotic Resistance Database. Nucleic Acids Research 45, D566-573. https://doi.org/10.1093/nar/gkw1004

Kocaoglu, O., Tsui, H. C., Winkler, M. E., Carlson, E. E. (2015). Profiling of β-lactam selectivity for penicillin-binding proteins in Streptococcus pneumoniae D39. Antimicrobial agents and chemotherapy, 59(6), 3548–3555. https://doi.org/10.1128/AAC.05142-14

LiverTox - Clinical and Research Information on Drug-Induced Liver Injury. National Institutes of Health. https://www.ncbi.nlm.nih.gov/books/NBK547993/

Penicillin G - Drug Bank. Drugbank.ca. http://www.drugbank.ca/drugs/DB01053

Penicillin G. Drugs.com. https://www.drugs.com/mtm/penicillin-g-sodium.html

Penicillin G (2012) Food and Drug Administration. https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/065068s013lbl.pdf

Penicillin G. PubChem. https://pubchem.ncbi.nlm.nih.gov/compound/5904

March 2025, Helen Gao, Shuchismita Dutta; Reviewed by Dr. Andrew Lovering
https://doi.org/10.2210/rcsb_pdb/GH/AMR/drugs/antibiotics/cellwall-biosynth/pbp/blm/penicillinG