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Beta-lactam antibiotics

β-lactam antibiotics (beta-lactam antibiotics) are antibiotics that contain a beta-lactam ring in their chemical structure. This includes penicillin derivatives (penams), cephalosporins and cephamycins (cephems), monobactams, carbapenems[1] and carbacephems.[2] Most β-lactam antibiotics work by inhibiting cell wall biosynthesis in the bacterial organism and are the most widely used group of antibiotics. Until 2003, when measured by sales, more than half of all commercially available antibiotics in use were β-lactam compounds.[3] The first β-lactam antibiotic discovered, penicillin, was isolated from a strain of Penicillium rubens (named as Penicillium notatum at the time).[4][5]

β-lactam antibiotic
Drug class
Core structure of penicillins (top) and cephalosporins (bottom) the 2 most common groups of β-lactam antibiotics . β-lactam ring in red.
Class identifiers
UseBacterial infection
ATC codeJ01C
Biological targetPenicillin binding protein
External links
MeSHD047090
Legal status
In Wikidata

Bacteria often develop resistance to β-lactam antibiotics by synthesizing a β-lactamase, an enzyme that attacks the β-lactam ring. To overcome this resistance, β-lactam antibiotics can be given with β-lactamase inhibitors such as clavulanic acid.[6]

Medical use edit

β-lactam antibiotics are indicated for the prevention and treatment of bacterial infections caused by susceptible organisms. At first, β-lactam antibiotics were mainly active only against Gram-positive bacteria, yet the recent development of broad-spectrum β-lactam antibiotics active against various Gram-negative organisms has increased their usefulness.[citation needed]

In uninflamed (normal) brain meninges, the penetration of beta-lactam antibiotics is low, at 0.15 of AUCCSF/AUCS ratio (the ratio of area under curve of cerebrosopinal fluid against area under curve of serum).[7]

Adverse effects edit

Adverse drug reactions edit

Common adverse drug reactions for the β-lactam antibiotics include diarrhea, nausea, rash, urticaria, superinfection (including candidiasis).[8]

Infrequent adverse effects include fever, vomiting, erythema, dermatitis, angioedema, pseudomembranous colitis.[8]

Pain and inflammation at the injection site is also common for parenterally administered β-lactam antibiotics.[citation needed]

Allergy/hypersensitivity edit

Immunologically mediated adverse reactions to any β-lactam antibiotic may occur in up to 10% of patients receiving that agent (a small fraction of which are truly IgE-mediated allergic reactions, see amoxicillin rash). Anaphylaxis will occur in approximately 0.01% of patients.[8][9] There is perhaps a 5–10% cross-sensitivity between penicillin-derivatives, cephalosporins, and carbapenems;[citation needed] but this figure has been challenged by various investigators.[who?][citation needed]

Nevertheless, the risk of cross-reactivity is sufficient to warrant the contraindication of all β-lactam antibiotics in patients with a history of severe allergic reactions (urticaria, anaphylaxis, interstitial nephritis) to any β-lactam antibiotic. Rarely, allergic reactions have been triggered by exposure from kissing and sexual contact with a partner who is taking these antibiotics.[10]

A Jarisch–Herxheimer reaction may occur after initial treatment of a spirochetal infection such as syphilis with a β-lactam antibiotic.

Mechanism of action edit

Inhibition of cell wall synthesis edit

 
Penicillin and most other β-lactam antibiotics act by inhibiting penicillin-binding proteins, which normally catalyze cross-linking of bacterial cell walls.[11]
 
 
In the absence of β-lactam antibiotics (left), the cell wall plays an important role in bacterial reproduction. Bacteria attempting to grow and divide in the presence of β-lactam antibiotics (right) fail to do so, and instead shed their cell walls, forming osmotically fragile spheroplasts.[12]

β-lactam antibiotics are bactericidal, and act by inhibiting the synthesis of the peptidoglycan layer of bacterial cell walls. The peptidoglycan layer is important for cell wall structural integrity,[6] especially in Gram-positive organisms, being the outermost and primary component of the wall. The final transpeptidation step in the synthesis of the peptidoglycan is facilitated by DD-transpeptidases, also known as penicillin binding proteins (PBPs). PBPs vary in their affinity for penicillin and other β-lactam antibiotics. The number of PBPs varies between bacterial species.[11]

β-lactam antibiotics are analogues of d-alanyl-d-alanine—the terminal amino acid residues on the precursor NAM/NAG-peptide subunits of the nascent peptidoglycan layer. The structural similarity between β-lactam antibiotics and d-alanyl-d-alanine facilitates their binding to the active site of PBPs. The β-lactam nucleus of the molecule irreversibly binds to (acylates) the Ser403 residue of the PBP active site. This irreversible inhibition of the PBPs prevents the final crosslinking (transpeptidation) of the nascent peptidoglycan layer, disrupting cell wall synthesis.[13] β-lactam antibiotics block not only the division of bacteria, including cyanobacteria, but also the division of cyanelles, the photosynthetic organelles of the glaucophytes, and the division of chloroplasts of bryophytes. In contrast, they have no effect on the plastids of the highly developed vascular plants. This is supporting the endosymbiotic theory and indicates an evolution of plastid division in land plants.[14]

Under normal circumstances, peptidoglycan precursors signal a reorganisation of the bacterial cell wall and, as a consequence, trigger the activation of autolytic cell wall hydrolases. Inhibition of cross-linkage by β-lactams causes a build-up of peptidoglycan precursors, which triggers the digestion of existing peptidoglycan by autolytic hydrolases without the production of new peptidoglycan. As a result, the bactericidal action of β-lactam antibiotics is further enhanced.[citation needed]

Guanine oxidation edit

Another possibility that has been proposed to account for much of the cytotoxicity of beta lactams focuses on the oxidation of the guanine nucleotide in the bacterial nucleotide pool.[15] The incorporation of oxidized guanine nucleotide into DNA could cause cytotoxicity. Bacterial cytotoxicity could arise from incomplete repair of closely spaced 8-oxo-2'-deoxyguanosine lesions in the DNA resulting in double-strand breaks.[15]

Potency edit

Two structural features of β-lactam antibiotics have been correlated with their antibiotic potency.[16] The first is known as "Woodward's parameter", h, and is the height (in angstroms) of the pyramid formed by the nitrogen atom of the β-lactam as the apex and the three adjacent carbon atoms as the base.[17] The second is called "Cohen's parameter", c, and is the distance between the carbon atom of the carboxylate and the oxygen atom of the β-lactam carbonyl.[18] This distance is thought to correspond to the distance between the carboxylate-binding site and the oxyanion hole of the PBP enzyme. The best antibiotics are those with higher h values (more reactive to hydrolysis) and lower c values (better binding to PBPs).[16]

Modes of resistance edit

By definition, all β-lactam antibiotics have a β-lactam ring in their structure. The effectiveness of these antibiotics relies on their ability to reach the PBP intact and their ability to bind to the PBP. Hence, there are two main modes of bacterial resistance to β-lactams:

Enzymatic hydrolysis of the β-lactam ring edit

If the bacterium produces the enzyme β-lactamase or the enzyme penicillinase, the enzyme will hydrolyse the β-lactam ring of the antibiotic, rendering the antibiotic ineffective.[19] (An example of such an enzyme is New Delhi metallo-beta-lactamase 1, discovered in 2009.) The genes encoding these enzymes may be inherently present on the bacterial chromosome or may be acquired via plasmid transfer (plasmid-mediated resistance), and β-lactamase gene expression may be induced by exposure to β-lactams.[citation needed]

 
Clavulanic acid
 
Amoxicillin

The production of a β-lactamase by a bacterium does not necessarily rule out all treatment options with β-lactam antibiotics. In some instances, β-lactam antibiotics may be co-administered with a β-lactamase inhibitor. For example, Augmentin (FGP) is made of amoxicillin (a β-lactam antibiotic) and clavulanic acid (a β-lactamase inhibitor). The clavulanic acid is designed to overwhelm all β-lactamase enzymes, and effectively serve as an antagonist so that the amoxicillin is not affected by the β-lactamase enzymes. Another β-lactam/β-lactamase inhibitor combination is piperacillin/tazobactam with a broad spectrum of antibacterial activity that includes Gram-positive and -negative aerobic and anaerobic bacteria. The addition of tazobactam to piperacillin has enhanced its stability against a wide range of β-lactamase enzymes including some Extended-Spectrum β-lactamases.[20]

Other β-lactamase inhibitors such as boronic acids are being studied in which they irreversibly bind to the active site of β-lactamases. This is a benefit over clavulanic acid and similar beta-lactam competitors, because they cannot be hydrolysed, and therefore rendered useless. Extensive research is currently being done to develop tailored boronic acids to target different isozymes of beta-lactamases.[21]

However, in all cases where infection with β-lactamase-producing bacteria is suspected, the choice of a suitable β-lactam antibiotic should be carefully considered prior to treatment. In particular, choosing appropriate β-lactam antibiotic therapy is of utmost importance against organisms which harbor some level of β-lactamase expression. In this case, failure to use the most appropriate β-lactam antibiotic therapy at the onset of treatment could result in selection for bacteria with higher levels of β-lactamase expression, thereby making further efforts with other β-lactam antibiotics more difficult.[22]

Possession of altered penicillin-binding proteins edit

As a response to the use of β-lactams to control bacterial infections, some bacteria have evolved penicillin binding proteins with novel structures. β-lactam antibiotics cannot bind as effectively to these altered PBPs, and, as a result, the β-lactams are less effective at disrupting cell wall synthesis. Notable examples of this mode of resistance include methicillin-resistant Staphylococcus aureus (MRSA)[23] and penicillin-resistant Streptococcus pneumoniae. Altered PBPs do not necessarily rule out all treatment options with β-lactam antibiotics.

Nomenclature edit

 PenamCarbapenamOxapenamPenemCarbapenemMonobactamCephemCarbacephemOxacephem
The β-lactam core structures. (A) A penam. (B) A carbapenam. (C) An oxapenam. (D) A penem. (E) A carbapenem. (F) A monobactam. (G) A cephem. (H) A carbacephem. (I) An oxacephem.

β-lactams are classified according to their core ring structures.[24]

By convention, the bicyclic β-lactams are numbered starting with the position occupied by sulfur in the penams and cephems, regardless of which atom it is in a given class. That is, position 1 is always adjacent to the β-carbon of β-lactam ring. The numbering continues clockwise from position one until the β-carbon of β-lactam is reached, at which point numbering continues counterclockwise around the lactam ring to number the remaining to carbons. For example, the nitrogen atom of all bicyclic β-lactams fused to five-membered rings is labelled position 4, as it is in penams, while in cephems, the nitrogen is position 5.

The numbering of monobactams follows that of the IUPAC; the nitrogen atom is position 1, the carbonyl carbon is 2, the α-carbon is 3, and the β-carbon 4.

Biosynthesis edit

To date, two distinct methods of biosynthesizing the β-lactam core of this family of antibiotics have been discovered. The first pathway discovered was that of the penams and cephems. This path begins with a nonribosomal peptide synthetase (NRPS), ACV synthetase (ACVS), which generates the linear tripeptide δ-(L-α-aminoadipyl)-L-cysteine-D-valine (ACV). ACV is oxidatively cyclized (two cyclizations by a single enzyme) to bicyclic intermediate isopenicillin N by isopenicillin N synthase (IPNS) to form the penam core structure.[25] Various transamidations lead to the different natural penicillins.

 
This figure outlines the different methods of β-lactam closure among the various classes of β-lactam compounds. Penams and cephems are cyclized oxidatively (first row); clavams and carbapenems are closed by ATP-utilizing amidation (second and third row); and some monobactams may be closed by a third method (fourth row).

The biosynthesis of cephems branch off at isopenicillin N by an oxidative ring expansion to the cephem core. As with the penams, the variety of cephalosporins and cephamycins come from different transamidations, as is the case for the penicillins.

While the ring closure in penams and cephems is between positions 1 and 4 of the β-lactam and is oxidative, the clavams and carbapenems have their rings closed by two-electron processes between positions 1 and 2 of the ring. β-lactam synthetases are responsible for these cyclizations, and the carboxylate of the open-ring substrates is activated by ATP.[26] In clavams, the β-lactam is formed prior to the second ring; in carbapenems, the β-lactam ring is closed second in sequence.[citation needed]

The biosynthesis of the β-lactam ring of tabtoxin mirrors that of the clavams and carbapenems. The closure of the lactam ring in the other monobactams, such as sulfazecin and the nocardicins, may involve a third mechanism involving inversion of configuration at the β-carbon.[27]

See also edit

References edit

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  20. ^ Gin A, Dilay L, Karlowsky JA, Walkty A, Rubinstein E, Zhanel GG (June 2007). "Piperacillin–tazobactam: a β-lactam/β-lactamase inhibitor combination". Expert Review of Anti-infective Therapy. 5 (3): 365–383. doi:10.1586/14787210.5.3.365. ISSN 1478-7210. PMID 17547502. S2CID 68837323.
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  27. ^ Townsend CA, Brown AM, Nguyen LT (1983). "Nocardicin A: stereochemical and biomimetic studies of monocyclic β-lactam formation". Journal of the American Chemical Society. 105 (4): 919–927. doi:10.1021/ja00342a047.

beta, lactam, antibiotics, lactam, antibiotics, beta, lactam, antibiotics, antibiotics, that, contain, beta, lactam, ring, their, chemical, structure, this, includes, penicillin, derivatives, penams, cephalosporins, cephamycins, cephems, monobactams, carbapene. b lactam antibiotics beta lactam antibiotics are antibiotics that contain a beta lactam ring in their chemical structure This includes penicillin derivatives penams cephalosporins and cephamycins cephems monobactams carbapenems 1 and carbacephems 2 Most b lactam antibiotics work by inhibiting cell wall biosynthesis in the bacterial organism and are the most widely used group of antibiotics Until 2003 when measured by sales more than half of all commercially available antibiotics in use were b lactam compounds 3 The first b lactam antibiotic discovered penicillin was isolated from a strain of Penicillium rubens named as Penicillium notatum at the time 4 5 b lactam antibioticDrug classCore structure of penicillins top and cephalosporins bottom the 2 most common groups of b lactam antibiotics b lactam ring in red Class identifiersUseBacterial infectionATC codeJ01CBiological targetPenicillin binding proteinExternal linksMeSHD047090Legal statusIn WikidataBacteria often develop resistance to b lactam antibiotics by synthesizing a b lactamase an enzyme that attacks the b lactam ring To overcome this resistance b lactam antibiotics can be given with b lactamase inhibitors such as clavulanic acid 6 Contents 1 Medical use 2 Adverse effects 2 1 Adverse drug reactions 2 2 Allergy hypersensitivity 3 Mechanism of action 3 1 Inhibition of cell wall synthesis 3 2 Guanine oxidation 4 Potency 5 Modes of resistance 5 1 Enzymatic hydrolysis of the b lactam ring 5 2 Possession of altered penicillin binding proteins 6 Nomenclature 7 Biosynthesis 8 See also 9 ReferencesMedical use editb lactam antibiotics are indicated for the prevention and treatment of bacterial infections caused by susceptible organisms At first b lactam antibiotics were mainly active only against Gram positive bacteria yet the recent development of broad spectrum b lactam antibiotics active against various Gram negative organisms has increased their usefulness citation needed In uninflamed normal brain meninges the penetration of beta lactam antibiotics is low at 0 15 of AUCCSF AUCS ratio the ratio of area under curve of cerebrosopinal fluid against area under curve of serum 7 Adverse effects editAdverse drug reactions edit Common adverse drug reactions for the b lactam antibiotics include diarrhea nausea rash urticaria superinfection including candidiasis 8 Infrequent adverse effects include fever vomiting erythema dermatitis angioedema pseudomembranous colitis 8 Pain and inflammation at the injection site is also common for parenterally administered b lactam antibiotics citation needed Allergy hypersensitivity edit Immunologically mediated adverse reactions to any b lactam antibiotic may occur in up to 10 of patients receiving that agent a small fraction of which are truly IgE mediated allergic reactions see amoxicillin rash Anaphylaxis will occur in approximately 0 01 of patients 8 9 There is perhaps a 5 10 cross sensitivity between penicillin derivatives cephalosporins and carbapenems citation needed but this figure has been challenged by various investigators who citation needed Nevertheless the risk of cross reactivity is sufficient to warrant the contraindication of all b lactam antibiotics in patients with a history of severe allergic reactions urticaria anaphylaxis interstitial nephritis to any b lactam antibiotic Rarely allergic reactions have been triggered by exposure from kissing and sexual contact with a partner who is taking these antibiotics 10 A Jarisch Herxheimer reaction may occur after initial treatment of a spirochetal infection such as syphilis with a b lactam antibiotic Mechanism of action editInhibition of cell wall synthesis edit nbsp Penicillin and most other b lactam antibiotics act by inhibiting penicillin binding proteins which normally catalyze cross linking of bacterial cell walls 11 nbsp nbsp In the absence of b lactam antibiotics left the cell wall plays an important role in bacterial reproduction Bacteria attempting to grow and divide in the presence of b lactam antibiotics right fail to do so and instead shed their cell walls forming osmotically fragile spheroplasts 12 b lactam antibiotics are bactericidal and act by inhibiting the synthesis of the peptidoglycan layer of bacterial cell walls The peptidoglycan layer is important for cell wall structural integrity 6 especially in Gram positive organisms being the outermost and primary component of the wall The final transpeptidation step in the synthesis of the peptidoglycan is facilitated by DD transpeptidases also known as penicillin binding proteins PBPs PBPs vary in their affinity for penicillin and other b lactam antibiotics The number of PBPs varies between bacterial species 11 b lactam antibiotics are analogues of d alanyl d alanine the terminal amino acid residues on the precursor NAM NAG peptide subunits of the nascent peptidoglycan layer The structural similarity between b lactam antibiotics and d alanyl d alanine facilitates their binding to the active site of PBPs The b lactam nucleus of the molecule irreversibly binds to acylates the Ser403 residue of the PBP active site This irreversible inhibition of the PBPs prevents the final crosslinking transpeptidation of the nascent peptidoglycan layer disrupting cell wall synthesis 13 b lactam antibiotics block not only the division of bacteria including cyanobacteria but also the division of cyanelles the photosynthetic organelles of the glaucophytes and the division of chloroplasts of bryophytes In contrast they have no effect on the plastids of the highly developed vascular plants This is supporting the endosymbiotic theory and indicates an evolution of plastid division in land plants 14 Under normal circumstances peptidoglycan precursors signal a reorganisation of the bacterial cell wall and as a consequence trigger the activation of autolytic cell wall hydrolases Inhibition of cross linkage by b lactams causes a build up of peptidoglycan precursors which triggers the digestion of existing peptidoglycan by autolytic hydrolases without the production of new peptidoglycan As a result the bactericidal action of b lactam antibiotics is further enhanced citation needed Guanine oxidation edit Another possibility that has been proposed to account for much of the cytotoxicity of beta lactams focuses on the oxidation of the guanine nucleotide in the bacterial nucleotide pool 15 The incorporation of oxidized guanine nucleotide into DNA could cause cytotoxicity Bacterial cytotoxicity could arise from incomplete repair of closely spaced 8 oxo 2 deoxyguanosine lesions in the DNA resulting in double strand breaks 15 Potency editSee also b lactam reactivity Two structural features of b lactam antibiotics have been correlated with their antibiotic potency 16 The first is known as Woodward s parameter h and is the height in angstroms of the pyramid formed by the nitrogen atom of the b lactam as the apex and the three adjacent carbon atoms as the base 17 The second is called Cohen s parameter c and is the distance between the carbon atom of the carboxylate and the oxygen atom of the b lactam carbonyl 18 This distance is thought to correspond to the distance between the carboxylate binding site and the oxyanion hole of the PBP enzyme The best antibiotics are those with higher h values more reactive to hydrolysis and lower c values better binding to PBPs 16 Modes of resistance editBy definition all b lactam antibiotics have a b lactam ring in their structure The effectiveness of these antibiotics relies on their ability to reach the PBP intact and their ability to bind to the PBP Hence there are two main modes of bacterial resistance to b lactams Enzymatic hydrolysis of the b lactam ring edit If the bacterium produces the enzyme b lactamase or the enzyme penicillinase the enzyme will hydrolyse the b lactam ring of the antibiotic rendering the antibiotic ineffective 19 An example of such an enzyme is New Delhi metallo beta lactamase 1 discovered in 2009 The genes encoding these enzymes may be inherently present on the bacterial chromosome or may be acquired via plasmid transfer plasmid mediated resistance and b lactamase gene expression may be induced by exposure to b lactams citation needed nbsp Clavulanic acid nbsp AmoxicillinThe production of a b lactamase by a bacterium does not necessarily rule out all treatment options with b lactam antibiotics In some instances b lactam antibiotics may be co administered with a b lactamase inhibitor For example Augmentin FGP is made of amoxicillin a b lactam antibiotic and clavulanic acid a b lactamase inhibitor The clavulanic acid is designed to overwhelm all b lactamase enzymes and effectively serve as an antagonist so that the amoxicillin is not affected by the b lactamase enzymes Another b lactam b lactamase inhibitor combination is piperacillin tazobactam with a broad spectrum of antibacterial activity that includes Gram positive and negative aerobic and anaerobic bacteria The addition of tazobactam to piperacillin has enhanced its stability against a wide range of b lactamase enzymes including some Extended Spectrum b lactamases 20 Other b lactamase inhibitors such as boronic acids are being studied in which they irreversibly bind to the active site of b lactamases This is a benefit over clavulanic acid and similar beta lactam competitors because they cannot be hydrolysed and therefore rendered useless Extensive research is currently being done to develop tailored boronic acids to target different isozymes of beta lactamases 21 However in all cases where infection with b lactamase producing bacteria is suspected the choice of a suitable b lactam antibiotic should be carefully considered prior to treatment In particular choosing appropriate b lactam antibiotic therapy is of utmost importance against organisms which harbor some level of b lactamase expression In this case failure to use the most appropriate b lactam antibiotic therapy at the onset of treatment could result in selection for bacteria with higher levels of b lactamase expression thereby making further efforts with other b lactam antibiotics more difficult 22 Possession of altered penicillin binding proteins edit As a response to the use of b lactams to control bacterial infections some bacteria have evolved penicillin binding proteins with novel structures b lactam antibiotics cannot bind as effectively to these altered PBPs and as a result the b lactams are less effective at disrupting cell wall synthesis Notable examples of this mode of resistance include methicillin resistant Staphylococcus aureus MRSA 23 and penicillin resistant Streptococcus pneumoniae Altered PBPs do not necessarily rule out all treatment options with b lactam antibiotics Nomenclature edit nbsp The b lactam core structures A A penam B A carbapenam C An oxapenam D A penem E A carbapenem F A monobactam G A cephem H A carbacephem I An oxacephem b lactams are classified according to their core ring structures 24 b lactams fused to saturated five membered rings b lactams containing thiazolidine rings are named penams b lactams containing pyrrolidine rings are named carbapenams b lactams fused to oxazolidine rings are named oxapenams or clavams b lactams fused to unsaturated five membered rings b lactams containing 2 3 dihydrothiazole rings are named penems b lactams containing 2 3 dihydro 1H pyrrole rings are named carbapenems b lactams fused to unsaturated six membered rings b lactams containing 3 6 dihydro 2H 1 3 thiazine rings are named cephems b lactams containing 1 2 3 4 tetrahydropyridine rings are named carbacephems b lactams containing 3 6 dihydro 2H 1 3 oxazine rings are named oxacephems b lactams not fused to any other ring are named monobactams By convention the bicyclic b lactams are numbered starting with the position occupied by sulfur in the penams and cephems regardless of which atom it is in a given class That is position 1 is always adjacent to the b carbon of b lactam ring The numbering continues clockwise from position one until the b carbon of b lactam is reached at which point numbering continues counterclockwise around the lactam ring to number the remaining to carbons For example the nitrogen atom of all bicyclic b lactams fused to five membered rings is labelled position 4 as it is in penams while in cephems the nitrogen is position 5 The numbering of monobactams follows that of the IUPAC the nitrogen atom is position 1 the carbonyl carbon is 2 the a carbon is 3 and the b carbon 4 Biosynthesis editTo date two distinct methods of biosynthesizing the b lactam core of this family of antibiotics have been discovered The first pathway discovered was that of the penams and cephems This path begins with a nonribosomal peptide synthetase NRPS ACV synthetase ACVS which generates the linear tripeptide d L a aminoadipyl L cysteine D valine ACV ACV is oxidatively cyclized two cyclizations by a single enzyme to bicyclic intermediate isopenicillin N by isopenicillin N synthase IPNS to form the penam core structure 25 Various transamidations lead to the different natural penicillins nbsp This figure outlines the different methods of b lactam closure among the various classes of b lactam compounds Penams and cephems are cyclized oxidatively first row clavams and carbapenems are closed by ATP utilizing amidation second and third row and some monobactams may be closed by a third method fourth row The biosynthesis of cephems branch off at isopenicillin N by an oxidative ring expansion to the cephem core As with the penams the variety of cephalosporins and cephamycins come from different transamidations as is the case for the penicillins While the ring closure in penams and cephems is between positions 1 and 4 of the b lactam and is oxidative the clavams and carbapenems have their rings closed by two electron processes between positions 1 and 2 of the ring b lactam synthetases are responsible for these cyclizations and the carboxylate of the open ring substrates is activated by ATP 26 In clavams the b lactam is formed prior to the second ring in carbapenems the b lactam ring is closed second in sequence citation needed The biosynthesis of the b lactam ring of tabtoxin mirrors that of the clavams and carbapenems The closure of the lactam ring in the other monobactams such as sulfazecin and the nocardicins may involve a third mechanism involving inversion of configuration at the b carbon 27 See also editList of b lactam antibiotics ATC code J01C Beta lactam antibacterials penicillins ATC code J01D Other beta lactam antibacterials Bacteria Cell wall Discovery and development of cephalosporins History of penicillin NitrocefinReferences edit Holten KB Onusko EM August 2000 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3 1031 1042 doi 10 1021 bi701577q PMID 18163649 Archived from the original on 2017 09 22 Retrieved 2017 10 28 Bachmann BO Li R Townsend CA 1998 b lactam synthetase a new biosynthetic enzyme Proceedings of the National Academy of Sciences of the United States of America 95 16 9082 9086 Bibcode 1998PNAS 95 9082B doi 10 1073 pnas 95 16 9082 PMC 21295 PMID 9689037 Townsend CA Brown AM Nguyen LT 1983 Nocardicin A stereochemical and biomimetic studies of monocyclic b lactam formation Journal of the American Chemical Society 105 4 919 927 doi 10 1021 ja00342a047 Retrieved from https en wikipedia org w index php title Beta lactam antibiotics amp oldid 1196770877, wikipedia, wiki, book, books, library,

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