Beta Lactam BIOCHEM - timmin Flashcards
Key Chemistry of Penicillins
Talk about angles?
Obviously bond angles are close to 90 degree because 4 angles add up to 360
However, we know that sp3 hybridized orbitals, want bond angles ~109 degrees
Even further, sp2 hybridized orbitals want 120 degrees.
Thus high energy due to ring strain!
Ring strain energy calculated at 166 kJ mol-1
As comparison C-C bond energy is ~ 350 kJ mol-1
Formation of Beta Lactam Ring . . . pro and con??
Ring of beta lactam is formed by condensation, forming an amide bond, this ring strain makes this compound unstable towards water. Bad thing. HOWEVER Also, makes the beta lactam reactive with nucleophiles other than water. As we will see, this is a good thing, as it is the basis of its action.
Chemistry of Penicillins- How do we make high energy
beta lactam??? (2 methods)
- Biosynthetically by fermentation (most common)
2. Synthetically (rare) e.g. some monobactams. Tough.
How are lactams made biosynthetically- where does the energy to make strained ring come from?
Non-ribosomal peptide synthetase, B lactam synthetases use energy from oxidation or ATP hydrolysis to cyclize, then various modifications
USES ENERGY FROM OXIDATION OR ATP HYDROLYSIS
MOA of PNC??
Penicillins irreversibly bind to a bacterial enzyme, a penicillin-binding transamidase.
This enzyme is involved in making the peptidoglycan layer important in the cell wall of both gram + and – bacteria.
The activated beta lactam forms an ester with a serine hydroxyl group at the active site of the transamidase. This is essentially (covalent) irreversible, so blocks formation of bacterial cell wall.
Without the presence of PCN… How does PBP (transamidase work to form PG cell wall??)
what about in the presence of PCN??
- Transamidase cleaves at a D-ala-D-ala segment in the forming peptidoglycan, this kind of stereochemistry doesn’t occur in mammals (d-amino acids rare)
- Transamidase-D-ala conjugate itself reacts with a pentaglycine subunit of peptidoglycan.
- Regenerates the active transamidase, and allows cell wall synthesis to proceed
however in the presence of PCN…Because penicillin-transamidase conjugate cannot do this to break down, it is locked up. Cell wall not made. (A variety of steric and charge-repulsion factors prevent pentaglycine reaction with transamidasepenicillin.)
Chemistry of Penicillins- Acid Stability… Penicillins are unstable to acid.. what happen in the presence of acid??
- Mechanism involves attack by internal nucleophile- oxygen of penicillin amide to produce a lactone, that then hydrolyzes to produce penicilloic acid intermediate
2.Penicilloic acid intermediate undergoes spontaneous decarboxylation (CO2 is a good leaving group) Decarboxylation is irreversible & so no way back to
lactam even if you could load up energy
Natural Penicillin . . . PNC G and PNC V! talk about the chemistry !
By incorporating an excess of specific compounds in the medium in which Penicillium is grown, can force it to specifically make a range of penicillins. (throwing in a specific compound of something else in a medium growing PNC can force it to have specificity!)
Excess Phenylacetic acid leads to benzylpenicillin (PNC G - peritoneal)
Excess Phenoxyacetic acid leads to phenoxymethylpenicillin (PNC V - oral)
How to make PNC more acid stable? (adding acid to culture and forcing it!!)
Some substitutions bring acid stability.
E.g. Ampicillin with aminophenylacetic acid. Electron withdrawing
NH3+ group lowers nucleophilicity of amide O. (lessens nuc attack and formation of lactone - b/c it spreads out the charges! not 100% of charge is on the O)
Allergy to PNC.. talk about what happen..
Same reactivity of beta-lactam can cause reaction with host protein serine residues. (PNC reacting with the host protein!) –> Formation of penicillin-protein conjugate.
Host can then make antibody against this PNC-protein conjugate! Attacks and causes reactivity!
Aztreonam (MONO) is only lactam where cross reactivity to other lactams isn’t an issue
Chemistry of Penicillins: Beta lactamase resistance… what does beta lactamase do to beta lactam ABX?
Bacterial resistance is associated with presence of the enzyme beta-lactamase (some bacteria make this) This enzyme break the beta lactam ring via hydrolysis!
Ring-opened lactam unable to react with PBP, no longer effective antibiotic
Beta Lactamase Inhibitor! drug that protects against beta lactamase.. how does it work? what are they?
Can inhibit some beta lactamase by use of clavulanic acid. Only
weakly active itself but acts synergistically with other beta lactams
Uses ‘dual anchor’ system to prevent reactivation of lactamase. First beta lactam cleavage makes reactive compound that reacts with 1st thing it sees, which is the beta lactamase
another example: Sulbactam
