Biology 216 - Lect6 (Antibiotics 2)
DNA ---> mRNA ---> protein
I. Inhibitors of transcription - most for research purposes
	 (ex Actinomycin D - binds DNA; To study RNA virus replication)
A. Rifamycin = rifampicin = rifampin - bind beta subunit 
	of procaryotic RNA polymerase
	Penetrates macs, so effective against intracellular TB.
	Resistance by mutated beta subunit.
II. Inhibitors of translation - most familiar antibiotics.
A. Inhibitors of tRNA charging enzymes (tRNA synthetases)
	1. Bactroban - irreversibly binds bacterial 
		isoleucyl tRNA synthetase
	    Topical for impetigo. Made by Ps fluorescens.
	2. Borrelidin - not selective - binds threonyl tRNA synthetase (not marketed)
B. Inhibitors of ribosomal activity - aminoglycosides, tetracyclines,...
	1. Aminoglycosides - strept, kn (amikacin resist modification), 
		neo, tobra, gent "mycin" - 
		most cidal, made by Streptomyces
		Streptomycin prototype.
	a. Bind 30S ribosomal subunit- Sm binds S12 protein (s = small)
	     Inhibits initiation and elongation by misreading at "A" site (amino acyl)
		 --> chain termination or nonsense.
	b. Resistance - mutated target (chromosomal) 
		Chemical modification (phosphorylation, 
		adenylate, acetylate) -usually R-factor.
	c. Most toxic - nephrotoxic, ototoxic
	d. Synergy with penicillin - Ps, endocarditis, osteomyelitis
	2. Tetracyclines (oxy, doxy, chloro) - 4 cyclics
	a. Broad spectrum; bacteriostatic; 
		bind 30S (prevent AA-tRNA binding)
		Toxic to both 70S and 80S - taken up by 
		    Mg transport of procaryote - trapped inside.
		Bind 30s (S4 and S18 proteins) and 
		     block aminoacyl tRNA binding to A-site
		Resistance by decreased permeability, export,
		      ribosome modification (rare)
	b. Problems - broad spectrum poor oral absorption 
		(normal flora wiped -> diarrhea)
		Deposits in calcifying tissues 
			(fetus and less than 6 years old not used)
			(Brown and affects bone growth and structure)
		Not with milk since binds Ca and not absorbed 
			(1 hour after milk is OK)
			(not absorbed, so wipes out lg intestinal flora)
		Sun sensitivity (absorbs max at 268 nm) - rash and sunburn.
	c. Clinical considerations - cheap, Mycoplasma, 
		major use in 1960s -> resistance, 
		livestock (fish, cattle, chickens) 
		(shown transfer from cattle to  human flora) 
	3. Chloramphenicol - penetrates tissues well -->
		 CSF, meningitis, abscesses.
	a. Binds 50S -> prevent peptide bond formation 
		by peptidyl transferase.
	b. Problems - rarely aplastic anemia 
		(marrow stops making RBC)
	c. Other - Used to amplify plasmid copy #; 
		Used in promoter studies.
       4. Erythromycin - substitute whenever 
		penicillin allergy. Ex. Gp A Strept
	a. Binds 50S - Blocks ribosomal translocation
		Resistance (L4 protein; methylation of 
		23S rRNA by plasmid)
III. Metabolic analogues - block pathways unique to bacteria
	Overhead pathway - synergy, Bactrim 
		(for UTI, Pneumocystis, otitis media....), SXT
	A. Sulfanilamide (sulfa drugs) - know structure 
		- competitive inhibitor of folate synthase.
		Bacteria synthesize folic acid, we must get it from 
		     our diets(folate for single C transfer - B-vitamine)
		Domagk - Prontosil inactive, but modified 
		     in vivo to active form.
	B. Trimethoprim - non-competitive inhibitor of 
		procaryotic dihydrofolate reductase.
		Synergistic with sulfa to block folate pathway
	C. Other: 
		Dapsone, Isoniazid - both not understood, 
		    	but used on TB.
  		Isoniazid - blocks synthesis of mycolic acids (Mycobacteria)
		- possible analog of NAD