Bacterial & Archaeal Cell Wall (BSC 310 Lecture 5) PDF

Lecture 5: Bacterial and Archaeal Cell Wall (Chapter 2 pg 44-51) Learning Objectives: Compare and contrast gram-positive, gram-negative, and archaeal cell wall structures What are the functional differences in cell membranes and cell walls? © 2018 Pearson Edu...

Lecture 5: Bacterial and Archaeal Cell Wall (Chapter 2 pg 44-51) Learning Objectives: Compare and contrast gram-positive, gram-negative, and archaeal cell wall structures What are the functional differences in cell membranes and cell walls? © 2018 Pearson Education, Inc.  Bacterial Cell Walls The concentration of solutes in the cytoplasm of bacterial cells causes significant osmotic pressure (~30 psi) so a cell wall is needed to prevent cell lysis Wall also confers shape and rigidity to the cell (and is a target for many antibiotics) Bacteria are divided into 2 major groups based on cell wall structure (which results in Gram reactions): 1) Gram-positive bacteria: thick cell wall layer of peptidoglycan and 1 membrane (cytoplasmic membrane) 2) Gram-negative bacteria: thin cell wall layer of peptidoglycan and 2 membranes (cytoplasmic membrane and outer membrane containing LPS) © 2018 Pearson Education, Inc.  Bacterial Cell Walls © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Peptidoglycan Peptidoglycan Rigid polysaccharide layer responsible for strength of wall. Not found in Archaea or Eukarya Typically composed of: 1) Alternating repeats of two modified glucose residues (N- acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in β-1,4 linkages 2) Amino acids L-alanine, D-alanine, D-glutamic acid, and either L-lysine or diaminopimelic acid (DAP) for peptide linages All are connected in an ordered way to form the glycan tetrapeptide  long chains form peptidoglycan © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Peptidoglycan © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Peptidoglycan Long chains of peptidoglycan form a sheet around the cell Chains are connected through Gram- Gram- cross-links (peptide bonds) of negative: positive: amino acids; the amount of cross- linking varies between species and determines rigidity Cross-linkage differs between gram-negative and gram-positive bacteria: Gram-negative: peptide bond between the DAP amino group and the terminal D-alanine carboxyl group Gram-positive: short peptide interbridges that vary between species (in Staphylococcus consists of 5 glycine residues) © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Peptidoglycan Peptidoglycan can be destroyed by: 1. Lysozyme: enzyme that cleaves the bonds between N- acetylglucosamine and N-acetylmuramic acid; found in many bodily secretions 2. Penicillin and other b-lactam antibiotics: prevents crosslinking of peptidoglycan chains Both lead to osmotic lysis of the cell Over 100 chemically distinct peptidoglycans have been described that vary in their cross-linking or interbridge structures; however, NAG and NAM (glycan portion) do not change © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Gram-positive organisms Composed of up to 90% peptidoglygan; several layers of peptidoglycan stacked on top of one another (peptidoglycan strands linked together into cable-like structures, cables are cross-linked providing strength to the cell wall) Have teichoic acids embedded in the cell wall that are covalently bonded to peptidoglycan The negative charge of the phosphates in teichoic acid are partly responsible for the negative charge of the cell surface Teichoic acids bind Ca2+ and Mg2+ prior to their transport Teichoic acids covalently bound to membrane lipids are called lipoteichoic acids © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Gram-negative organisms The outer membrane is a second lipid bilayer Outer membrane also contains polysaccharides that are linked to the lipid bilayer, resulting in the name lipopolysaccharide (LPS) LPS does not provide strength, but does act as an additional barrier against harmful agents (e.g. antibiotics) LPS consists of 1) the O-specific polysaccharide, 2) a core polysaccharide, and 3) lipid A (lipid A = endotoxin, the toxic component of LPS) © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Gram-negative organisms LPS replaces much of the phospholipid in the outer layer of the outer membrane bilayer; inner layer is more typical Braun lipoprotein attaches the outer membrane to the peptidoglycan Thus, gram-negative bacteria contain 1) an outer membrane (contains LPS), 2) a thin peptidoglycan layer, and 3) a cytoplasmic membrane The space between the membranes is referred to as the periplasm © 2018 Pearson Education, Inc.  Bacterial Cell Walls: Gram-negative organisms Periplasm and Porins: The outer membrane is generally not permeable to proteins and other large molecules; it keeps proteins whose activities occur outside the cytoplasmic membrane from diffusing away These proteins remain in the periplasm and include hydrolytic enzymes, binding proteins (transport), and chemoreceptors Small molecules can cross the outer membrane via porins which function as channels for solutes Some porins are specific, others are nonspecific Porins consist of 3 identical subunits Notice the tiny channel in the middle – each porin complex provides 3 large and 1 small pore © 2018 Pearson Education, Inc.  Prokaryotes that lack cell walls A few prokaryotes lack cell walls including: Mycoplasma, a group of bacterial pathogens Thermoplasma, archaeal thermophiles Contain either unusually tough cytoplasmic membranes or live in osmotically protected environments May use sterols or lipoglycans to add strength and rigidity to their membranes © 2018 Pearson Education, Inc.  Archaeal Cell Walls No peptidoglycan, no outer membrane; and display a variety of chemistries in their cell walls: Pseudomurein: Found in cell walls of certain methanogenic Archaea Polysaccharide similar to peptidoglycan Backbone is repeating subunits of N-acetylglucosamine (like peptidoglycan) and N- acetyltalosaminuronic acid (different, replaces N- acetylmuramic acid) Contains β-1,3 glycosidic bonds instead of β-1,4 and all amino acids are the L-sterioisomer Cannot be destroyed by lysozyme or penicillin © 2018 Pearson Education, Inc.  Archaeal Cell Walls Cell walls of some Archaea lack pseudomurein and contain other polysaccharide polymers instead Most common type of archaeal cell wall is the paracrystalline surface layer (S-Layer) Consist of interlocking protein or glycoprotein molecules that form a paracrystalline structure of various symmetries (hexagonal, tetragonal, trimeric) For some organisms, cell wall consists only of the S-Layer (sufficiently strong to withstand osmotic lysis) In many organisms, S-layer is present in addition to other cell wall components, usually polysaccharides Always the outermost layer, allows small molecules to pass through while excluding large molecules © 2018 Pearson Education, Inc. TEM of S-Layer fragment