The Objectives of This Chapter Are To

The objectives of this chapter are to: N Describe the steps of membrane synthesis. N Outline the process of peptidoglycan synthesis. N Introduce the concepts of protein structure and function. N Highlight important aspects of protein synthesis and export. N Describe the structure and function of cell appendages. © 2007 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. 11 Assembly of Bacterial Cell Structures ...Topologically, all the layers of the envelope are closed surfaces and must be physically continuous for cell integrity and viability to be maintained. ...All the constituents of the envelope must grow coordinately and with special regard to their location... The overall process is similar in gram-positive and gram-negative bacteria but differs in detail... —Neidhardt, Ingraham and Schaechter, Physiology of the Bacterial Cell: A Molecular Approach, 1990 he generation of precursor metabolites and the reactions that convert these to the monomeric building blocks of a cell were Tdiscussed in Chapter 10. The precursor metabolites originate ei- ther from CO2 fixation, gluconeogenesis, the tricarboxylic acid cycle, glycolysis, or allied pathways. Through a limited number of well-in- tegrated reactions, the major monomers—including the amino acids, nucleotides, sugars, and fatty acids—are synthesized from these precursors. Rapid and orderly growth depends on the polymerization or assembly of monomeric building blocks to form macromolecules. Among the essential polymerization reactions are the formation of proteins from amino acids, polysaccharides from sugars, and nucleic acids from nucleotides. Phospholipids are derived from fatty acids. Once formed, macromolecules (DNA, RNA, proteins, and phospholipids) are assembled to generate a cell (Figure 11.1). Note that proteins and RNA make up the major part of a living cell. The actual number of molecules of each macromolecular component present in an Escherichia coli cell is listed in Table 11.1. Replication of DNA, RNA synthesis, and the role of the nucleic acids in protein synthesis will be discussed in Chapter 13. The follow- ing is a brief discussion of the synthesis and assembly of cell membranes and protein structures and the assembly and export of constituents to the cell envelope (peptidoglycan layers), outer membrane (in gram-negative bacteria), plus cell appendages. © 2007 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. 288 Chapter Eleven The precursor …the types of monomers, Further reactions …that interact to metabolites formed in or building blocks, of all polymerize the produce the structures glycolysis, the TCA cellular components. monomers to form that make up the E. coli cycle, and related macromolecules… cell. pathways enter biosynthetic pathways that produce… Lipids Fatty acids ˜8 Lipopolysaccharides Inclusion 2– Glycogen bodies Glucose-6-phosphate PO4 Sugars ˜25 Fructose-6-phosphate + Peptidoglycan Envelope Pentose-5-phosphate HS– Erythrose-4-phosphate + 2– PO4 Glycerol-3-phosphate NH3 Flagella 3-Phosphoglycerate Glucose Phosphoenolpyruvate Amino acids Fueling Biosynthetic Pyruvate ˜25 Protein Pili reactions Acetyl-CoA reactions α-Ketoglutarate Succinyl-CoA Cytosol Oxaloacetate Poly- Nucleotides 8 ribosomes Figure 11.1 Synthesis of cell structures from glucose ˜ RNA An overview of the anabolic reactions that lead from glu- Nucleoid cose to the structures in an E. coli cell. The numbers of dif- DNA ferent monomer types needed are indicated; the size of Polymerization Assembly each box is proportional to the amount of material required reactions by an E. coli cell. The overall macromolecular composition of an TABLE 11.1 Escherichia coli cell Percentage of Number of Types of Total Dry Weight Molecules Molecules Molecule of Cell per Cell Possible Protein 55.0 2,360,000 ~4200 RNA 20.5 23S rRNA 18,700 1 16S rRNA 18,700 1 Perry Staley Lory Microbiology 2/e 5S rRNA 18,700 1 Sinauer Associates transfer RNA 205,000 ~60 Elizabeth Morales Illustration Services Figure 11.01.eps Date 02-16-07 messenger RNA Variable ~1,380 DNA 3.1 ~2.1 1 Lipida 9.1 22,000,000 4 Lipopolysaccharide 3.4 1,200,000 1 Peptidoglycan 2.5 1 1 Glycogen 2.5 4,360 1 Soluble organic poolb 2.9 Large ~850 Inorganic poolc 1.0 Large ~20 aThe phospholipids are of four general classes, which may exist in a variety of types based on fatty acid chain. bMetabolites, vitamins, and precursors. cAnions, cations. Adapted from Neidhardt, Ingraham, and Schaechter, 1990. © 2007 Sinauer Associates, Inc. This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. Assembly of Bacterial Cell Structures 289 11.1 Membrane Synthesis able cell may be controlled by proteins in the membrane. This process may require ATP, but that is uncertain at One structure that is present in all Bacteria and Archaea the present time. is the cytoplasmic membrane, also called the cell mem- Modification of bacterial membranes can occur inde- brane. The basic structure of a cytoplasmic membrane pendently of cell growth and division. A constant was described in Chapter 4 (see Figures 4.44 and 4.45); turnover of membrane proteins or phospholipids (or it serves as the primary boundary of the cell’s cytoplasm. both) occurs as organisms adjust to changing environ- The bacterial cytoplasmic membrane is similar in its ba- mental conditions. Cells increase the proportion of mem- sic structure to other membranes—for example, those brane unsaturated fatty acids as a response to a decrease surrounding eukaryotic cells, nuclei, mitochondria, and in growth temperature. This occurs because a functional other organelles. The membranes of Archaea are quite membrane must be fluid and because the double bonds different, as will be discussed in Chapter 18. in unsaturated fatty acids are more fluid at lower temper- A cytoplasmic membrane in a growing cell must as- atures. A cell may also vary the fatty acid chain length. In similate new membrane components (phospholipids and E. coli, C18 fatty acids are more abundant at high growth proteins) and integrate these into the membrane, which temperatures, while C16 predominates at lower temper- is increasing in surface area. During integration of the atures. Newly formed solute transport proteins or respi- newly synthesized phospholipids, the permeability and ratory enzymes may also be inserted into the cytoplasmic barrier functions of the membrane must be maintained. membrane as a microorganism encounters a different sub- strate or other changes in growth conditions. Synthesis of Lipids Membrane lipids compose about 10% of the cell dry SECTION HIGHLIGHTS weight (see Table 11.1) and thus represent a considerable Assembly of phospholipids occurs at the sur- expenditure of cell energy. Enzymes for lipid synthesis are face of the cytoplasmic membrane and re- generally located in the cytoplasmic membrane with the quires the carrier molecule ACP. The lipid and exception of those needed for precursor biosynthesis (glyc- protein composition of a cell may be adjusted erol-3-phosphate) and fatty acid synthesis; these are dis- independently of cell growth in order to adapt cussed in Chapter 10. Since lipid types differ among bac- to temperature and nutritional changes. teria with respect to fatty acid chain length, saturation, and polar head group, the reader is directed to advanced text- books and scientific literature for detailed information. As a general plan, long-chain fatty acids (C14–18) are assembled in the cytoplasm from acetyl-CoA precursor mole- 11.2 Peptidoglycan Synthesis cules via small acyl-carrier protein (ACP) intermediates. The degree of fatty acid saturation may vary depending Peptidoglycan (murein) is the main structural compo- on the cell growth condition; the fatty acids are then ester- nent of the bacterial cell wall. It is the source of strength ified with glycerol at the inner surface of the cytoplasmic and provides the characteristic cell size and shape. The membrane. A polar head group is then added to join two composition, structure, and function of peptidoglycan fatty acids, yielding the mature phospholipid (see Figure in both gram-positive and gram-negative bacteria were 4.45). E. coli, for example contains three types of phospho- discussed in Chapter 4. The position beyond the cyto- lipids (phosphatidylglycerol, phosphatidylethanolamine, plasmic membrane (Figure 11.2) presents interesting and cardiolipin (diphosphatidylglycerol), where the polar questions regarding its synthesis and assembly. head groups are glycerol, ethanolamine, and phos- The assembly of peptidoglycan precursor units oc- phatidylglycerol, respectively. Other microbes, for exam- curs in the cytoplasm and the cytoplasmic membrane ple Bacillus subtilis, may contain different head groups: (Figure 11.3). At the inner surface of the cell membrane, glucose and glucose—O—glucose in addition to ethan- both N-acetylglucosamine (NAG) and N-acetylmuramic olamine and glycerol. acid (NAM) are synthesized and then coupled to bac- The newly synthesized phospholipids are incorpo- toprenol (undecaprenol phosphate, Udc; Figure 11.4). rated into the inner leaflet of the cytoplasmic membrane Bactoprenol is a long-chain hydrocarbon that can enter bilayer, and in a relatively short time appear in the outer the hydrophobic core

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