Primary Structure of Biological Macromolecules Determines Function

Drawing of a typical bacterial cell, by Vaike Haas, University of Wisconsin-Madison Primary Structure of Biological Macromolecules Determines Function Procaryotic structural components consist of macromolecules such as DNA, RNA, proteins, polysaccharides, phospholipids, or some combination thereof. The macromolecules are made up of primary subunits such as nucleotides, amino acids and sugars (Table 1). It is the sequence in which the subunits are put together in the macromolecule, called the primary structure, that determines many of the properties that the macromolecule will have. Thus, the genetic code is determined by specific nuleotide base sequences in chromosomal DNA; the amino acid sequence in a protein determines the properties and function of the protein; and sequence of sugars in bacterial lipopolysaccharides determines unique cell wall properties for pathogens. The primary structure of a macromolecule will drive its function, and differences within the primary structure of biological macromolecules accounts for the immense diversity of life. Table 1. Macromolecules that make up cell material Macromolecule Primary Subunits Where found in cell Proteins amino acids Flagella, pili, cell walls, cytoplasmic membranes, ribosomes, cytoplasm Polysaccharides sugars (carbohydrates) capsules, inclusions (storage), cell walls Phospholipids fatty acids membranes Nucleic Acids (DNA/RNA) nucleotides DNA: nucleoid (chromosome), plasmids rRNA: ribosomes; mRNA, tRNA: cytoplasm Procaryotic Cell Architecture At one time it was thought that bacteria and other procaryotes were essentially "bags of enzymes" with no inherent cellular architecture. The development of the electron microscope in the 1950s revealed the distinct anatomical features of bacteria and confirmed the suspicion that they lacked a nuclear membrane. Procaryotes are cells of relatively simple construction, especially if compared to eucaryotes. Whereas eucaryotic cells have a preponderance of organelles with separate cellular functions, procaryotes carry out all cellular functions as individual units. A procaryotic cell has five essential structural components: a nucleoid (DNA), ribosomes, cell membrane, cell wall, and some sort of surface layer, which may or may not be an inherent part of the wall. Structurally, there are three architectural regions: appendages (attachments to the cell surface) in the form of flagella and pili (or fimbriae); a cell envelope consisting of a capsule, cell wall and plasma membrane; and a cytoplasmic region that contains the cell chromosome (DNA) and ribosomes and various sorts of inclusions (Figure 1). Figure 1. Cutaway drawing of a typical bacterial cell illustrating structural components. See Table 2 below for chemical composition and function of the labeled components. Table 2. Summary of characteristics of typical bacterial cell structures Structure Flagella Function(s) Swimming movement Predominant chemical composition Protein Pili Sex pilus Stabilizes mating bacteria during DNA transfer by conjugation Protein Common pili or fimbriae Attachment to surfaces; protection against phagotrophic engulfment Protein Capsules (includes "slime layers" and glycocalyx) Attachment to surfaces; protection against phagocytic engulfment, occasionally killing or digestion; reserve of nutrients or protection against desiccation Usually polysaccharide; occasionally polypeptide Cell wall Gram-positive bacteria Prevents osmotic lysis of cell protoplast and confers rigidity and shape on cells Peptidoglycan (murein) complexed with teichoic acids Gram-negative bacteria Peptidoglycan prevents osmotic lysis and confers rigidity and shape; outer membrane is permeability barrier; associated LPS and proteins have various functions Peptidoglycan (murein) surrounded by phospholipid protein-lipopolysaccharide "outer membrane" Plasma membrane Permeability barrier; transport of solutes; energy generation; location of numerous enzyme systems Phospholipid and protein Ribosomes Sites of translation (protein synthesis) RNA and protein Inclusions Often reserves of nutrients; additional specialized functions Highly variable; carbohydrate, lipid, protein or inorganic Chromosome Genetic material of cell DNA Plasmid Extrachromosomal genetic material DNA Figure 2 . Electron micrograph of an ultra-thin section of a dividing pair of group A streptococci (20,000X). The cell surface fimbriae (fibrils) are evident. The bacterial cell wall is seen as the light staining region between the fibrils and the dark staining cell interior. Cell division in progress is indicated by the new septum formed between the two cells and by the indentation of the cell wall near the cell equator. The streptococcal cell diameter is equal to approximately one micron. Electron micrograph of Streptococcus pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with permission. The Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller University.