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Lecture 3 Cell Biology ٢٢٢ ١ Cellulose It is a polymer of glucose that forms plant cell walls cellulose is the most abundant organic molecule on Earth Cell Biology ٢٢٢ ٢ Chitin Is a polysaccharide used by insects and crustaceans to build an exoskeleton Found in cell wall of fungi Cell Biology ٢٢٢ ٣ Hyaluronic acid Cell Biology ٢٢٢ Found in connective tissues 1,3 glycosidic bond 1,4 glycosidic bond ٤ Inulin Cell Biology ٢٢٢ Found in some plantes, formed of fructose units (b 1,2 glycosidic bond) ٥ Glycogen Cell Biology ٢٢٢ Is a storage polysaccharide composed of glucose, which is hydrolyzed by animals when glucose is needed Highly branched molecules ٦ Starch Cell Biology ٢٢٢ Is a storage polysaccharide composed of glucose monomers and found in plants Starch helix may be unbranched or branched ٧ Other polysaccharides Cell Biology ٢٢٢ Formed of Mannose units Found in Yeast (branched) and in Some plantes ( nonbranched ٨ Paramylum Homopolsaccharides, found in Euglina Cell Biology ٢٢٢ ٩ Dextrin Branched polysaccharides , produced by some bacteria Cell Biology ٢٢٢ ١٠ Complex polysaccharides Polysaccharides can combine with other classes of macromolecules to form complex polysaccharides: - Glycoproteins: polysaccharides + proteins Glycolipids: polysaccharides + lipids Cellular functions Cell-surface receptor molecules; typically reside on external surfaces of the membrane Glycolipids important in cell walls of gram-negative bacteria Cell Biology ٢٢٢ ١١ PROTEINS Are a polymer built from various combinations of 20 amino acid monomers - Are the most abundant macromolecules in cells found throughout cell - Have an important structural and enzymatic roles - Transmit information between cells (e.g : protein hormones) - Provide a defense against infection (e.g antibodies) - Account for more than 50% of the dry mass of cells. Proteins divided into two groups based on its function in the cells: Structural proteins : Integral parts of cellular structures (Fibrous proteins (Collagen) , cartilage, skin and bone (keratin). tubulin, actin like protiens, microtubules and ,Microfilaments Dynamic proteins : Catalytic proteins; catalysts for chemical reactions, cell metabolism (hormones , insulin, erthrproetin and thyroxine, hemoglobinhemocyanin-myoglobin Cell Biology ٢٢٢ ١٢ PROTEINS proteins can be divided into three main classes, which correlate with typical tertiary structures: 1- globular proteins: Almost all are soluble and many are enzymes. 2- fibrous proteins, Fibrous proteins are often structural, such as collagen, the major component of connective tissue, or keratin, the protein component of hair and nails 3- membrane proteins. Membrane proteins often serve as receptors or provide channels for polar or charged molecules to pass through the cell membrane. Cell Biology ٢٢٢ ١٣ Amino Acids Most consist of carbon, hydrogen, oxygen, and nitrogen; 2 of 22 contain sulfur, 1 contains selenium All contain two important functional groups Carboxylic acid group (-COOH) Amino group (-NH2) Amino acid monomers held together by covalent bonds (peptide bonds) Polypeptides: thousands or millions of amino acids Have two distinct ends: one terminating in an α amino group (the amino- or N- terminus) and the other is an α carboxylic group (carboxyl or C-terminus) - protein consists of a specific amino acid sequence - The amino acid sequence of protein determines its threedimensional structure and its chemical reactivity . - Side chains of amino acids impart its chemical properties Cell Biology ٢٢٢ ١٤ Cell Biology ٢٢٢ ١٥ Peptide bond The amino acids in a polypeptide chain are linked by peptide bonds This bond links the carboxyl group of one amino acid to the amino group of the next amino acid Cell Biology ٢٢٢ ١٦ Conjugated proteins A conjugated protein is a protein that functions in interaction with other chemical groups Nucleoproteins: chromosome Chromoprotein: haemoglobin Phosphproteins: found in milk Glycoproteins : contain oligosaccharide chains covalently attached to their polypeptide sidechains, they play a role in cell-cell interactions Lipioproteins: enzymes, transporters Cell Biology ٢٢٢ ١٧ Levels of protein structure Protein can have four levels of structure 1- Primary structure 2- Secondary structure 3-Tertiary structure 4-Quaternary structure Primary structure Primary structure: sequence of amino acids in a polypeptide -The correct amino acid sequence is determined by the cell’s genetic information -The slightest change in this sequence affects the protein’s ability to function -Sickle cell disease is manifested by an inability of hemoglobin in red blood cells to carry oxygen, the primary function of hemoglobin. This blood disorder is the result of change in a single amino acid. Cell Biology ٢٢٢ ١٨ Four levels of protein structure Four Levels of Protein Structure Primary structure Amino acids Hydrogen bond Secondary structure Alpha helix Pleated sheet Tertiary structure Polypeptide (single subunit of transthyretin) Cell Biology ٢٢٢ ١٩ 2- Secondary structure Cell Biology ٢٢٢ Secondary structure: the regular arrangement of amino acids within localized regions of the polypeptide folds in polypeptide that form a more stable structure, often involving hydrogen bonding between R groups There are two types of secondary structure: helical structure called an alpha helix (α-helix) (region of polypeptide chain coils around itself pleated sheet (β sheet): two parts of polypeptide chain lie side by side with hydrogen bonds between them. ٢٠ Tertiary structure Tertiary structure: additional folding of polypeptide to result in greater stability and unique three-dimensional shape Forms exposed regions or grooves in the molecule that are important for binding other molecules Disulfide bonds: bonds between -SH groups from two different amino acids In most proteins combination of alpha helix and beta sheets connected by loop regions of polypeptide chain, fold into compact globular structures called Domains (the basic unit of tertially structure) Cell Biology ٢٢٢ ٢١ Quaternary structure Cell Biology ٢٢٢ Quaternary structure: occurs in proteins composed of two or more polypeptides Subunit: each polypeptide in the protein, held together by either/both covalent and noncovalent linkages Homodimer: protein containing two identical subunits Heterodimer: protein containing two nonidentical subunits ٢٢