Macromolecules I, II and III

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Reading: Campbell and Reece, 2002: Chapter 5 - Structure and Function of  Macromolecules

Student Objectives: As a result of this lecture and the assigned reading, you should understand the following:

  1. The four major types of organic macromolecules are: carbohydrates, lipids, proteins, and nucleic acids.


  2. Organic macromolecules are polymers created through dehydration synthesis reactions that chemically link the specific monomers together with covalent bonds.  It is the variety in polymers that accounts for the the uniqueness of each organism; the monomers used to make polymers are essentially universal throughout the biological realm. 


  3. Carbohydrates generally have molecular formulas that are some multiple of CH2O, and carbohydrates range from single small sugar molecules (monosaccharides) to long polymers of sugar monomers (polysaccharides).  Polysaccharides may be straight or branched molecules of hundreds or thousands of sugar monomers.


    1. Carbohydrates can function as sources of energy, recognition or signaling molecules, and/or structural molecules.


    2. Monosaccharides have two or more -OH groups and either an aldehyde or a ketone group. Some sugars are highly negatively charged (e.g., N-acetylglucosamine or N-acetylgalactosamine) because they are commonly sulphated (SO3-) or because they contain a carboxyl group (COO-).



  4. Lipids consist mainly of C and H atoms linked by nonpolar covalent bonds; consequently, lipids are not attracted to polar water molecules, and lipids are hydrophobic.


    1. The main function of fats is energy storage; other functions of lipids include membrane structure, hormone signaling, and insulation.


    2. Fats and oils are large lipids made from glycerol and fatty acids.  Triglyceride fats consist of three (3) fatty acid chains hooked to a glycerol molecule.  For saturated fats, every C atom of the carbon skeleton (except the carboxyl carbon) carries 2 H atoms (the maximum number of hydrogens). In contrast, unsaturated fats contain double bonds and less than the maximum number of hydrogens possible.


    3. Phospholipids, the major components of cellular membranes, are structurally similar to fats except they contain a phosphate group and only 2 fatty acid chains attached to the glycerol.


    4. Steroids are lipids with the carbon chain bent to form fused rings. Cholesterol is a common substance in animal cell membranes, and animal cells also use cholesterol as a precursor for making other steroids, including male and female sex hormones.


  5. Proteins are biological polymers constructed from amino acid monomers. Each different protein has a unique structure and function, and protein diversity is based upon these different arrangements of a universal set of amino acids.


    1. There are seven (7) major functional classes of proteins: 1) structural proteins; 2) contractile proteins; 3) storage proteins; 4) defense proteins 5) transport proteins 6) signaling proteins 7) enzymes.


    2. Amino acids have the same basic structure, with the amino group and carboxyl group bonded to a central C atom (the alpha C). This central carbon also has an attached H atom and a chemical group called the "R" group. It is this "R" group that is the variable part of the amino acid and determines the specific properties of each of the 20+ amino acids in proteins.  Amino acids are linked together by dehydration synthesis, with the resulting covalent linkages called peptide bonds.


    3. The specific shape that determines a protein's function comprises four (4) successive levels of structure, each determined by the previous level.  The primary structure is the sequence of amino acids forming the polypeptide chain. The secondary structure consists of polypeptide chain coils or folds held in place by hydrogen bonding between the - N - H and the - C = O groups along the backbone of the chain. Coiling or folding of a polypeptide chain usually results in one of two repeating structures, either an alpha-helix or a beta-pleated sheet.  Tertiary structure is the overall 3-dimensional shape of a polypeptide; tertiary structure is maintained by bonding (hydrogen, ionic and covalent [disulfide bridges]) and hydrophobic or hydrophilic interactions between the "R" groups of various amino acids in the polypeptide chain.  Quaternary structure is produced by the bonding interactions of two (2) or more polypeptide subunits. Quaternary structure is maintained by hydrogen bonding, ionic interactions, and hydrophobic interactions.


  6. Nucleic Acids are polymers of nucleotides. There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).


    1. Nucleic acids function in information coding, storage and transfer.  DNA does not directly control protein synthesis, instead it works through intermediates, RNA molecules.


    2. Each nucleotide monomer has three (3) parts: a five carbon sugar; a phosphate group; and a nitrogenous base. Nucleic acids contain one of two 5-carbon sugars, either deoxyribose (in DNA) or ribose (in RNA).  Linked to the one end of the sugar is a phosphate group, and linked to the other end of the pentose is one of a number of nitrogenous bases. DNA has the nitrogenous bases adenine (A), guanine (G), thymine (T), and cytosine (C). RNA has A, G, C and uracil (U) (instead of thymine).


    3. A nucleic acid polymer, a polynucleotide, forms from monomers covalently linked by dehydration synthesis. The phosphate group of one nucleotide bonds to the sugar of the next nucleotide, with the result a repeating sugar-phosphate backbone. RNA is normally a single polynucleotide strand, while DNA is a double-stranded molecule. Nucleic acids form complementary base pairs stabilized by hydrogen bonds, with guanine pairing with cytosine and adenine pairing with thymine or uracil.