EFB325 Cell Physiology

Building blocks & Polymers - Polysaccharides and Nucleic acids

Assembly of macromolecules can be simple or intricate

• some polymers have a specific order of the different monomer subunits which is critical for their biological function; nucleic acids and proteins (=informational)
• other polymers consist of a single or a few types of monomers and the specific order does not determine function as it does with informational polymers; cellulose, starch, or glycogen (=structural or storage)

Synthesis of biopolymers-common themes:

• much more energy is required to synthesize the monomers, relatively little is needed to add on to the polymer
• stepwise polymerization of identical or similar monomer units
• the reaction involving the addition of each monomer occurs through the removal of a molecule of water=condensation
• the monomer units must be in an activated form for condensation to occur; can involve coupling to a carrier molecule to produce a high-energy activated monomer
• ATP or other high-energy molecule is the source of chemical energy to activate the monomer
• polymerization is catalyzed by enzymes that can ensure the proper monomer is being added
• polymers have a directionality to the chain, head-to-tail

Polysaccharides
Carbohydrates and polysaccharides function in energy storage, as an immediate fuel source to power chemical reactions, as structural elements, and in signaling

• see Panel 2-3 in Essential Cell Biology
• not informational molecules, polymer of the same molecule or of only a few (monosaccharides)
• usually hexoses (6-carbon sugars), glucose, fructose, galactose
• can be either an aldehyde (aldose, like glucose) or ketone (ketose, like fructose)
• numbering of sugars starts at oxidized end, carbons are numbered
• the orientation of -OH on carbon 1 determines alpha (-OH is down) vs. beta (-OH is up)
• monosaccharides can be modified by the addition of other side groups: an amino group at carbon 2 of glucose = glucosamine; nitrogen-linked acetate on carbon 2=N-acetylglucosamine (GlcNAc); carboxylic acid on carbon 6 = glucuronic acid
• monosaccharides can be linked together by a glycosidic bond, a covalent bond which forms through a condensation reaction
• disaccharides=maltose (glu-glu, alpha glycosidic bond); sucrose (glu-fru, alpha bond); lactose (glu-gal, beta glycosidic bond)
• synthesis of polymers requires activated monomers-glucose with UDP attached

Storage polysaccharides

• in animals=glycogen; in plants=starch (amylose=unbranched or amylopectin=branched)
• both are glucose polymers, with alpha 1-4 linkages; alpha 1-6 branches (not amylose though)
• coil into a loose helix (not ordered)

Structural polysaccharide

• in plants=cellulose, glucose polymer linked by beta 1-4 bonds
• forms rigid, linear rods; come together to form microfibrils
• principal component of plant cell walls
• in insects & crustaceans=chitin, polymer of N-acetylglucosamine, also rigid, beta 1-4

Nucleic acids
The nucleic acid polymers function to maintain, replicate, and express genetic information.  The monomers (the nucleotides) can serve to carry energy, as coenzymes (assist in reactions), or in signaling.

Nucleic acid polymers contain genetic information, so the precise sequence of monomers is critical

Nucleotides and deoxynucleotides

• composed of three chemical groups: a base, a 5-carbon (pentose) sugar, and phosphate group(s)
• see Panel 2-6 in Essential Cell Biology

Bases are guanine (G), adenine (A), thymine (T), cytosine (C), and uracil (U)

• G, A, T, C are found in DNA; G, A, U, C are found in RNA
• G & A are purines; C, T, & U are pyrimidines
• base is linked to the 1' carbon position of the sugar by a N-glycosidic bond

Sugars are ribose (in RNA) or deoxyribose (in DNA)

• only difference is -OH at 2' carbon position in ribose; -H at that carbon in deoxyribose
• both ribose and deoxyribose have -OH at the 3' carbon position

The base and sugar combination with no phosphates=nucleoside

Phosphate group can include one, two, or three phosphates

• nucleic acid polymers are synthesized from the triphosphate monomer, which holds the most energy (NTP or dNTP)
• phosphate group is linked at the 5' carbon position of the sugar by a phosphoester bond

Polymers are: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)

• DNA monomers are deoxynucleotides = dATP, dCTP, dGTP, dTTP
• RNA monomers are nucleotides = ATP, CTP, GTP, UTP

The nucleotides in DNA and RNA polymers are linked by a phosphodiester bond between the 5' carbon of the sugar from one nucleotide and the 3' carbon of the sugar from the next

• covalent bond=sugar-phosphate backbone
• confers a directionality to the strand
• each phosphate in the backbone has a negative charge

Purine bases can hydrogen bond with pyrimidine bases

• A can form two hydrogen bonds with either T or U
• G can form three hydrogen bonds with C
• allows for specific pairing of the bases
• causes DNA strands to bind together in a double helix; the two strands are in anti-parallel orientation; one is 5'-3', complementary strand is 3'-5'

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