Chapter 27 Nucleosides, Nucleotides, and Nucleic Acids I Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall Organic Chemistry 4 th Edition Paula Yurkanis Bruice
Nucleic Acids
In nucleic acid, the phosphate group is phosphodiester
The Bases in Nucleic Acids Adenine, guanine, cytosine, and thymine are found in DNA Adenine, guanine, cytosine, and uracil are found in RNA
Nucleosides = Base + Sugar
Nucleotides = Base + Sugar + Phosphate
Different Forms of Nucleotides
The energy released from ATP hydrolysis is used to drive the phosphorylation of D-gluose (coupled reactions) ATP as a Chemical Energy Source
Breakage of the Phosphoanhydride Bond in ATP
Without ATP, the OH – group cannot be displaced
This reaction does not occur without ATP because of the poor leaving group
Phosphoryl Transfer Reaction Mechanism I
Phosphoryl Transfer Reaction Mechanism II
Phosphoryl Transfer Mechanism III
Pyrophosphate can be further hydrolyzed to phosphate
Why is the hydrolysis of a phosphoanhydride bond so exergonic?
1. Greater electrostatic repulsion in ATP 2. More solvation in the products 3. Greater resonance stabilization in the products
The interactions between ATP, Mg 2+, and arginine and lysine residues at the active site of an enzyme Since all the negative charges in ATP are neutralized, ATP is readily approached by nucleophiles
Other Important Nucleotides Cyclic AMP serves as a link between several hormones and certain enzymes that regulate cellular function
The Nucleic Acids Biosynthesis of DNA occurs in the 5’ 3’ direction
Complementary Base Pairing in DNA
The sugar–phosphate is on the outside, and the bases are on the inside
The DNA Double Helix
Hydrolysis of RNA
The Three Helical Forms of DNA Hydrogen bonding and base stacking interactions hold the DNA helix together
Replication of DNA
Transcription of DNA A stretch of bases representing a portion of a gene is called an exon A stretch of bases that contain no genetic information is called an intron mRNA is spliced prior to leaving the nucleus
A Transfer RNA
The Proposed Mechanism for Aminoacyl-tRNA Synthetase
The Substrate-Binding Site for tRNA His
A protein is synthesized from its N-terminal end to its C-terminal end Each amino acid is specified by a three-base sequence known as the genetic code
Translation
Why does DNA contain thymine instead of uracil? The synthesis of thymine is energetically expensive
Because cytosine can be converted to uracil …. Having T’s in place of U’s in DNA allows the U’s that are found in DNA to be recognized as mistakes
DNA Sequencing
Laboratory Synthesis of DNA
Oligonucleotide Synthesis with Phosphoramidites
The amino groups of the bases must be protected
Removal of the Protecting Group by Ammonia
Utilization of H-phosphate monomer to synthesize DNA strand
Hoogsteen Base Pairing