Cau tao ADN

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

P
O
-O
O-
O
P

O
O-
O-
+ H
2
O
P
O
-O
O-
OH
2
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
P
O
-O
O-
O
P
O
O-
P
O

-O
O-
P
O
-O
O-
OH
+

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