26 Chapter 2
Lys Asn (^) Gly Gln Thr Asn
Polypeptide strand
(^) Cys Tyr
Gln Ser Primary structure
a Helix b-Pleated sheet
Secondary structure
Hydrogen
bond
Tertiary structure^
Disulfide bond
Quaternary structure
(^) ®
Learning
Cengage ©
Figure 2- 10 The four levels of protein structure.
bonded to the sugar. There are two categories of ni-trogen
bases, which consist of a complex ring struc-ture of carbon
and nitrogen atoms. Purines consist of a fused double ring
of nine atoms. The two purine nitrogen bases are adenine
and guanine. Pyrimidines consist of a single ring of six
atoms. The three py-rimidine nitrogen bases are thymine,
cytosine, and uracil (Figure 2-11). The DNA molecule has
adenine, thymine, guanine, and cytosine. The RNA
molecule substitutes uracil for thymine and also has
adenine, cytosine, and guanine. In the DNA molecule,
adenine joins thymine, whereas cytosine always joins
guanine in forming the double helical chain. We will
discuss this structure in detail in Chapter 4.
Adenosine Triphosphate
Adenosine triphosphate (ATP) is the high-energy molecule
or fuel that runs the cell’s machinery. All the food we eat
(which is a form of chemical energy) must be transformed
into another form of chemical energy (ATP) that allows our
cells to maintain, re-pair, and reproduce themselves. The
ATP molecule consists of a ribose sugar, the purine
adenine, and three phosphate groups (Figure 2-12). The
energy of the molecule is stored in the second and third
phos-phate groups.
The breakdown of the glucose molecule and other
nutrients provides the energy to make ATP molecules