Biology 102 Fall 2001
R. Brundage
Lecture 4
Carbon Compounds in Cells II
I.Amino Acids and the Primary Structure of Proteins
A.Proteins function as enzymes, in cell movements, as storage and transport agents, as
hormones, as antibodies, and as structural material.
B.Structure of Amino Acids
1.Amino acids are small organic molecules with an amino group,
a carboxyl group, and one of twenty varying R groups.
2.All of the parts of an amino acid molecule are covalently bonded
to a central carbon atom.
C.Primary Structure of Proteins
1.Primary structure is defined as ordered sequences of amino acids
each linked together by peptide bonds to form polypeptide chains.
2.There are 20 kinds of amino acids available in nature.
3.The sequence of the amino acids is determined by DNA and is unique
for each kind of protein.
a.Fibrous proteins have polypeptide chains organized as
strands or sheets; they contribute to the shape,
internal organization, and movement of cells.
b.Globular proteins, including most enzymes, have their
chains foldedinto compact, rounded shapes.
II.Emergence of the Three-Dimensional Structure of Proteins
A.Three-dimensional structure is determined by how amino acid sequences present
their atoms for hydrogen bonding.
B.Second Level of Protein Structure
1.Secondary structure refers to the helical coil (as in hemoglobin) or
sheetlike array (as in silk) that results from hydrogen bonding
of side groups on the amino acid chains.
2.The peptide bonds between the amino acids of primary structure allow
slight bending to permit secondary structure.
C.Third Level of Protein Structure
1.Tertiary structure is the result of folding due to interactions among
R groups along the polypeptide chain.
2.The result is a more compact, globular shape in complex proteins.
D.Fourth Level of Protein Structure
1.Quaternary structure describes the complexing of two of more
polypeptide chains.
2.Hemoglobin is a good example of four interacting chains that
form a globular proteins; keratin and collagen are complex
fibrous proteins.
E.Glycoproteins and Lipoproteins
1.Lipoproteins have both lipid and protein components; they transport fats
and cholesterol in the blood.
2.Glycoproteins consist of oligosaccharides covalently bonded to proteins;
they are abundant on the exterior of animal cells, as cell products,
and in the blood.
F.Structural Changes by Denaturation
1.High temperatures or changes in pH can cause a loss of a protein’s normal
three-dimensional shape (denaturation).
2.Normal functioning is lost upon denaturation, which is often irreversible
(for example, a cooked egg).
III.Nucleotides and the Nucleic Acids
A.Nucleotides with Roles in Metabolism
1.Each nucleotide consists of a five-carbon sugar (ribose or deoxyribose),
a nitrogen-containing base, and a phosphate group.
2.Adenosine phosphates are chemical messengers (cAMP) or
energy carriers (ATP).
3.Nucleotide coenzymes transport hydrogen atoms and electrons
(examples: NAD+ and FAD).
B.Nucleic Acids–DNA and RNA
1.Nucleic acids are polymers of nucleotides.
a.Four different kinds of nucleotides are strung together to form
large single or double-stranded molecules.
b.Each strand’s backbone consists of joined sugars and phosphates
with nucleotide bases projecting toward the interior.
2.The two most important nucleic acids are DNA and RNA.
a.DNA is a double-stranded helix carrying encoded hereditary
instructions.
b.RNA is single stranded and functions in translating the
code to build proteins.