CONCEPTUAL EXAMPLE 15.1
Classification of Monosaccharides
Shown below are structures of (left to right) erythrulose, gulose, mannose, and ribose.
CHO
HO
CHO
CH2OH
HO
C
H
HO
C
H
C
O
HO
C
H
HO
C
H
C
H
H
C
OH
H
C
HO
C
H
H
C
CH2OH
CH2OH
CHO
HO
C
H
OH
H
C
OH
OH
H
C
OH
CH2OH
CH2OH
Classify erythrulose and gulose as an aldose or a ketose.
Solution
Erythrulose is a ketose. The carbonyl (C O) group is on the second carbon atom from the top; it is between
two other carbon atoms, a situation that defines a ketone. Gulose is an aldose. The carbonyl group is on an
“end” carbon atom, as it is in all aldehydes.
Exercise 15.1A
Classify mannose and ribose as an aldose or a ketose.
CONCEPTUAL EXAMPLE 15.1
Classification of Monosaccharides
continued
Exercise 15.1B
How does the structure of gulose differ from that of glucose? How does the structure of mannose differ from
that of glucose?
EXAMPLE 15.2
Names of Peptides
Give the (a) alternate designation using one-letter abbreviations and the (b) full name for the pentapeptide MetGly-Phe-Ala-Cys. You may use Table 15.3.
Solution
a.
b.
M-G-F-A-C
The endings of the names for all except the last amino acid are changed from ine to yl. The name is
therefore methionylglycylphenylalanylalanylcysteine.
Exercise 15.2A
Give the (a) alternate designation using one-letter abbreviations and the (b) full name for the tetrapeptide HisPro-Val-Ala.
Exercise 15.2B
Use (a) three-letter abbreviations and (b) one-letter abbreviations to indicate the amino acids in the peptide
threonylglycylalanylalanylleucine.
EXAMPLE 15.3
Numbers of Peptides
How many different tripeptides can be made from the three amino acids methionine, valine, and phenylalanine?
Write them, using the three-letter designations in Table 15.3.
Solution
Write out the various possibilities:
Met-Val-Phe
Val-Met-Phe
Met-Phe-Val
Val-Phe-Met
There are six possible tripeptides.
Phe-Val-Met
Phe-Met-Val
Exercise 15.3A
How many different tripeptides can be made from two methionine units and one valine unit?
Exercise 15.3B
How many different tetrapeptides can be made from the four amino acids methionine, valine, tyrosine, and
phenylalanine?
EXAMPLE 15.4
Nucleotides
Consider the following nucleotide. Identify the sugar and the base. State whether it appears in DNA or in RNA.
Solution
The sugar has two H atoms on the second C atom and therefore is deoxyribose. The base is a pyrimidine
(one ring). The amino (NH2) group helps identify it as cytosine.
EXAMPLE 15.4
Nucleotides continued
Exercise 15.4A
Consider the following nucleotide. Identify the sugar and the base. State whether it appears in DNA
or in RNA.
Exercise 15.4B
A nucleotide is composed of a phosphate unit, ribose, and thymine. Does it appear in DNA, RNA, or neither?
Explain.