Chapter overviewChapter overview
WORKSHEET 2.2 Amino acid analysis
Section 2.2 Exercises
4 Amino acids combine in condensation reactions to form dipeptides and polypeptides.
a State the name of the functional group formed by the condensation reaction between two amino acids.
b Draw the structure of a dipeptide formed when alanine and glycine react together and circle the functional group that joins the two amino acid residues.
c State the name of the other product formed (other than a dipeptide) when alanine and glycine react together.
5 The side groups of three amino acids are shown in the table below.
Abbreviation Amino acid Side group Ser
Val Cys
Serine Valine Cysteine
–CH2OH –CH(CH3)2 –CH2SH
a Draw the molecular structures of two possible products of the condensation reaction between serine and valine.
b The notation SerCysVal represents a tripeptide formed from serine, cysteine and valine.
i Draw a possible molecular structure for CysSerVal.
ii State how many structures are possible for tripeptides formed from serine, cysteine and valine.
6 The diagram below shows a segment of a protein.
C
H O H H CH2
C
O
O HO
O O
OH
N
CH2SH CH2
H H H H H
C
C C
N N C C N C C
NH2 O
CH2 C
a State how many amino acids were used to produce this segment.
b State how many peptide (amide) links are included.
7 Complete the following table, which links the structure of a protein to the bonding involved.
Protein structure Bonding or intermolecular forces involved in the structure
Primary Secondary Tertiary
8 Describe how the amino acids present in a protein can be separated using paper chromatography.
CHAPTER 2 HUMAN BIOCHEMISTRY 9 a Explain why amino acids move along an electrophoresis gel.
b Explain how the different isoelectric points of amino acids can affect their movement during electrophoresis.
c Explain how the different molar masses of amino acids can affect their movement during electrophoresis.
d A mixture of aspartic acid, glutamic acid, valine, lysine and tyrosine were separated using gel electrophoresis at pH 5.7. Draw the fi nished electrophoresis gel, showing the fi nal positions of the fi ve amino acids.
10 List the major functions of proteins in the body, giving one example of a protein that performs each function.
The name carbohydrate is derived from the observation that many members of this group have the empirical formula Cx(H2O)y, where x and y are whole numbers. Examples include the sugar found in grapes (glucose, C6H12O6) and cane sugar (sucrose, C12H22O11). Carbohydrates are often called saccharides (from the Latin, saccharum, for sugar) because of the sweet taste of these simple members of the group. Carbohydrates are among the most abundant components of living things. They serve several functions: as an energy source and storage (starch, glycogen and glucose), as a structural material (cellulose) and as an essential component of the genetic material (ribose and deoxyribose in nucleic acids).
Glucose is one of the simplest of carbohydrate types, known as monosaccharides (simple sugars). All monosaccharides have the empirical formula CH2O. They contain a carbonyl (C=O) group and have at least two hydroxyl (–OH) groups.
Fructose, found in fruits and honey, is also a monosaccharide, as is galactose. All are white, crystalline solids with a sweet taste, and all have the formula C6H12O6. They are structural isomers of each other and are known as hexoses because they have six carbon atoms in their formula.
Figure 2.3.1 Structures of the monosaccharides glucose, galactose and fructose.
CH2OH
HOCH2
CH2OH HO
H H
H
HO OH H OH
OH H
H HO
OH
glucose fructose
H
O O H
CH2OH HO H
H
H OH H OH
OH galactose H O
Note that most of the carbon atoms are not drawn in these simplifi ed structures. In each case, a carbon atom is to be found where four lines intersect; that is, at the vertices of the hexagon or pentagon.
In solution, three isomers of monosaccharides are in equilibrium—two with ring structures and a straight-chain molecule.
2.3 CARBOHYDRATES
B.3.1
Describe the structural features of monosaccharides.
© IBO 2007
B.3.2
Draw the straight-chain and ring structural formulas of glucose and fructose. © IBO 2007
1
Figure 2.3.2 Glucose exists in solution in equilibrium between three isomers.
While the ring structures of glucose appear to be very similar, there is a fundamental difference between the two. When the orientation of the hydroxyl groups on carbons numbered 1 and 4 are compared, the difference between α-glucose and β-glucose becomes apparent. In α-glucose, both hydroxyl groups are pointing ‘downwards’, whereas in β-glucose the hydroxyl group on carbon 1 is pointing ‘upwards’ and that on carbon 4 is pointing ‘downwards’. This difference is most important when the polymers of glucose—cellulose and starch are made. While free rotation around all bonds except the C=O bond in the linear glucose molecule is possible, the hydroxyl groups are fi xed in their positions (pointing up or down) once the ring structure is formed.
Disaccharides (di- meaning two) form by condensation reactions between two monosaccharides. Like the condensation reaction between amino acids, the combination of a H atom from one monosaccharide and an –OH group from the other monosaccharide results in the formation of a water molecule. The linkage formed between the two monomers is a glycosidic or ether linkage, and the product formula is C12H22O11. Sucrose, the most common disaccharide, is added as a sweetener to foods. The condensation reaction between α-glucose and β-fructose produces sucrose. Lactose is present in the milk of mammals, making milk an energy source for the young. It is less sweet than sucrose, possibly a design of nature to protect the taste buds of the young. The condensation reaction between α-glucose and β-galactose produces lactose.
Notice the very slight difference in structure between glucose and galactose, in the arrangement of the atoms around carbon number 4. Maltose is found in grains, particularly in barley. Barley is the source of maltose for the brewing of beer. Amylose, a polymer of maltose is hydrolysed to produce maltose which is fermented to produce ethanol and carbon dioxide. The condensation reaction between two α-glucose molecules produces maltose.
The condensation reaction between monosaccharides does not necessarily stop with the disaccharide. The reaction may continue on to produce a polymer, a polysaccharide containing thousands of glucose units. These polysaccharides are less soluble than the smaller saccharides and do not taste sweet.
PRAC 2.2