1. CH2 H N C OH O
Amino Acids
and
Dipeptides H N C O R1 OH R2

2. Polymer:
large molecules consisting of large numbers of repeating units connected by covalent bonds
Monomer:
a small molecule that may become chemically bonded to other monomers to form a polymer
amino acid =
monomer
polypeptide =
polymer

3. General structure of an amino acid
R group H N C OH O R
2.2.2
carboxyl group
amino group
(acidic)
(basic)

4. Different amino acids have different R groups
Their different properties depend on their
R groups
Hydrophobic
(‘Water-hating’) R groups
CH2 H N C OH O
phenylalanine (aromatic)
CH3 H N C OH O
alanine
Do not need to know names or structure of amino acids – interest only

5. Hydrophillic
(‘Water-loving’) R groups
CH2 H N C OH O SH
cysteine
CH2 H N C OH O
serine

6. First isolated from asparagus juice!
CH2 H N C OH O
CH2 H N C OH O
NH2
asparagine (basic)
aspartic acid (acidic)
First isolated from asparagus juice!

7. There are 20 amino acids naturally incorporated into proteins
Alanine
Arginine
Asparagine
Aspartic acid
Cysteine
Glutamic acid
Glutamine
Glycine Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Proline
Serine Threonine
Tryptophan
Tyrosine
Valine
You do NOT need to know their names!

8. Other amino acids do exist…
CH2 H N C OH O I
Thyroxine:
Other amino acids do exist…
used in the human body as a hormone to regulate metabolic rate
Some other amino acids do occur in proteins e.g. selenocysteine (some enzymes), hydroxyproline (in collagen) – these are produced by modification of other amino acids after incorporation into proteins. Other amino acids occur as intermediaries in metabolic pathways (ornithine – urea production). D-alanine, D-glutamic acid, mesodiaminopimelic acid are found in the peptide cross-links in bacterial peptidoglycan: only L-forms are found in proteins. Also dopamine (neurotransmitter), creatine (energy store in muscles)
…but are not normally found in proteins

9. These are the essential amino acids
Humans can synthesis some amino acids by altering other amino acids, however…
…some amino acids cannot be synthesised and are therefore needed in the diet.
These are the essential amino acids
Arginine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Interest only but see Option A 2.8

10. No meat, fish, poultry, eggs, cheese, milk, beans, or peas
Phenylketonuria or
Maple Syrup Urine Disease
Patients lack the enzyme
phenylalanine hydroxylase – needed
to dispose of the amino acid phenylalanine properly
Penylalanine rapidly builds up in the blood stream
- it converted to unusual metabolites which give the patient’s urine a characteristic smell
Interest only: students may have seen ‘contains a source of phenylalanine’ on food packaging
- it also rapidly causes brain damage!
Treatment:
A diet low in phelylalanine
No meat, fish, poultry, eggs, cheese, milk, beans, or peas

11. Formation of a dipeptide: carboxyl group of one amino acid reacts with amino group of second amino acid H N C OH O R2 R1
water molecule formed H N C O R1 OH R2
H2O
2.2.6
a dipeptide
peptide linkage

12. 2 amino acids  dipeptide + water
Formation of a dipeptide produces a molecule of water, therefore… H N C OH O R1 R2
H2O
…this is a condensation reaction H N C O R1 OH R2
Part of 2.2.5
2 amino acids  dipeptide + water

13. dipeptide + water  2 amino acids
Splitting a dipeptide to form two amino acids consumes one molecule of water, therefore… H N C O R1 OH R2
H2O H N C OH O R1 R2
…this is a hydrolysis reaction
Part of 2.2.5
dipeptide + water  2 amino acids

14. three amino acids linked together
Tripeptide:
three amino acids linked together
Oligopeptide:
a short chain of a few amino acids
Polypeptide:
a chain of many amino acids
A protein may consist of one or more polypeptide chains
Structure of hemoglobin – a protein consisting of four polypeptides