The primary structure of a protein refers to the specific sequence of amino acids in the protein chain linked together by peptide bonds. The unique amino acid sequence determines how the protein folds into its 3D structure and performs its function. While different sequences can result in very different protein shapes and functions, the sequence is precisely determined by the mRNA template which is transcribed from DNA.

1. Primary Structure – the sequence of amino acids
PROTEIN STRUCTURE

2. PRIMARY STRUCTURE
 Sequence of amino acids in protein linked by
covalent amide (peptide) bonds

3. CONDENSATION AMINO ACIDS: AMIDES OR
PEPTIDE BONDS
O O O
H2N C + H2N C + H2N C Amino acids condense
HC OH CH OH
CH2 OH
to form amides – or
CH3 alanine glycine glutamine CH2
H2C peptide bonds
Peptide or amide bond C
H2N O
O
+ O Protein chain made up
H3N C H O
HC N C of amino acids
H HC N C - covalently bonded by
CH3 CH O
CH3
CH2
amide bonds
H2C
C
H2N O

4. SEQUENCE MATTERS
 Functioning proteins have
O O
O
unique primary structure –
H2N C + H2N C + H2N C
sequence of amino acids
HC OH CH2 OH CH OH
CH3 alanine glycine glutamine CH2
in the chain
H2C
 This primary sequence
Peptide or amide bond C determines how the
H2N O
O protein folds into 3-D
O
H3N
+
C H O shape required for protein
HC N C
H HC N C -
function
CH3 CH O
CH3  Sequence has a free
N-terminal amino acid CH2 C-terminal amino acid
H2C amino end (N-terminal
alanine (free amino end) glutamine (free carboxyl end)
C
amino acid)
H2N O
 Sequence has free
carboxyl end (C-terminal
amino acid

5. SEQUENCE MATTERS
O O O
H2N C + + H2N  When 3 amino acids
H2N C C
HC OH CH2 OH CH OH reacted together, 6
CH3 alanine glycine glutamine CH2 possible different
H2C
tripeptides result
C
H2N O
Possible sequences
 Sequence of amino acids
in proteins determined by
ala-gly-gln
sequence of nucleotides
ala-gln-gly on m-RNA, which is
gly-ala-gln transcribed from DNA
gly-gln-ala
gln-ala-gly
gln-gly-ala

6. Vasopressin (antidiuretic
hormone
• 9 amino acid polypeptide
forms cyclic structure
• N terminal amino acid:
glycine
• No C terminal amino acid
(due to ring structure)
• Conserves body fluids
, reduces loss of water in
urine
• Leads to constriction of
blood vessels, increase in
blood pressure
POLYPEPTIDE HORMONES

7. Oxytocin
• 9 amino acid polypeptide with
cyclic structure
• N-terminal amino acid: glycine
• No C-terminal amino acid
• Well studied effects:
• Stimulates milk production
• Stimulates muscle contraction
for birth
• Promotes maternal behavior
• Possible association with
development of human
bonding an interpersonal
relationships
• Compare structure to
vasopressin: 2 amino acids
different – very different
physiological effects!
POLYPEPTIDE HORMONES

8. Lysozyme
• Enzyme breaks down
bacterial cell walls
• Found in egg
whites, human tears
• Single polypeptide chain
of 129 amino acids
• N-terminal amino acid is
lysine
• C-terminal amino acid is
leucine
Disulfide bonds considered
to be part of tertiary
structure.
PROTEIN PRIMARY STRUCTURE