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PROTEINS
Definition
• Protein from the Greek proteois, meaning
primary.
• They play key roles in constructing and
maintaining living cells (Cell, Hormones
and Enzymes).
• Our genes code for proteins.
• Proteins are polymers of amino acids.
• Ribosome (Synthesis of Proteins).
Proteins
Roles played by proteins include
Enzymes (biological catalysts)
􀁺 1. Hormones
􀁺 2. Storage proteins
􀁺 3. Transport proteins
􀁺 4. Structural proteins
􀁺 5. Protective proteins
􀁺 6. Contractile proteins
􀁺 7. Toxic proteins
Functions:
1.
Nutritional: growth, digestion, metabolism (enzymes).
2.
Physical structure: cheese, bread, foaming agent, gel-forming.
Food Proteins
•
•
•
•
Palatable
Digestible
Non-toxic
Available Economically
Proteins in the Diet
• 9 of the 20 amino acids must be obtained from the diet
• These are referred to as the essential amino acids.
– Histidine
– Isoleucine
– Leucine
– Lysine
– Methionine
– Phenylalanine
– Threonine
– Tryptophan
– Valine
• Proteins are also the major source of nitrogen in the diet
• Protein are polymers
of α-amino acids
• The amino acids used
to make proteins are 2aminocarboxylic
acids.
• The α (alpha) carbon
is the carbon to which
a functional group is
attached.
Groups of Amino Acids
1.
Aliphatic amino acids
2.
Hydroxy amino acids
3.
Acidic amino acids
4.
Amide amino acids
5.
Basic amino acids
6.
Sulfur-containing amino acids
7.
Aromatic amino acids
8.
Secondary amino acids
Aliphatic Amino Acids
Glycine (GLY)
Alanine (ALA)
H
NH 3
+
C
H
H
-
COO
NH 3
+
-
C COO
CH 3
Aliphatic Amino Acids
Leucine (LEU)
Valine (VAL)
H
+
NH 3
C
H
-
COO
CH
CH 3 CH 3
+
NH 3
C
COO
CH 2
CH
CH 3 CH 3
-
Aliphatic Amino Acids
Isoleucine (ILE)
H
NH 3
+
C
-
COO
CH
CH CH2CH3
3
Amino Acids with Alcohol
Threonine (THR)
Serine (SER)
H
H
NH
+
3
C
COO-
CH2 OH
NH
+
3
C
COO-
CHOH
CH 3
Acidic Amino Acids
Aspartic Acid (ASP)
Glutamic Acid (GLU)
H
H
NH 3
+
-
C COO
CH2
-
COO
NH 3
+
C
COO
CH 2
CH 2
-
COO
-
Amino Acids with Amides
Asparagine (ASN)
Glutamine (GLN)
H
H
NH 3
+
C
-
COO
CH 2
C
O
NH2
NH 3
+
C
COO
CH 2
CH 2
C
O
NH2
-
Basic Amino Acids
Lysine (LYS)
Arginine (ARG)
H
NH 3
+
C
H
-
COO
NH 3
CH2
+
-
C COO
CH 2
CH2
CH2
CH 2
CH2
CH 2 NH +
2
NH 3
+
NH
C
NH 2
Basic Amino Acids
Histidine (HIS)
H
NH 3
+
C
COO
CH2
HN NH
+
-
Sulfuric Amino Acids
Cysteine (CYS H)
Cystine (CYS-CYS)
H
NH 3
+
C
COO
CH 2
SH
-
-
-
COO
COO
NH 3
+
CH CH 2
S S
CH 2
CH NH 3
+
Sulfuric Amino Acids
Methionine (MET)
H
NH3
+
-
C COO
CH2
CH2
S
CH3
Aromatic Amino Acids
Tyrosine (TYR)
Phenylalanine (PHE)
H
H
NH3
+
-
C COO
CH 2
NH 3
+
-
C COO
CH 2
OH
Aromatic Amino Acids
Tryptophan (TRY)
H
NH 3
+
C
COO
CH 2
N
H
-
Secondary Amino Acids
Proline (PRO)
Hydroxyproline (HYPRO)
-
COO
-
COO
H2 N
+
H2 C
CH
CH2
CH2
H2N
+
H2 C
CH
CH 2
CH
OH
Amide Linkage
+
H3N
R1 O
C C O
R2 O
C C O
+
+
H3N
H
H
-
H
+
H3N
C
R2 O
O
C
N
+
-
H
O
C
C
R1
C
C
O-
H H
Amide Linkage
R1
H3N
H2O
R2 O
+
N
C
H
H
C
O
-
Amide Linkage and Peptides
-
O
COO
R3
C H H
CH
R1 LINKAGE
C N
Ncharacter
C N double-bond
C
AMIDE
- Some
H H
R2
O
H
1 unit of amino acid
Protein Structure
Primary Structure: due to covalent peptide bonds of amino acids.
H
+
HN3
C
O
C
R
N-terminal amino acid
R
H
O
(N C
C )n N C
H H
H
R
O
R O
C N C C OH H
C-terminal amino acid
Protein Denaturation
Denaturation
o
o
o
Changes in 2 , 3 , or 4 structure by
•
Heat.
•
Heavy metals (Hg is most common).
•
pH (trichloroacetic acid or phosphotungstic acid)
•
Salt (NaCl or ammonium sulfate [NH4]2 SO4)
Reasons for Precipitating Proteins
1.
Purify, concentrate protein.
2.
Remove protein which cause turbidity or emulsion troublesome.
Protein Denaturation
Effects of Denaturation
1. Decreased solubility.
2. Alteration of size and shape
3. greater reactivity
4. Decreased biological activity (enzyme +
immune proteins)
5. Increased sensitivity to electrolytes.
6. Nutritive value.
Protein Quality
What is Essential Amino Acids?
Amino acids which the body cannot make (or make
enough of) for protein synthesis due to lack of enzymes.
Essential Amino Acids:
Histidine,
Methionine,
Isoleucine,
Leucine,
Lysine,
Phenylalanine, Threonine, Valine
Limiting Amino Acid
The essential amino acid which is lacking in the protein to have a
balanced protein.
Product
Limiting Amino Acid
Corn
Lysine
Oats
Lysine
Rice
Lysine
Wheat
Lysine
Cow’s Milk
Methionine
Potato
Green Pea
Methionine
Methionine
Cotton Seed
Isoleucine
Beef
Valine
Chicken
Tryptophan