Each amino acid aa shares a common structure ie an amine NH 2 group an acid group COOH and a central carbon atom bonded to hydrogen and to a side chain R The side chains qualify aas as acidic basic neutral aromatic and sulphurcontaining amino acids ID: 914830
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Slide1
Slide2PROTEINS
Proteins are polymers of amino acids referred to as polypeptides.
Each amino acid (aa) shares a common structure; i.e. an amine (NH
2
) group, an acid group (COOH), and a central carbon atom bonded to hydrogen and to a side chain (R).
The side chains qualify aas as acidic, basic, neutral, aromatic, and sulphur-containing amino acids.
Slide3Slide4Slide5Slide6Slide7Slide81. There is a total of 20 amino acids, which can be transferred by ribonucleic acid (RNA) for protein biosynthesis.
2. There are 7 neutral acids (tryptophan is not dissociated); 3 polar acids, which carry an OH; and 2 sulphur amino acids
Slide9-The hydrocarbon groupings, -CH
3
, -CH
2
, =CH- have hydrophobic character, which increases with chain length. In decreasing order these are Trp, Phe, Ile, Leu, Pro, Val, Met, Ala (also Met).
-The polar non-dissociated groups, e.g. -OH (Ser, Thr, Tyr), -SH (Cys), and-CO-NH
2
may form hydrogen bonds,
Slide10particularly with water, because of their hydrophilic nature
-Two molecules of Cystein can be oxidised to one molecule of cystine with an –S=S bond between the –SH
HOOC-CHNH
2
-CH
2
-SH + HS-CH
2
-CHNH
2
-COOH Cystein + Cystein HOOC-CHNH2CH2-S=S-CH2-CHNH2-COOH + H2 Cystine molecule
Slide11The dissociated groups e.g. –COOH (Asp, Glu) and NH
2
or –NH (Lys, Arg, His) increase the hydrophilic character and influence the pH of proteins.
3
.
Amino acids with the exception of glycine, have an optical activity due to the presence of an asymetric carbon
Slide124. Essential Amino acids
-Are those with branched chain (or aromatic) (Val, Leu, Ileu, Phe, Trp), also with one hydroxyl (Thr), one basic amino acid (Lys) and one sulphur amino acid (Met). They are not synthesized by the majority of higher animals. Thyrosine may replace Phe, and cystein may replace Met.
Slide13Proteins:
are composed of aas joined by peptide bonds, which connect the end of one aa with the amino end of the other.
Slide14Most proteins are a few dozen to several hundred aas long.
Proteins are either conjugated or nonconjugated.
Conjugated proteins are proteins combined with nonprotein substances e.g. CHOs and lipids into complex molecules.
Slide15-glycoprotein CHO
-lipoprotein lipids (fat)
-phosphoprotein phosphate molecules
-metaloprotein metal ions
-chromoproteins colour pigments
Nonconjugated proteins are not bound to any nonprotein substances
Slide16Food protein structures and Shapes
Food Proteins
Structure
Shape
Egg albumen
Globular
Spherical
Meat and legume globulins
Globular
Spherical
Collagen
Fibrous
Elongated
Elastin
Fibrous
Elongated
Glycoprotein: ovomucoid
Conjugated
Prot. bound to carbohydrate and haemagglutinin
Slide17Food protein structures and shapes (continued)
Food proteins
Structure
Shape
Lipoproteins: chylomicron
Conjugated
Prot. bound to lipid, LDL and VLDL*
Metalloproteins:
haemoglobin
Conjugated
Protein bound to metal e.g. ferritin and myoglobin
Phosphoprotein: casein
Conjugated
Protein bound to phosphorus
*LDL = Low density lipoprotein; VLDL = Very low density lipoprotein
Slide18Protein Structures
Within the protein itself, 4 specific levels of structure have been identified. These levels describe the composition of a polypeptide and its shape.
Primary (1º) structure – is the linear sequence of aas in order within a polypeptide e.g. gly-gly-pro- etc.
Slide19Slide20Secondary (2º) structure
refers to conformation taken by the polypeptide chain, stabilized by two H bonds between the skeletal structures and not the side chains i.e. between the H of –NH and the O of –C=O. The aas together assume either an
α
-halix or
β
-sheet configuration within a polypeptide.
Slide21Slide22Tertiary (3º) protein configuration describes the overall three-dimensional shape achieved by the folding of the entire protein molecule. The structure is stabilised by hydrogen and disulphide bonds between aas in close proximity with one another due to the folded situation of the 3º structure.
Slide23Slide24Quaternary (4º) structure:
If a protein contains more than one polypeptide chain in its structure, its overall spatial structure is quaternary.
Slide25
Slide26Proteins are typically either globular or fibrous in shape.
Globular proteins are soluble in water e.g. enzymes.
Fibrous proteins are generally insoluble in water e.g. collagen
Slide27Differences exist in the degree of stability of food proteins.
Native protein is protein as exists in its original form.
This form is rather fragile and many reagents and conditions can cause slight or extensive changes in the structure.
Slide28Denaturation is the change caused to the original structure of a protein. It generally leads to the unfolding of the tertiary structure of the protein.
Denaturation can be caused by:
-Acids, alkalis, alkaloids, heavy metal salts, urea and methanol.
-Iodide, bromide, and chloride ions
Slide29