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(Question 1: Egg Protein )
Principles
- Amino acids and the primary structure
- Peptides and the secondary structure
- Proteins and the tertiary and quatenary structure
- Protein functionality and enzymes
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Amino
Acids and the Primary Structure
There are 20 common amino acids are classified by their functional group, or their "R" group. depending on the nature of the R group they are categorized as: Nonpolar (aliphatic), Aromatic, Polar uncharged, Negatively charged or positively charged.
The following are illustrations of one amino acid from each category
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Leucine |
Tryptophan |
Asparagine |
Glutamate |
Arginine |
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Peptides and the Secondary Structure
Amino acids are joined together by the peptide bond:
The peptide bond is rigid and planar. This occurs because the C-N bond has 40% double bond character. This means that that bond is unable to rotate freely. This is a key factor is the spatial arrangement of of the peptide.
A peptide is then a series of amino acids joined together by a series of peptide bonds. The properties of a peptide are based on the amino acid sequence of the peptide from the amino terminus to the carboxyl terminus. This is the peptide's or proteins' primary structure. The polypeptide is flexible and has no definite conformation. However, the primary structure contains all the information to form the secondary and tertiary structure. Most polypeptides range from 2 - 50 amino acid residues in length. Most proteins have 50 - 2000 amino acid residues. However, they can be longer such as apolipoprotein B which has 4,536 amino acid residues.
The secondary structure is the local conformations of the peptide chain. There are two common repetitive forms:
The most common structure is the alpha helix.
This structure gives stability to the unit because there is hydrogen bonding
between the various peptide bond residues. To the left is a diagram of
an alpha helix. Notice the that the left handed helical backbone is made
up of the peptide chain. The R groups point out from the structure at a
90 degree angle. Therefore, steric hindrance between grourps is minimized.
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of the formation of an alpha helix secondary structure:Alpha/Beta
to Tertiary
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This is a diagram of a Beta Sheet.
Beta sheets are a combination of two or more beta strands. The strands
are held together and stabilized by hydrogen bonding. There are two types
of sheets: either parallel or anti parallel depending on the orientation
of the peptide chain.
These common repeating units (helixes and beta sheets) are joined by
unstructured loops and reverse turns. This combination of structures will
then fold to form the active form of the protein.
Proteins and the Tertiary and Quantenary Structure
The tertiary structure is the folded conformation of the protein. The structure is stabilized most commonly by hydrophobic bonding on the inside (exclusion of water) and the formation of disulfide bonds. The outer amino acid residues are often polar in nature and thus allow the protein to function within the cell. The hormone, insulin is one of the smallest proteins. It is only 51 residues long and consists of two polypeptide chains. Below is the ribbon model and the space filled model of it's 3D structure.
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Often proteins are made up of more than one polypeptide chains. The
joining together of these tertiary units is the quantenary
structure. Below are diagrams of myoglobin, which only
has tertiary structure (because it is composed of one chain) and hemoglobin.
Hemolglobin is composed of four chains.
Myoglobin |
Hemoglobin (2 alpha units, 2 Beta units) |
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Protein Functionality and Enzymes
Protein function involves the binding involves the binding of other molecules called ligands. Ligands may be any kind of molecule even other proteins. Ligands bind to the protein at a certain site referred to as the binding site. The binding site is particular to the ligand with respect to shape, charge and other chemical properties.
Some proteins contain other chemical groups besides amino acids. They are called conjugated proteins. The non-amino acid part of a conjugated protein is usually called the prosthetic group. Therefore, conjugated proteins are classified on the chemical nature of their prosthetic group.
- Class - Prosthetic Group - Example
- Lipoproteins - Lipids - Chylomicrons
- Glycoproteins - Carbohydrates - Immunoglobulin
- Phosphoproteins - Phosphates - Caseine
- Hemoproteins - Heme (Iron porphyrin) - Hemoglobin
- Flavoproteins- Flavin nucleotides - Succinate Dehydrogenase
- Metalloproteins - Fe, Zn, Ca, Mb or Cu -Ferritin (Fe)
