Proteins (2.9)
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Amino acids are the monomers from which polypeptides and proteins are made.
Polypeptides are linear chains of more than two amino acids, whereas proteins consist of one or more polypeptides folded into a specific 3D functional structure.
Amino acids contain a central carbon atom, attached to a hydrogen atom, an amino group (), a carboxylic acid group () and a variable side chain group (R group).
There are 20 amino acids common to all organisms and they differ only by their R group.
Essential amino acids come from the diet and non-essential ones can be synthesized in the body.
Polypeptides are formed when amino acid monomers are linked to each other during condensation reactions by peptide bonds (strong covalent bonds).
The peptide bond is formed between the carboxyl group of one amino acid and the amino group of another amino acid.
The reaction is catalysed by peptidyl transferase.
There are four levels of protein structure:
Primary structure refers to the specific sequence and number of amino acids in a polypeptide chain, determined by the DNA base sequence.
This linear sequence determines how the protein folds into its three-dimensional structure and therefore its function. A change in the amino acid sequence can alter the protein’s structure and affect its function.
Secondary structure refers to the localised folding of a polypeptide chain into -helices and -pleated sheets
In an -helix, hydrogen bonds form between atoms in the polypeptide backbone, causing the chain to coil into a spiral.
In a -pleated sheet, several polypeptide chains lie parallel to one another and are held together by hydrogen bonds. These hydrogen bonds stabilise the secondary structure of the protein.
Tertiary structure is the overall three-dimensional shape of a protein, determined by interactions between the R groups of amino acids. The shape of a protein is closely related to its function.
Tertiary structure is stabilised by:
- Hydrogen bonds between polar R groups.
- Ionic bonds between oppositely charged R groups (impacted by ).
- Disulfide bonds / bridges are covalent bonds between cysteine residues (impacted by ).
- Hydrophobic interactions, where non-polar R groups cluster together away from water.
- Hydrophilic interactions, where polar R groups are attracted to water and orientate towards the outside of the protein.
Quaternary structure occurs when two or more polypeptide chains (which may be identical or different) associate to form a functional protein.
The subunits are held together by the same interactions found in tertiary structure (hydrogen bonds, ionic bonds, disulfide bonds and hydrophobic / hydrophilic interactions), but these occur between polypeptides rather than within one polypeptide.
For example, haemoglobin consists of two -globin and two -globin subunits and is only functional when all four polypeptides are associated.
Globular proteins are compact, roughly spherical in shape, and soluble in water meaning that they can be transported in the circulatory system.
Examples include haemoglobin, enzymes and antibodies.
Fibrous proteins are long, insoluble polypeptide chains with structural roles, such as in keratin and collagen.
They are not folded into compact shapes and have repetitive amino acid sequences. Their strength comes from extensive hydrogen bonding and additional cross-links, such as disulfide bonds, between chains.
These proteins form strong fibres and are well suited to support and protection.
Collagen is a fibrous protein found in connective tissues such as skin, tendons, ligaments, cartilage and the nervous system.
It consists of three polypeptide chains wound together in a triple helix, like a rope, making it strong yet flexible. The chains are held together by hydrogen bonds and covalent cross-links, and are staggered to avoid weak points.
Collagen molecules form fibrils and fibres and are insoluble in water.