Proteins
- Know the structure of an amino acid (structures of specific amino acids are not required.)
- Understand the formation of polypeptides and proteins (as amino acid monomers linked by peptide bonds in condensation reactions).
- Understand the role of ionic, hydrogen and disulphide bonding in the structure of proteins
- Understand the significance of the primary,secondary tertiary and quaternary structure of a protein in determining the properties of fibrous and globular proteins, including collage and haemoglobin
- Understand how the structure of collagen and haemoglobin related to their function
Proteins
Amino Acids
Polypeptides
In a polypeptide there is always
Protein Structure
Secondary, tertiary and quaternary structures are largely held together by hydrogen bonds. The structure is dependent on the integrity of these bonds. If the bonds do break the chain will fold into random coils and the protein loses its function. This process is called denaturing and happens generally at >50 degrees or at low or high pH. Covalent bonds and the primary structure remain intact.
Conjugated Proteins
Amino Acids
- Proteins are made of amino acids. Amino acids contain Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur
- They contain both an amino group and an acid group
- The general structure of an amino acid molecule is an amino group, a hydrogen, an R group and a carboxyl acid group.
- There is a central carbon atom (α carbon) from which the four groups come off
Polypeptides
- Amino acids are joined together by peptide bonds. The reaction involves the formation of a molecule of water in another condensation polymerisation reaction
In a polypeptide there is always
- one end with a free amino (NH2) group, called the N terminus
- one end with a free carboxyl (COOH) group, called the C-terminus
Protein Structure
- Primary Structure
- The sequence of amino acids in the polypeptide chain. The primary structure determines the rest of the protein structure. Most polypeptide chains contain hundreds / thousands of amino acids
- Secondary Structure
- The most basic level of protein folding, and consists of a few basic structures. It is held together by hydrogen bonds between the carboxyl groups and the amino groups.
- The α-helix. The polypeptide chain is wound round to form a helix. There are so many hydrogen bonds that is very stable
- The β – pleated sheet. The polypeptide chain zig-zags back and forward forming a sheet of antiparallel strands. Held by hydrogen bonds.
- Tertiary Structure
- This is the complete structure formed by the folding u of a polypeptide chain. Each protein has a unique tertiary structure which relates to property and structure. It also decides the active site.
- The tertiary structure is held by bonds in the R-group
- Hydrogen bonds, which are weak but numerous
- Ionic bond between oppositely charged R-groups , these ionic bonds are stronger than hydrogen but not covalent
- Covalent bonds of sulphur called sulphur bridges between cysteine which are very strong
- Quaternary Structure
- Most working proteins are composed of more than one polypeptide chain. Quaternary is the arrangement of the different chains
- There are many different varieties; tetrahedral, rotating motor, Y, triple helix, double helix
- Most working proteins are composed of more than one polypeptide chain. Quaternary is the arrangement of the different chains
- They have a compact, roughly spherical structure. This includes enzyme, membrane proteins, receptors, transport proteins and storage proteins
- Haemoglobin is a globular protein found in red blood cells. One molecule is composed of four globular polypeptide chains called globins. There are two α chains with 141 amino acids each and there are two β chains with 146 amino acid each. This gives a total of 574 amino acids. Each chain contains a small non-polypeptide group called haem, which has an iron atom in the centre. They are attached to the globins by covalent sulphur bridges. One oxygen molecule can bind to each iron atom, so a haemoglobin molecule can bind up to four oxygen molecules. The haemoglobin can also “offload” the oxygen easily due to the polypeptide chains providing a suitable environment. This means haemoglobin can be an effective oxygen transport protein
- They are long and thin, much like ropes. They are generally used in the structure of things; collagen (bones), keratin (hair), tubulin (cytoskeleton), actin (muscle), fibrin (blood clots) and fibroin (silk). They are composed of many polypeptide chains
- Collagen is a fibrous protein found in bone and cartilage. A single molecule of collagen contains three long polypeptide chains linked by multiple hydrogen bonds and a triple helix. Each polypeptide chain is about 10,000 amino acids long. The amino acids are in a sequence of three.
- Collagen molecules bind together to form a fibril
- Collagen fibrils link to form fibres
- Collagen fibres have high tensile strength and give strength and flexibility to cartilage, ligaments, bone, skin and blood vessels. Collagen is also found in the cornea and lens of the eye
- Synthesis of collagen requires vitamin C, ( a lack of vitamin C leads to scurvy)
Secondary, tertiary and quaternary structures are largely held together by hydrogen bonds. The structure is dependent on the integrity of these bonds. If the bonds do break the chain will fold into random coils and the protein loses its function. This process is called denaturing and happens generally at >50 degrees or at low or high pH. Covalent bonds and the primary structure remain intact.
Conjugated Proteins
- This occurs when a protein is joined with / conjugated to another molecule called a prosthetic molecule
- This affects the performance and function of the molecule
- For example haemoglobin
- Are proteins with a carbohydrate prosthetic group
- The carbohydrate part allows the molecule to hold on to a lot of water and makes it harder for the molecule to be broken down
- Many lubricants (mucus) are glycoproteins
- Proteins conjugated with lipids
- Important in the transport of cholesterol
- There are two types Low density / High density Lipoproteins
- High density lipoproteins contain more proteins