Chapter 3- 3.7- Types of protein Flashcards
What are the two main groups of proteins ?
Globular proteins
Fibrous proteins
What are globular proteins?
Globular proteins are compact, water soluble, and usually roughly spherical in shape.
They form when proteins fold into their tertiary structures in such a way that the hydrophobic R-groups on the amino acid are kept away from the aqueous environment.
The hydrophilic R-groups are on the outside of the protein. This means the proteins are soluble in water.
This solubility is important for many different functions of globular proteins. They are essential for regulating many of the processes necessary to life. This includes processes such as chemical reactions, immunity, muscle contraction and any more.
What is insulin ?
Insulin is a globular protein. It is a hormone involved in the regulation of blood glucose concentration. Hormones are transported in the bloodstream so need to be soluble. Hormones also have to fit into specific receptors on cell-surface membranes to have their effect and therefore need to have precise shapes.
What are conjugated proteins ?
They are globular proteins that contain a non-protein component called a prosthetic group.
Proteins without prosthetic groups are called simple proteins.
There are different types of prosthetic groups.
Lipids of carbohydrates can combine with protein forming lipoproteins or glycoproteins.
Metal ions and molecules derived from vitamins also form prosthetic groups.
These are called cofactors when they are necessary for the proteins to carry out their functions.
Haem groups are example of prosthetic groups. They contain an iron II ion (Fe^2+).
Catalase and haemoglobin both contain Haem groups.
What is haemoglobin ?
Haemoglobin is the red, oxygen-carrying pigment found in red blood cells.
It is a quaternary protein made from four polypeptides, two alpha and two beta subunits.
Each subunit contains a prosthetic haem group. The iron II ions present in the Haem groups are each able to combine reversibly with an oxygen molecule.
This is what enables haemoglobin to transport oxygen around the body.
It can pick oxygen up in the lungs and transport it to the cells that need it, where it is released.
What determines the role the protein will play in the body ?
The complex tertiary and quaternary structure
What is catalase ?
Catalase is an enzyme. Enzymes catalyse reactions, meaning they increase reaction rates, and each enzyme is specific to a particular reaction or type of reaction.
Catalase is a quaternary protein containing four Haem prosthetic groups.
The presence of the Iron II ions in the prosthetic groups allow catalase to interact with hydrogen peroxide and speed up its breakdown. Hydrogen peroxide is a common byproduct of metabolism but is dating to cells and cell components of allowed to accumulate. Catalase makes sure this doesn’t happen.
What are fibrous proteins ?
Fibrous proteins are formed from long, insoluble molecules. This is due to the presence of a high proportion of amino acids with hydrophobic R-groups I’m their primary structures.
They contain a limited range of amino acids, usually with small R-groups. The amino acid sequence in the primary structure is usually quite repetitive.
This leads to very organised structures reflected in the role fibrous proteins often have.
Keratin, elastin, and collagen are examples of fibrous proteins.
Fibrous proteins tend to make strong, long molecules which are not folded into complex three-dimensional shapes like globular proteins.
What is keratin ?
Keratin is a group of fibrous proteins present in hair, nails and skin.
It has a large proportion of the sulphur containing amino acid, cysteine.
This results in any strong disulphide bonds/ bridges forming strong, inflexible, and insoluble materials.
The degree of disulphide bonds determines the flexibility- hair contains fewer bonds making it more flexible than nails, which contains more bonds.
The unpleasant smell produced when hair or skin is burnt is due to the presence of relatively large quantities of sulphur in these proteins.
What is elastin?
Elastin is a fibrous protein found in elastic fibres (along with small protein fibres).
Elastin fibres are present in the walls of blood vessels and in the alveoli of the lungs- they give these structures the flexibility to expand when needed, but also to return to their normal size.
Elastin is a quaternary protein made from many stretchy molecules called tropoelastin.
What is collagen?
Collagen is another fibrous protein. It is a connective tissue found in skin, tendons, ligaments, and the nervous system.
There are a number different forms but all are made up of three polypeptides wound together in a long and strong rope-like structure.
Like rope, collagen has flexibility.
What is the structure of elastin?
Elastin is made by linking many soluble tropoelastin protein molecules to make a very large, insoluble, and stable, cross-linked structure.
Tropoelastin molecules are able to stretch and recoil without breaking, acting like small springs. They contain alternate hydrophobic and lysine-rich areas.
Elastin is formed when multiple tropoelastin molecules aggregate via interactions between the hydrophobic areas.
The structure is stabilised by cross-linking covalent bonds involving the amino acid lysine, but the polypeptide structure still have flexibility.
Elastin confers strength and elasticity to the skin and other tissues and organs in the body.
What is the structure of collagen?
Collagen molecules have three polypeptides chains wound around each other in a triple helix structure to form a tough, rope-like protein.
Every third amino acid in the polypeptide chains is glycine, which is a small amino acid. Its small size allows the three protein molecules to form a closely packed triple helix.
Many hydrogen bonds form between the polypeptide chains forming long quaternary proteins with staggered ends.
These allow the proteins to join end to end, forming long fibres called tropocollagen.
The tropocollagen fibrils cross-link to produce strong fibres.
Collagen also contains high proportions of the amino acids proline and hydroxyproline. The R-groups in these amino acids repel each other and this adds to the stability of collagen.
In some tissues, multiple fibres of collagen aggregate into larger bundles. This is the structure found in ligaments and tendons.
In skin, collagen fibres form a mesh that is resistant to tearing.
