Peptide Chain Diversity
- There is a large variety of proteins available to living organisms
- This is because:
- There are 20 naturally occurring amino acids that form the basic structure of a polypeptide chain
- Polypeptides can vary in length from a few to thousands
- The structure and amino acid sequence can also vary
- The genetic code, meaning DNA base sequence, codes for the number and order of amino acids in a polypeptide, and there is a huge variety of options for DNA base sequence
- 20 amino acids can give an almost infinite number of polypeptides
- Polypeptides are assembled at a ribosome by condensing individual amino acids onto a growing chain, one by one
- This allows a choice of 20 amino acids each time one is added
- The mRNA codon determines which amino acid is added
- For a polypeptide chain of 50 amino acids in length (considered a very short protein), there would be 2050 possible combinations of amino acids
- This gives 1.13 x 1065 combinations
- Given that the average length of a protein is 300 amino acids, the number of possible combinations is so large, we can consider it to be infinite
Role of proteins
- The range of proteins available means that they are very versatile so that they have many different roles in cells, tissues and organs, such as:
- Speeding up cellular reactions, or catalysis, is performed by enzymes
- Blood clotting, where blood proteins interact with oxygen to form a gel-like scab across a wound
- Strengthening fibres in skin, hair, tendons, blood vessels e.g. collagen, keratin
- Transport of vital metabolites e.g. oxygen which is carried by haemoglobin
- Formation of the cytoskeleton, a network of tubules within a cell that cause chromosomes to move during the cell cycle
- Cell adhesion, where cells in the same tissue stick together
- Hormones, chemical messengers that are secreted in one part of the body to have an effect elsewhere
- Compaction of DNA in chromosomes for storage, caused by histone proteins
- The immune response produces antibodies, the most diverse group of proteins
- Membrane transport channel and carrier proteins that determine which substances can pass across a membrane
- Cell receptors, which are binding sites for hormones, chemical stimuli such as tastes, and for other stimuli such as light and sound
Examples of polypeptides
Rubisco
- Ribulose Bisphosphate Carboxylase
- An enzyme that catalyses the fixing of CO2 from the atmosphere during photosynthesis
- Composed of 16 polypeptide chains as a globular protein
- This is the source of all organic carbon, so Rubisco is arguably the most important enzyme in nature!
- The most abundant enzyme on Earth as it's present in all leaves
- Rubisco is a very slow catalyst, but it's the most effective to have evolved so far to fulfil this vital function
Insulin
- A hormone produced and secreted by β-cells in the pancreas
- Binds to insulin receptors (on liver, fat and muscle cells) reversibly, causing absorption of glucose from the blood
- Composed of 2 polypeptide chains as a short, globular protein
Immunoglobulins
- Also known as antibodies
- They have a generic 'Y' shape, with specific binding sites at the two tips of the 'Y'
- They bind to specific antigens
- The binding areas of immunoglobulins are highly variable, meaning that antibodies can be produced against millions of different antigens
- Immunoglobulins (as the name suggests) are globular and are the most diverse range of proteins
Rhodopsin
- A pigment in the retina of the eye
- A membrane protein that is expressed in rod cells
- Contains a light-sensitive part, retinal, which is derived from Vitamin A
- A photon of light causes a conformational change in rhodopsin, which sends a nerve impulse along the optic nerve to the central nervous system
Collagen
- A fibrous protein made of three separate polypeptide chains
- The most abundant protein in the human body - approximately 25%
- Fibres form a network in skin, blood vessel walls and connective tissue that can resist tearing forces
- Plays a role in teeth and bones, helping to reduce their brittleness
Spider Silk
- The silk used by spiders to suspend themselves and create the spokes of their webs is as strong as steel wire though considerably lighter
- Contains rope-like, fibrous parts but also coiled parts that stretch when under tension, helping to cause extension and resist breaking
- Does not denature easily at extremes of temperature
- Has many attractive aspects for engineering and textile product design thanks to its strength and low weight
- Can be genetically engineered to be expressed in goats' milk as spiders can't be farmed on a large enough scale
- Other kinds of spider silk protein are tougher though lack the tensile strength, e.g. the silk they use to encase their prey after capture