The Variety of Proteins (SL IB Biology)

• 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