The Variety of Proteins (DP IB Biology: SL): Revision Note

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

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