2.3.3 The Variety of Proteins

• Living organisms synthesize many different proteins with a wide range of functions
• Proteins are so versatile that they have many different roles in cells, tissues and organs
• All of the following functions are performed by proteins:
  • 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 eg. collagen, keratin
  • Transport of vital metabolites eg. 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

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 - approx 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, eg. the silk they use to encase their prey after capture

• The proteome is the full range of proteins that a cell or organism is able to produce
• By contrast, a genome is the complete set of genes present in a cell/organism
  • The full genome is present within every cell of an organism, but not every gene is expressed in every cell. Which genes are expressed, depends on the cell type

• The proteome is usually larger than the genome of an organism
• Every individual has a different proteome
  • Because of small differences in the amino acid sequence of proteins

• The proteome varies during an organism's lifetime as certain proteins are not needed throughout the organism's life
  • • An example is fetal haemoglobin. The gene for that protein is not expressed after the baby is around 3 months old, as the baby expresses adult forms of haemoglobin, which are encoded by separate genes

  • This is also due to a large amount of modification of proteins that can take place after synthesis (often in the Golgi apparatus)
    • For example, adding a carbohydrate part to form glycoproteins, which are important in cell signalling

• Splicing of RNA during transcription can allow one gene to code for many proteins