• 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