• 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

  Exam Tip

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

Proteome

• 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

Exam Tip