2.4 Proteins Flashcards
How are proteins formed?
Amino acids are linked by condensation reactions to form polypeptides. This process happens on ribosomes by a process called translation. Polypeptides are the main component of proteins and sometimes they are the only component.
A condensation reaction involves the OH of the carboxyl group and the H of an amine group. This forms a peptide bond.
How many amino acids are there in humans?
There are 20 different amino acids in polypeptides synthesised on ribosomes. The difference between them is the R group.
HOWEVER some proteins contain amino acids that are not in the basic repertoire of twenty. In most cases this is due to one of the twenty being modified after a polypeptide has been synthesised. There is an example of modification of amino acids in collagen, a structural protein used to provide tensile strength in tendons, ligaments, skin and blood vessel walls. Collagen polypeptides made by ribosomes contain proline at many positions but at some of them it is converted to hydroxyproline which makes collagen more stable.
Why are those 20 amino acids used for nearly all organisms?
We are not sure but there are many hypothesises:
- These 20 amino acids were the ones produced by chemical processes on Earth before the origin of life, so all organisms used them and have continued to use them. Other amino acids might have been used, if they had been available.
- They are the ideal 20 amino acids so natural selection will always favour organisms that use them and not other organisms.
- All life has evolved from a single ancestral species, which used these 20 amino acids. Because of the way that polypeptides are made by ribosomes it is difficult for any organism to chance the repertoire of amino acids.
Why are there so many different types of polypeptide?
Because the 20 amino acids can be linked together in any sequence which gives a huge range of possible polypeptides.
The number of amino acid sequences is immense but living organisms only actually produce a small fraction of these. Even so, a typical cell produces polypeptides with thousands of different sequences and must store information needed to do this.
How does the amino acid sequence come to be?
Amino acid sequences are coded for by genes.
How are proteins and polypeptides linked?
A protein may consist of a single polypeptide or more than one polypeptide linked together.
Integrin has 2 for example, and collagen has 3, hemoglobin has 4.
What are fibrous proteins?
Fibrous proteins like collagen are elongated with a repeating structure.
What are globular proteins?
Globular proteins fold up as they are made, to develop the final conformation. This is stabilised by bonds between the R groups of the amino acids that have been brought together by folding.
How do amino acids ensure the shape of a protein?
In globular proteins bonds between the R groups stabilise the folding as the protein is made. If the proteins are soluble in water then there will be hydrophilic R groups on the outside and hydrophobic R groups on the inside also keeping the shape of the molecule.
In fibrous proteins the amino acid sequence prevents folding up and ensures that the chain of amino acids remains in elongated form.
Why does pH effect enzymes?
In extreme pH’s the charges on the R groups are changed, this breaks the ionic bonds within the protein or causes new ionic bonds to form. The 3 dimensional structure then changes and some proteins dissolve and the protein no longer functions.
Why does temperature effect enzymes?
Heat causes an increase in energy which causes the molecules to vibrate more. These vibrations within the molecules break intermolecular bonds or interactions.
Why do enzymes stay denatured?
This is because soluble proteins often become soluble and form a precipitate, this is because these are the hydrophilic proteins that were kept away from water on the inside but where revealed by the shape changing.
What are proteins used for?
- Enzymes
- Muscle contraction - actin and myosin cause contraction of muscles
- Cytoskeletons - tubulin is the subunit of microtubules that give animals their shape and pull of chromosomes during mitosis.
- Tensile strengthening - fibrous proteins give tensile strength needed in skin, tendons, ligaments and blood vessel walls.
- Blood clotting - plasma proteins act as clotting factors.
- Transport of nutrients and gases.
- Cell adhesion - membrane proteins cause adjacent animal cells to stick together in tissues.
- Membrane transport
- Hormones
- Receptors
- Packing of DNA
- Immunity - making antibodies
What is rubisco?
The name is an abbreviation for ribulose bisphosphate carboxylase. The shape and chemical properties of its active site allow it to catalyse the reaction that fixes carbon dioxide from the atmosphere, which provides the source of carbon from which all carbon compounds needed by living organisms can be produced. It is present in high concentrations in leaves.
What is insulin?
This is a hormone produced as a signal to many cells in the body to absorb glucose and help reduce the glucose concentration of the blood. It binds reversibly to receptors on membranes. The shape and chemical properties of the insulin molecule correspond precisely to the binding site on the receptor, so insulin binds to it, but not other molecules. Insulin is secreted by B cells in the pancreas and is transported by the blood.
