27 WJEC Biology AS Level - Marianne Izen - 2nd Edition Flashcards
What are the four ways by which the tertiary structure of proteins is maintained?
Hydrogen bonds
Ionic bonds
Disulphide bonds
Hydrophobic interactions
Explain how to perform the test for proteins.
The test for protein is called the biuret test
To test a solution for protein, a few drops of biuret reagent (sodium hydroxide and copper (II) sulphate) are added, although they can be added separately.
The sodium hydroxide and copper sulphate react to make blue copper hydroxide.
If a protein is present, the copper hydroxide interacts with the peptide bonds in the protein to make biuret, which is purple.
So, the colour change for a positive biuret test is blue → purple.
Explain how to interpret the test for proteins.
The colour change for a positive biuret test is blue → purple.
At low protein concentration, the colour change is difficult to detect by eye.
The more concentrated the protein, the darker the purple colour, so the test is qualitative.
It could be used as a semi-quantitative test, comparing the intensity of purple in two identically treated solutions.
Measuring the absorbance of the purple biuret in a colorimeter, using a yellow (580 nm) filter gives a numerical estimate of relative concentration of proteins present in a sample.
This is also semi-quantitative as an actual protein concentration is not measured.
To measure the actual concentration, making the test quantitative, a biosensor is needed.
Describe the tertiary structure of protein
The a-helix of the secondary protein structure can be folded and twisted to give a more complex, compact 3D structure.
This is the tertiary structure.
How is the tertiary structure of a protein maintained?
The shape of tertiary structure of a protein is maintained by:
Hydrogen bonds.
lonic bonds.
Disulphide bonds
Hydrophobic interactions.
These bonds are important in giving globular proteins, e.g. enzymes, their shape.
Draw a table linking the bond types and the level of protein structure
Describe the structure of collagen
A single fibre, sometimes called tropocollagen, consists of three identical polypeptide chains twisted around each other, like a rope.
The three chains are linked by hydrogen bonds, making the molecule very stable.
A - 2
B - 3
C - 1
D - 4
Describe the quartenary structure of proteins
Some polypeptide chains are not functional unless they are in combination.
In some cases, they may combine with another polypeptide chain, such as the insulin molecule, which has two chains.
They may also be associated with non-protein groups and form large, complex molecules, such as haemoglobin.
What do the roles of proteins depend upon?
The roles of proteins depend on their molecular shape.
Describe fibrous proteins
Fibrous proteins have long, thin molecules and their shape makes them insoluble in water, so they have structural functions, as in bone.
The polypeptides are in parallel chains or sheets, with many cross-linkages forming long fibres, for example keratin, the protein in hair.
Fibrous proteins are strong and tough.
Collagen is a fibrous protein, providing the strength and toughness needed in tendons.
Describe globular proteins
Globular proteins are compact and folded into spherical molecules.
This makes them soluble in water and so they have many different functions, including enzymes, antibodies, plasma proteins and hormones.
Haemoglobin is a globular protein, consisting of four folded polypeptide chains, at the centre of each of which is the iron-containing group, haem.
Describe hydrophobic interaction
The R groups of some amino acids contain-CH, and -C2H5, groups, which are hydrophobic.
These groups cluster in the middle of the protein molecule.
Their positioning together, away from water molecules, is called hydrophobic Interaction.
What class of molecules does the haem group belong to?
The haem group is in the class of non-protein molecules called porphyrin rings.
Describe the porphyrin ring in chlorophyll.
In chlorophyll, a porphyrin ring also combines with protein, but the metal ion it contains is magnesium, not iron.
