Proteins

Question 1

What are proteins?

Answer 1

Proteins are polypeptides made up of monomers called amino acids

Question 2

What elements do proteins contain?

Answer 2

Carbon, hydrogen, oxygen and nitrogen

Question 3

What is the general structure of an amino acid?

Answer 3

A central carbon atom bonded to a carboxyl group, amine group, a R group and a hydrogen atom

Question 4

How are peptide bonds formed?

Answer 4

The OH in the carboxyl group of one amino acid and the H in the amine group of another amino acid react together. They form a dipeptide and a water molecule is also produced (condensation reaction). The remaining carbon atom from the first amino acid bonds to the nitrogen atom of the second amino acid

Question 5

How is a polypeptide formed?

Answer 5

When many amino acids are joined together by peptide bonds

Question 6

Which enzyme catalyses the formation of peptide bonds?

Answer 6

Peptidyl transferase

Question 7

What is the primary structure of a protein?

Answer 7

It is the sequence of amino acids (determined by DNA) Sequence of amino acids- folding of the polypeptide- shape of the protein- function of the protein.
Peptide bonds are involved

Question 8

What is the secondary structure of a protein?

Answer 8

Oxygen, hydrogen and nitrogen atoms interact to form hydrogen bonds

Question 9

What are the two shapes of the secondary structure?

Answer 9

Alpha helix (coil shape)
Beta pleated sheet (polypeptide chains lie parallel to each other and are joined by hydrogen bonds)

Question 10

What is the tertiary structure of a protein?

Answer 10

The folding of a protein into its final shape. Involves R group interactions.

Question 11

Which R group interactions can take place?

Answer 11

1- Hydrogen bonds
2- Ionic Bonds (between oppositely charged R groups)
3- Hydrophobic and hydrophilic interactions (between polar and non polar R groups)
4- Disulfide bonds (covalent bonds between R groups that have sulfur atoms)
In order of weakest from strongest

Question 12

What is the quaternary structure of a protein?

Answer 12

Proteins that have more than one polypeptide chain (subunit)
The same bonds occur in the quaternary structure as the tertiary, but they are between two different protein molecules rather than one.

Question 13

How are peptide bonds broken?

Answer 13

A water molecule is needed to break the peptide bond (hydrolysis reaction). This is catalysed by the enzyme protease

Question 14

What test do you carry out for the identification of proteins?

Answer 14

Biuret test
Add biuret reagent (sodium hydroxide and copper sulfate) to the sample
If the solution turns from blue to purple, proteins are present.

Question 15

What is the purpose of thin layer chromatography?

Answer 15

It can be used to separate and identify amino acids based on their solubility (more soluble amino acids travel further)
The rate at which amino acids move through the mobile phase depends on the hydrogen bonds they have with the stationary
phase

Question 16

What is the mobile and stationary phase?

Answer 16

The mobile phase picks up the amino acids and moves through the stationary phase- separating the amino acids
Stationary phase- silica gel is used and amino acids are added
Mobile phase- end is submerged in an organic solvent, which moves through the silica gel

Question 17

How can you calculate Rf value?

Answer 17

Distance travelled by compound/ distance travelled by solvent front

Question 18

What are globular proteins?

Answer 18

Compact, water soluble, spherical in shape

Question 19

Why are globular proteins soluble in water and spherical in shape?

Answer 19

When globular proteins fold into their tertiary structure, the hydrophobic R groups are kept away from the aqueous environment and the hydrophilic R groups are on the outside of the protein.
This means they can be easily be transported around organisms

Question 20

What is a conjugated protein?

Answer 20

Globular proteins with a non protein component (prosthetic group)
Example of prosthetic group- haem group which have an Fe2+ ion.

Question 21

Insulin (G)

Answer 21

• Hormone involved in the regulation of BGC
• Needs to be soluble as hormones are transported in the bloodstream
• Need to have a precise shape as hormones have specific receptors on cell surface membranes
• 2 polypeptide chains, held together by disulfide bonds

Question 22

Haemoglobin (G)

Answer 22

• Red, oxygen carrying pigment found in red blood cells
• Made from 4 polypeptides, 2 alpha and 2 beta subunits
• Contains a prosthetic haem group, the iron in each haem group can combine reversibly with oxygen (enables it to carry oxygen around the body)

Question 23

Catalase (G)

Answer 23

• Is an enzyme, which catalyses reactions (speeds up the rate)
• Quaternary protein, containing 4 prosthetic haem groups
• Fe2+ in the haem group interacts with hydrogen peroxide and speeds up its breakdown since it is damaging to cells

Question 24

What are fibrous proteins?

Answer 24

Made from long, insoluble molecules. Tend to make strong, long molecules which are not folded into 3D shapes like globular proteins

Question 25

Why are fibrous proteins insoluble and very organised?

Answer 25

There are a high proportion of amino acids which have hydrophobic R groups in their primary structure- Insoluble
There are a limited range of amino acids, quite repetitive- organised
This makes them suitable for structural roles

Question 26

Keratin (F)

Answer 26

• Present in hair, skin and nails
• Has a large proportion of cysteine (sulfur containing amino acid) resulting in many strong disulfide bonds, forming strong and insoluble material

Question 27

Elastin (F)

Answer 27

• Found in elastic fibres (walls of blood vessels and alveoli of lungs)
• Gives the structures flexibility to expand when needed and then return to their normal size
• Quaternary protein

Question 28

Collagen (F)

Answer 28

-Connective tissue found in skin, tendons, ligaments
-Made of 3 polypeptides wound together in a rope like structure (held together by hydrogen bonds)
-When the triple helixes are parallel to each other, cross links are formed- fibrils-fibres
Molecules are staggered so there are no weak spots
The triple helix is packed very tightly as every 3rd amino acid is glycine ( R group H- smallest )