BM: Proteins

Question 1

What are the monomers of proteins?

Answer 1

Amino acids

Question 2

What are amino acids?

Answer 2

The monomer unit which combine to form polypeptides.

Polypeptides then combine to form proteins.

Question 3

What is formed when two amino acids join?

Answer 3

A dipeptide

Question 4

What is formed when more than two amino acids join together?

Answer 4

Polypeptides.

Question 5

What are proteins made up?

Answer 5

One or more polypeptides.

Question 6

What is the general structure of an amino acid?

Answer 6

Question 7

How many amino acids have been discovered?

How many of these are naturally occuring.

Answer 7

About 100 have been identified.

20 of which naturally occur in all living organisms - this provides evidence for evolution.

Question 8

What is the only difference between the 20 naturally reoccuring amino acids?

Answer 8

The ‘R’ variable group.

Glycine is the only amino acid that doesn’t have carbon in its R group. It consists of just one hydrogen atom.

Question 9

What bond is formed by the joing of two amino acids?

Answer 9

A peptide bond.

Question 10

Draw a condensation reaction between two amino acids:

Answer 10

Question 11

How do amino acids join together?

Answer 11

Through condensation reactions.

A water molecule is released and a peptide bond forms.

Question 12

How many structural levels do all proteins have?

How many can some larger proteins have?

Answer 12

All have 3 levels - primary, secondary and tertiary.

Some have 4 levels - an additional quaternary structure.

Question 13

Through what process can many amino acids join together?

Answer 13

Polymerisation.

This results in the formation of a polypeptide.

Question 14

What forms the primary structure of a protein?

Answer 14

The sequence of amino acids in the polypeptide chain.

Question 15

What determines the sequencing of amino acids in the primary structure of a protein?

Answer 15

DNA.

Question 16

What determines the ultimate shape and function of a protein?

Answer 16

The primary structure.

A change in sequencing of amino acids results in bonds forming in different places, and the protein folding differently.

This changes its shape which is very specific to its function.

Question 17

What causes hydrogen bonds to form in the secondary structure of the protein?

Answer 17

• Linked amino acids contain both -NH and -C=O groups on either side of peptide bond.
• H of -NH group has an overall positive charge.
• O of -C=O group has an overall negative charge.
• Therefore, a weak hydrogen bond is formed between them.

Question 18

Describe the secondary structure of a protein:

Answer 18

• Hydrogen bonds form between amino acids in chain.
• Therefore, chain becomes a 3D shape by either:
  
  • Coiling into an alpha helix
  • Folding into a beta pleated sheet.

Question 19

Draw a diagram illustrating the difference between an alpha helix and a beta pleated sheet:

Answer 19

Question 20

What bonds are there within the primary structure of a protein?

Answer 20

Peptide

Question 21

What bonds are there within the secondary structure of a protein?

Answer 21

Hydrogen

Question 22

What bonds are there within the tertiary structure of a protein?

Answer 22

• Disulfide - fairly strong and not easily broken
• Hydrogen - numerous but easily broken
• Ionic - formed between carboxyl and amino groups not involved in peptide bond - weaker than disulfide and broken by changes in pH.

Question 23

Describe the tertiary structure of a protein:

Answer 23

• Polypeptide chain is coiled or folded further.
• More bonds form between different parts of the chain, such as ionic and hydrogen.
• Disulfide bridges form.
• Can result in final 3D structure.

Question 24

How do disulfide bridges form?

Answer 24

When two molecules of the amino acid cysteine come close together - sulfur atom in one molecule bonds to sulfur atom in another.

Question 25

What is important in determining how a protein functions?

Answer 25

Its 3D structure - this is distinctive allowing it to recognise, and be recognised by, other molecules.

This is first determined by the sequencing of amino acids.

Question 26

Describe the quaternary structure of some proteins:

Answer 26

• Two or more polypeptide chains joining together.
• Sometimes, there may also be a non-protein group associated with the molecule.
  
  • Eg, the haem group in haemoglobin.

Question 27

Give examples of proteins that have a quaternary structure:

Answer 27

• Haemoglobin
• Insulin
• Collagen

Question 28

Draw a diagram to illustrate the structural levels of a protein:

Answer 28

Question 29

Name 4 different examples of proteins:

Answer 29

1. Enzymes
2. Antibodies
3. Transport proteins
4. Structural proteins

Question 30

Explain the structure of enzymes:

Answer 30

Usually roughly spherical due to tight folding of polypeptide chains.

Question 31

Describe the structure of antibodies:

Answer 31

Two light polypeptide chains and two heavy polypeptide chains bonded together, with a variable region so they can be highly specific.

Question 32

Describe the structure of transport proteins:

Answer 32

Hydrophobic amino acids and hydrophilic amino acids are both present, causing the protein to fold up and form a channel.

Question 33

Describe the structure of structural proteins:

Answer 33

Long polypeptide chains lying parallel to each other with cross-links between them, making them very strong.

Question 34

What is the test for proteins?

Answer 34

Biuret test

Question 35

What does the biurets test detect?

Answer 35

The presence of a peptide bond.

Question 36

How would you perform the Biuret test?

Answer 36

1. Place sample in test tube.
2. Add an equal volume of sodium hydroxide solution at room temperature, to make the solution alkaline.
3. Add a few drops of very dilute (0.05%) copper (II) sulfate solution.

Question 37

What are the possible results of the Biuret test?

Answer 37

Positive = solution turns purple

Negative = solution remains blue

The colours are pale, so you will need to look carefully.