1.6 - Proteins

Question 1

What is an amino acid?

Answer 1

Amino acids arc the basic monomer units which combine to make up a polymer called a polypeptide

Question 2

What can polypeptides be combined to form?

Answer 2

Polypeptides can be combined to form proteins.

Question 3

How many naturally occurring amino acids are there?

Answer 3

There are 20 amino acids that occur in all living organisms. - evidence for evolution

Question 4

What is the general structure of an amino acid?

Answer 4

H       H
   \      /
      N
       |
R -  C - H
       |
O = C - O
             |
            H

Question 5

What is an amino group?

Answer 5

amino group (-NH ) - a basic group from which the amino part of 2
the name amino acid is derived

Question 6

What is a carboxyl group?

Answer 6

carboxyl group (-COOH) - an acidic group which gives the amino
acid the acid part of its name

Question 7

What is an R - Group?

Answer 7

a variety of different chemical groups. Each amino acid has a different R group. These 20 naturally occurring amino acids differ only in their R (side) group.

Question 8

What do two amino acid monomers join to form?

Answer 8

A dipeptide.

Question 9

How are the two amino acid monomers joined?

Answer 9

condensation reaction. The water
is made by combining an - OH from the carboxyl group of one amino acid with an - H from the amino group of another amino acid.

Question 10

What is the bond that links two amino acids?

Answer 10

peptide bonds between the carbon of one amino acid and the nitrogen of another.

Question 11

What is the process in which many amino acids are joined together?

Answer 11

Polymerisation = Polypeptide.

Question 12

What is the primary structure of a protein?

Answer 12

The sequence of amino acids in a polypeptide chain forms the primary structure of any protein.

Question 13

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

Answer 13

DNA

Question 14

What is the secondary structure of a protein?

Answer 14

Hydrogen bonds are formed and this causes the long polypeptide chain to be twisted into a 3-D shape - ⍺-helix or β-pleated sheet.

Question 15

What is the tertiary structure of proteins?

Answer 15

The ⍺-helices of secondary structure can be twisted and folded even more to give a complex and specific 3-D structure.

Question 16

What type of bonds help maintain the Tertiary Structure of a protein?

Answer 16

1. Disulfide Bridges
2. Ionic Bonds
3. Hydrogen Bonds

Question 17

What is a Disulfide Bridge?

Answer 17

A disulfide bridge is a strong bond that can form between two cysteines. The strength of disulfide bridges helps stabilize a protein. Disulfide bridges are especially common in proteins that are secreted from cells.

Question 18

What is the Quarternary Structure of a protein?

Answer 18

A complex molecule that contains more than one polypeptide chain that are linked in various ways.There may also be non-protein (prosthetic) groups associated with
the molecules such as the iron-containing haem group in haemoglobin.

Question 19

What determines the function of a protein?

Answer 19

The sequence of amino acids determines the 3-D structure of the protein. The 3-D shape then determines the function of the protein.

Question 20

What is the test for Proteins?

Answer 20

The most reliable protein test is the Biuret test, which detects peptide
bonds.

Question 21

Outline the steps of the Biruet Test.

Answer 21

1. Place a sample of the solution w be tested in a test tube and add an
   equal volume of sodium hydroxide solution at room temperature.
2. Add a few drops of very dilute (0.05%) copper(II) sulfate solution
   and mix gently.
3. A purple coloration indicates the presence of peptide bonds and
   hence a protein. If no protein is present, the solution remains blue.

You can simply refer to adding Biuret reagent to test for protein.

Question 22

What are the two types of proteins?

Answer 22

1. Fibrous

2. Globular

Question 23

What are Fibrous Proteins?

Answer 23

Fibrous proteins, such as collagen, have structural functions. Fibrous proteins form long chains which run parallel to one another. These chains are linked by cross-bridges and so form very stable molecules.

Question 24

What are Globular Proteins?

Answer 24

Globular proteins, such as enzymes and haemoglobin, carry out metabolic functions.

Question 25

What is the molecular structure of a fibrous protein?

Answer 25

1. The primary structure is an unbranched polypeptide chain.
2. In the secondary structure the polypeptide is very tightly wound.
3. Lots of the amino acid glycine helps close packing.
4. In the tertiary structure the chain is twisted into a second helix.
5. Its quaternary structure is made up of three such polypeptide chains wound together in the same way as individual fibres would in a rope

Question 26

Collagen is found in tendons. Tendons join muscles to bones. When a muscle contracts the bone is pulled in the direction of the contraction.

Explain why the quaternary structure of collagen makes it a suitable molecule for a tendon.

Answer 26

It has three polypeptide chains wound together to form a strong, rope-like structure that has strength in the direction of pull of a tendon.

Question 27

The individual collagen polypeptide chains in the fibres are held together by bonds between amino acids of adjacent chains.

Suggest how the cross·linkages between the amino acids of polypeptide chains increase the strength and stability of a collagen fibre.

Answer 27

Prevents the individual polypeptide chains from sliding past one another and so they gain strength because they act as a single unit

Question 28

The points where one collagen molecule ends and the next begins are spread throughout the fibre rather than all being in the same position along it.

Explain why this arrangement of collagen molecules is necessary for the efficient functioning of a tendon.

Answer 28

The junctions between adjacent collagen molecules are points of weakness. If they all occurred at the same point in a fibre, this would be a major weak point at which the fibre might break.