Proteins

Question 1

1. What elements are proteins made from?

Answer 1

Carbon, hydrogen, oxygen, nitrogen and often sulphur and sometimes phosphorous

Question 2

2. What are the various roles of proteins?

Answer 2

Biochemical - enzymes, hormones, antibodies, buffers Structural - membranes, hair, muscle fibres, nails, bones, connective tissue Energy - only under conditions of starvation

Question 3

3. How are proteins formed?

Answer 3

They are formed when amino acid subunits combine through the formation of peptide bonds and condensation reactions (removing water). The long chains of amino acids form polypeptides

Question 4

4. Describe the structure of an amino acid

Answer 4

Each of the 20 different amino acids has an amino group -NH at one end, a carboxyl group -COOH at the other end and an “R” group in the middle, which may be as simple as an H atom or may be more complex with sulphur as part of the group.

Question 5

5. What is the importance of the “R” group

Answer 5

The “R” groups determine the role and properties of the polypeptide or protein, for example, its solubility, its buffering ability, its enzymatic abilities (it could be part of the active site), its role as a hormone or antibody, the bonds it forms etc

Question 6

6. What is a dipeptide and how is it formed?

Answer 6

A dipeptide is formed when just two amino acids combine through condensation and the formation of a peptide bond. Like all condensation reactions, it is reversed by hydrolysis (addition of water)

Question 6

7. Is there a difference between a polypeptide and a protein?

Answer 6

A polypeptide is formed when up to 100 amino acids combine through condensation reactions to form a long chain. A protein consists of one or more of these chains which are then either, folded, branched or cross-linked

Question 7

9. What do we mean by the secondary structure of a protein?

Answer 7

This is the way in which the chain of amino acids folds or turns on itself to make shapes or patterns, namely alpha helices and beta pleated sheets

Question 8

8. What do we mean by the primary structure of a protein?

Answer 8

The primary structure is the order or sequence of the amino acids which are joined together via peptide bonds (and through condensation reactions)

Question 9

11. What do we mean by the tertiary structure of a protein?

Answer 9

This is how the polypeptide folds and coils further to form a complex, 3-dimentional shape, all held in place by bonds between the different amino acid R-groups. These bonds include relatively weak hydrogen bonds, slightly stronger ionic bonds and the very strong disulphide bridges (between 2 sulphur atoms). Within the structure hydrophobic interactions act to maintain tertiary structure also.

Question 9

10. Describe the a-helices and B-pleated sheets?

Answer 9

• The a-helix consists of twists or turns in the chain of amino acids which are held in place by hydrogen bonds at regular intervals, forming a spiral * The B-pleated sheets consist of anti-parallel chains of amino acids lying adjacent to each other and held in place by hydrogen bonds. The result is a flat, folded structure

Question 10

14. Describe fibrous proteins

Answer 10

They consist of long, parallel fibres of polypeptide chains, cross-linked to form stable and strong structures, often structural in function.

Question 10

13. What are the two main types of proteins?

Answer 10

Globular (eg. Enzymes, antibodies and haemoglobin) and Fibrous (eg. Collagen and keratin)

Question 11

12. What do we mean by the quaternary structure of a protein?

Answer 11

This level of organisation only exists if there is more than one polypeptide chain. These chains tend to be linked by lots of sulphur bonds (as a further part of them becoming biologically active, they may contain non-protein or brosthetic groups)

Question 12

15. Describe globular proteins

Answer 12

They consist of polypeptide chains folded to give a spherical 3-d shape which is very specific, and leads to a biochemical functions.

Question 13

18. Give an example of another common conjugated protein

Answer 13

Glycoprotein, which consists of carbohydrate plus protein. An example would be mucin, found in the mucus of the respiratory tract, where it traps dirt, dust and bacteria.

Question 13

6. Explain how collagen is a fibrous protein

Answer 13

Each collagen molecule consists of 3 identical helical polypeptides wound around each other and held together by hydrogen bonds to form fibrils. Cross-links join these fibrils to form fibres. The whole structure has high tensile strength, making it ideal for ligaments, cartilage and skin.

Question 14

17. Explain how haemoglobin is a globular AND conjugated protein

Answer 14

Haemoglobin is made of 4 polypeptide chains of “globin” (so it is therefore quaternary), all tightly folded, giving a globular shape. At the centre of each chain lies a non-protein “haem” group, containing iron. Having this non-protein group, makes haemoglobin a conjugated protein. The iron ions in the haem group carry oxygen.

Question 15

19. What is a prion?

Answer 15

A protein which “mis-folds” to form a secondary structure of polypeptides rich in beta-pleated sheets

Question 16

22. Name 3 prion diseases

Answer 16

CJD (humans), BSE (cows) and Scrapie (sheep)

Question 16

21. What makes the normal PrP protein change into the Prpsc prion?

Answer 16

• It may be genetic and caused by mutations in DNA * It may change spontaneously (and then they replicate) * It can be transmitted via eating contaminated meat (the prion is infectious)

Question 17

20. Where is the normal form of the protein, PrP, found?

Answer 17

It is a protein found in neurones in the brain which helps in synaptic transmission

Question 18

23. What are the typical effects of prion disease?

Answer 18

They are neurodegenerative spongiform diseases, impairing brain function i.e. memory, personality, behaviour, coordination etc.

Question 19

24. Is there an incubation period?

Answer 19

The incubation period of prion disease is between 5 and 20 years.