Section 8 - Amino Acids, Proteins and DNA

Question 1

What makes amino acids amphoteric?

Answer 1

They have a basic amino group (NH₂) and an acidic carboxyl group (COOH).

Question 2

What kind of isomerism do amino acids display?

Answer 2

Optical - they have a chiral carbon.

Question 3

Which amino acid does not display isomerism?

Answer 3

Glycine - its R group is another hydrogen/

Question 4

What is a zwitterion?

Answer 4

A dipolar ion - it has a positive and negative charge in different parts of the molecule.

Question 5

What is the isolectric point?

Answer 5

The pH when the average overall charge of the amino acid is zero.

Question 6

What is a suitable way to identify unknown amino acids?

Answer 6

Thin-layer chromatography.

Question 7

What is the name for condensation polymers of amino acids?

Answer 7

Proteins

Question 8

What is the link in a protein chain?

Answer 8

Peptide link

Question 9

What are the conditions for hydrolysis of proteins?

Answer 9

• hot aqueous 6M HCl acid
• heat
• reflux
• 24 hours

Question 10

What is the primary structure of a protein?

Answer 10

The sequence of amino acids in the polypeptide chain.

Question 11

What is the secondary structure of a protein?

Answer 11

The shape of the chain - the peptide links form hydrpgen bonds creating either an alpha helix (spiral) or beta-pleated (folded) sheet.

Question 12

What is the tertiary structure of a protein?

Answer 12

The extra bonds formed between different parts of the polypeptide chain, which gives the protein a 3-dimensional shape.

Question 13

Which types of bonds help to keep proteins in shape?

Answer 13

Hydrogen and Disulfide bonds.

Question 14

How do hydrogen bonds form in proteins?

Answer 14

They exist between the -OH and -NH₂ groups.

Question 15

How do disulfide bonds form in proteins?

Answer 15

Cysteine contains a thiol group (-SH). These can lose their H atoms and join together forming a disulfide bond, linking together parts of the protein chain.

Question 16

Which factors can affect the formation of hydrogen and disulfide bonds?

Answer 16

• temperature
• pH

Question 17

Define enzymes

Answer 17

Biological catalysts which speed up chemical reactions

Question 18

What type of reactions do enzymes catalyse in the human body?

Answer 18

Metablolic reactions

Question 19

What is an active site?

Answer 19

The area on an enzyme that the substrate fits into so that it can interact with the molecule.

Question 20

Why are enzymes stereospecific and what does this mean?

Answer 20

They have chiral centres and only one enantiomer of a substrate will fit into the active site.

Question 21

What is an inhibitor?

Answer 21

A molecule which has a similar shape to the substrate which bonds to the active site but doesn’t react - just blocks the substrate from getting in.

Question 22

What is the amount of enzyme inhibition affected by?

Answer 22

• how strongly the the inhibitor bonds to the active site
• the relative concentrations of inhibitor and substrate

Question 23

How do inhibiting drugs work?

Answer 23

• block active site of enzyme and stop it working
• eg. some antibiotics block the active site of enzymes in bacteria which make their cell walls so that eventually they burst.

Question 24

Why is it a problem if inhibiting drugs are chiral?

Answer 24

Active sites of enzymes are stereospecific

Question 25

What does DNA stand for?

Answer 25

deoxyribonucleic acid

Question 26

What is DNA made from?

Answer 26

Lots of monomers called nucleotides

Question 27

What are nucleotides made up from?

Answer 27

• phosphate group
• pentose sugar
• base

Question 28

How is the sugar-phosphate backbone bonded?

Answer 28

Covalent bonding

Question 29

How is the sugar-phosophate backbone formed?

Answer 29

Condensation Polymerisation

Question 30

What is the structure and bonding of DNA?

Answer 30

It has a double-helix structure which is held together by hydrogen bonds between the bases.

Question 31

What does Adenine pair with?

Answer 31

Thymine

Question 32

What does guanine pair with?

Answer 32

Cytosine

Question 33

What atoms do hydrogen bonds form between in DNA?

Answer 33

Polar positive Hs (an H bonded to anything highly electronegative eg. N) and the lone pair on a nearby O or N atom.

Question 34

How many hydrogen bonds can A and T form?

Answer 34

2

Question 35

How many hydrogen bonds can G and C form?

Answer 35

3

Question 36

What is cisplatin?

Answer 36

An anti-cancer drug which is a complex of platinum (II) with two chlorine ligands and two ammonia ligands in a square planar shape.

Question 37

What causes cancer?

Answer 37

Cells in the body dividing uncontrollably and forming tumours

Question 38

How does cisplatin work?

Answer 38

• an N atom on a guanine base in DNA forms a co-ordinate bond with cisplatin’s Pt ion, replacing one of the Cl ion ligands.
• a second N atom from a nearby guanine can bond to the Pt and replace the second Cl ion.
• this causes the strands to kink so the DNA can’t unwind and be copied properly - so the cell can’t replicate.

Question 39

What is the problem with cisplatin?

Answer 39

It can bind to DNA in normal cells too, eg hair cells and blood cells, which have to replicate frequently.

This means it causes hair loss and suppresses the immune system.

Question 40

How can you reduce the side effects of cisplatin?

Answer 40

• give low doses
• target it to the tumour