3.3.13 Amino Acids, Proteins and DNA

Question 1

What is an amino acid?

Answer 1

A carbon atom bonded to a hydrogen, R group, amine group and carboxylic acid group

Question 2

What properties do all amino acids have? (2)

Answer 2

Amphoteric (act as both a base and an acid)

Chiral (four different groups attached to carbon)

Question 3

What is a zwitterion?

Answer 3

A dipolar ion where the amine group is protonated to NH3+ and the carboxylic acid is deprotonated to COO-
Only exist at isoelectric point (where overall charge is zero)

Question 4

What happens to the zwitterion under acidic conditions?

Answer 4

NH3+

COOH

Question 5

What happens to the zwitterion under basic conditions?

Answer 5

NH2

COO-

Question 6

What are proteins?

Answer 6

Condensation polymers of amino acids - a sequence of amino acids connected by peptide links

Question 7

What is the primary structure of proteins?

Answer 7

The sequence of amino acids in a chain

Question 8

What is the secondary structure of proteins? (2)

Answer 8

The shape of the chain (α-helix / β-pleated sheet)

Due to hydrogen bonds

Question 9

What is the tertiary structure of proteins?

Answer 9

The way the chain is coiled or folded

Due to hydrogen bonding, disulphides bridges and ionic attractions

Question 10

Where do hydrogen bonds exist in proteins?

Answer 10

Between polar groups such as -OH and -NH2

Question 11

What is disulphide bonding?

Answer 11

Bonding that occurs between two sulphur atoms present in amino acids, forming a covalent bond

Question 12

What factors affect the shape of a protein? (2)

Answer 12

Temperature

pH

Question 13

What are enzymes?

Answer 13

Proteins that act as biological catalysts

Question 14

What is special about the active site of an enzyme? (2)

Answer 14

Lock and key model - active site only catalyses reactions with substrates of a specific shape
They are stereospecific - only work on one enantiomer of a substrate

Question 15

How do inhibitors work? (2)

Answer 15

Molecules with a similar shape to the substrate act as inhibitors as they bond with the active site without reacting
No substrate can fit in the active site so the reaction is blocked

Question 16

How are drugs designed? (2)

Answer 16

By using computers to model active sites and designing potential drug molecules to fit the specific shapes
This method is quick because hundreds of shapes can be tested before trying anything in a laboratory

Question 17

What is a nucleotide?

Answer 17

The monomer that makes up DNA; made up of a phosphate group, pentose sugar (2-deoxyribose) and a base

Question 18

What is the structure of a single strand of DNA like?

Answer 18

A polymer chain of nucleotides, linked by covalent bonds to form a sugar-phosphate backbone, with bases attached to the sugars in the chain

Question 19

How does DNA exist? (2)

Answer 19

As two complimentary strands, arranged as a double helix

The bases pair up as AT or GC due to the arrangement of atoms allowing hydrogen bonding to exist between them

Question 20

What is cisplatin?

Answer 20

A complex of platinum (II) containing two chloride ion ligands and two ammonia ligands in a square planar shape
The two chloride ions are next to each other (Z isomer)

Question 21

How is cancer caused?

Answer 21

By the mutation and uncontrollable division of cells, forming tumours

Question 22

How does cisplatin act as an anti-cancer drug? (4)

Answer 22

In order for a cell to divide, the DNA helix unwinds so it can be replicated
Cisplatin stops this from happening by forming a coordinate bond between the platinum ion and a nitrogen atom in guanine, replacing one of the chloride ion ligands
A second nitrogen atom from a different guanine molecule can replace the other chloride ion ligand
The presence of the cisplatin causes the strands to kink so they can’t unwind and be copied properly

Question 23

Why can cisplatin cause adverse effects? (2)

Answer 23

It can also bind to DNA in normal cells, stopping healthy cells that normally replicate frequently from multiplying (e.g. hair and blood)
It can cause hair loss, suppress the immune system and cause kidney damage

Question 24

How can the side effects of cisplatin be lessened? (2)

Answer 24

By giving low dosages

By targeting the tumour directly

Question 25

Why is cisplatin still used as an anti-cancer drug despite the side effects?

Answer 25

The balance of long-term positive effects outweigh the negative short-term effects