Amino Acids and Proteins

Question 1

What is an Isoelectric point ?

Answer 1

the pH at which the amino acid exists as a zwitterion

Question 2

Explain why amino acids are solids at room temp where as most organic compounds of similar Mr are liquids

Answer 2

This is due to the strong ionic attractions between zwitterions
which need more energy to break
than the intermolecular forces acting between the molecules of most organic compounds.

Question 3

What form is an amino acid in in the solid state ?

Answer 3

in its zwitterion form

Question 4

What is an amino acid described as ?

Answer 4

amphoteric

Question 5

When an amino acid is at a pH below its isoelectric point. What does it behave as ?

Answer 5

as a base and forms a positive ion

Question 6

When an amino acid is at a pH above its isoelectric point. What does it behave as ?

Answer 6

as an acid and forms a negative ion

Question 7

Why do amino acids have different Rf values

Answer 7

different polarities and therefore different solubilities in the solvent

Question 8

What are proteins ?

Answer 8

polymers of amino acids joined by peptide links

Question 9

What type of hydrolysis is used to break down proteins to release amino acids ?

Answer 9

6M HCl, 24hour reflux

Question 10

How can amino acids formed by hydrolysis be separated ?

Answer 10

separated by column chromatography and identified using thin layer chromatography

Question 11

Describe primary structure

Answer 11

the sequence of amino acids in a polypeptide chain

Question 12

Describe secondary structure

Answer 12

2D folding of the polypeptide chain due to hydrogen bonds
The polypeptide chain can fold into an alpha helix (spiral) or a beta pleated sheet

Question 13

Describe Tertiary structure

Answer 13

The 3D folding of the polypeptide chain due to hydrogen, ionic and disulphide bonds
These interactions occur between the R groups on specific amino acids

Question 14

What two types of bond are responsible for for maintaining tertiary structure ?

Answer 14

Hydrogen bonds
Disulphide bonds

Question 15

Explain how a disulphide bond is formed

Answer 15

cysteine contains a thiol group- SH
Two cysteines close together in space can form a S-S disulphide link which is covalent.

Question 16

What type of protein are enzymes ?

Answer 16

globular

Question 17

What is the word used to describe enzymes ?

Answer 17

stereospecific

an enzyme may catalyse the reaction of an L enantiomer but not the reaction of the corresponding D enantiomer

Question 18

Explain the lock and key hypothesis

Answer 18

enzyme has an active site with a specific shape. This means it can only bind to a substrate which is complimentary shape in order for an Enzyme-Substrate complex to be formed.

Question 19

Explain how enzymes lower activation energy

Answer 19

weaken bonds within the substrate or holds substrate molecules closer together in the ES complex so that chemical bonds can form more easily.

Question 20

Explain why only one enantiomer is able to bind to active site of an enzyme

Answer 20

the active site is stereospecific because the amino acids that form the active site contain chiral centres.
D and L enantiomers have a different arrangement of atoms in space. Therefore cannot bind

Question 21

Explain how an enzyme inhibitor drug works

Answer 21

similar shape to substrate
compete with substrate to bind to active site of enzyme
bind more strongly.
They block the active site so that substrate molecules can no longer bind.
This reduces the number of Enzyme substrate complexes formed and therefore the activity of the enzyme

Question 22

How are new drugs often found ?
What is wrong with this method ?

Answer 22

trial and error
this process can take a very long time

Question 23

What is a faster method to design drugs ?

Answer 23

to use computer based drug design

Question 24

Explain how computer based drug design works

Answer 24

an enzymes active site is modelled
How well a potential drug molecule will interact with the active site can then be predicted
hundreds of molecules can be studied quickly and any that might have the right shape can be then be synthesised and tested

Question 25

What is the advantage of computer based drug design ?

Answer 25

faster

Question 26

What are enzymes ?

Answer 26

Globular proteins that catalyse specific biochemical reactions due to the lowering of activation energy

Question 27

Where do Hydrogen bonds occur in the Tertiary structure ?

Answer 27

formed between polar groups on the amino acids