1.6->1.7 Proteins and enzymes

Question 1

Polypeptide

Answer 1

polymer made up of amino acid monomer sub-units
can be combined to form proteins
forms primary structure of protein- determines shape and fucntion
sequence is determined by DNA

Question 2

Amino acids

Answer 2

100 have been identified
20 naturally occur in proteins

each amino acid has a central C atom attached to four chemical groups:
1. amino group(-NH2-)
2. carboxyl group(-COOH)
3. hydrogen atom(-H)
4. R(side) group- one out of a variety of different chemical groups

Question 3

Peptide bond

Answer 3

when two amino acid monomers combine to form a dipeptide
condensation reaction
- removal of water molecule by combining -OH from carboxyl group from one amino acid with -H from amine group of the other

now link between carbon atom and nitrogen

Question 4

Secondary structure of protein

Answer 4

hydrogen bonds are formed between -NH and -C=O as oxygens overall negative charge and hydrogen as overall positive charge

this causes long polypeptide chain to be twisted into 3d shape e.g. alpha helix coil

Question 5

Tertiary structure of proteins

Answer 5

alpha-helices of secondary protein structure can be twisted and folded even more to give complex, compact, specific 3d structure

maintained by number of different bonds(location depends on primary structure):

1. disulphide bridges- fairly strong and cannot easily be broken
2. ionic bonds- between carboxyl and amino groups not involved in forming peptide bonds- weak and easily broken by changes in pH
3. hydrogen bonds- numberous but easily broken

Question 6

Quaternary structure

Answer 6

large proteins form complex molecules containing various number of individual polypeptide chains that are linked in various ways

associated with non-protein(prosthetic) groups
e.g. iron-containing harem group in haemoglobin

Question 7

Test for protein

Answer 7

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

Question 8

Fibrous proteins

Answer 8

form long chains which run parallel to one another
chains are linked by cross-bridges so form very stable molecules
e.g. collage

Question 9

Molecules structure of collagen

Answer 9

1. unbranched polypeptide chain
2. Polypeptide chain is very tightly wound
   Lots of amino acid glycine helps close packing
3. chain is twisted into second helix
4. 3 polypeptide chains are wound together as individual fibres are wound together in a rope

Question 10

Collagen

Answer 10

found in tendons
tendons join muscles to bones
when a muscle contracts the bone is pulled in the direction of the contraction

Question 11

enzymes

Answer 11

globular tertiary proteins that act as catalysts
alter the rate of a chemical reaction without undergoing permanent changes
can be reused

Question 12

activation energy level

Answer 12

minimum amount of energy needed to activate a reaction to take place

Question 13

free energy

Answer 13

energy of a system that is available to perform work

Question 14

active site

Answer 14

functional area made up of relatively small number of amino acids
form small depression in enzyme molecule

Question 15

induced fit model of enzyme action

Answer 15

active site forms as enzyme and substrate interact
proximity of substrate(change in environment) leads to change in enzyme that forms functional active site
enzyme is flexible and can mould itself around substrate

Question 16

how enzymes catalyse energy

Answer 16

reduce activation energy

Question 17

lock and key theory

Answer 17

particular enzyme’s active site shape is specific to corresponding substrate that fits inside the depression during random collisions, forming an enzyme-substrate complex
substrate molecule is held within active site by bonds that temporarily form between certain amino acids and groups on substrate molecule

Question 18

limitation of lock and key

Answer 18

enzyme is not rigid structure but flexible
other molecules could bind to enzymes at sites other than active site, altering the activity of enzyme(competitive inhibitors)