1.2.4 - Proteins

Question 1

what are proteins?

Answer 1

amino acids joined together in long chains by condensation reactions

Question 2

what is the basic structure of amino acids?

Answer 2

an amino group (-NH2) and a carboxyl group (-COOH) attached to a carbon atom

Question 3

what is R in the general formula of amino acids?

Answer 3

R defines what amino acid it is (+ it affects how it bonds to other proteins)

Question 4

what is a peptide bond?

Answer 4

the bond formed by condensation reactions between amino acids

Question 5

where is the peptide bond found in a dipeptide?

Answer 5

between the carboxyl group of one amino acid and the amino group of another

Question 6

define dipeptide

Answer 6

two amino acids joined by a peptide bond

Question 7

what is a polypeptide chain?

Answer 7

long chain of amino acids joined by peptide bonds

Question 8

how does a protein form?

Answer 8

when a polypeptide folds/coils with other polypeptide chains

Question 9

how are hydrogen bonds formed between amino acids?

Answer 9

• opposite charges attract between negative charge present on the oxygen in carboxyl group and the positive charge present on the hydrogen of the amino groups

Question 10

are hydrogen bonds weak or strong?

Answer 10

• weak
• the large number of them is what holds the protein together
• they will break easily if the pH or temperature conditions change

Question 11

what molecules are in a disulfide bond?

Answer 11

• two cysteine molecules forms a disulfide bond when they are close to each other in the structure of a polypeptide

Question 12

how are disulfide bonds formed?

Answer 12

oxidation reaction takes place between two sulfur containing groups

Question 13

are disulfide bonds weak or strong?

Answer 13

• they form a strong covalent bond (holding the polypeptide in place)
• much stronger than hydrogen bonds (but occur much less often)

Question 14

how are ionic bonds formed in amino acids?

Answer 14

• formed between strongly positive and negative amino acid side chains (the links are known as salt bridges and help to stabilise the tertiary structure of a protein)

Question 15

are ionic bonds weak or strong?

Answer 15

• strong

- not as common as other structural bonds

Question 16

what is the primary structure of a protein?

Answer 16

• the sequence of amino acids that make up a polypeptide chain held together by peptide bonds

Question 17

what is the secondary structure of a protein?

Answer 17

• the arrangement of the polypeptide chain into a regular, repeating structure that is held together by hydrogen bonds

(sometimes there is no regular structure and the polypeptide forms a random coil)

Question 18

what is the tertiary structure of a protein?

Answer 18

• the 3D folding of the secondary structure held together by hydrogen bonds, disulfide bonds and ionic bonds between the amino acids

Question 19

what is the quaternary structure of a protein?

Answer 19

• the 3D arrangement of more than one tertiary polypeptide

e. g important enzymes + the blood pigment haemoglobin

Question 20

define denaturation

Answer 20

the loss of the 3D shape of a protein (e.g as a result of changes in temperature/pH)

Question 21

why is the 3D structure of proteins important?

Answer 21

• because it’s important to the way they work

- denaturation will cause them to stop working properly

Question 22

what are fibrous proteins?

Answer 22

• long, parallel polypeptide chains with occasional cross-linkages that form into fibres
• little to no tertiary structure

Question 23

what are two characteristics of fibrous proteins?

Answer 23

• insoluble in water

- very tough

Question 24

what is collagen?

Answer 24

• a fibrous protein that gives strength to tendons, ligaments, bones and skin

Question 25

is collagen weak or strong?

Answer 25

• extremely strong

- due to being made up of three polypeptide chains (each up to 1000 amino acids long)

Question 26

what is the structure of collagen?

Answer 26

• Primary structure = repeating sequences of glycine with two other amino acids (proline or hydroxyproline)
• they are arranged in a unique triple helix held together by a large number of hydrogen bonds
• these collagen molecules are found together in fibrils held together to form collagen fibres

Question 27

what happens when the collagen triple helix doesn’t fold properly?

Answer 27

• it will lack tensile strength and will be brittle and break very easily
• e.g in the genetic disease osteogenesis imperfecta

Question 28

what is the structure of haemoglobin?

Answer 28

• a globular protein

- large molecule made up of 574 amino acids arranged in 4 polypeptide chains held together by disulfide bonds

Question 29

what makes haemoglobin a conjugated protein?

Answer 29

• it is arranged around a haem group

Question 30

what is a conjugated protein?

Answer 30

protein molecules that are joined to a prosthetic group (affecting the performance and functions of the molecules)

Question 31

other than haemoglobin, what is an example of a conjugated protein?

Answer 31

• chlorophyll

- prosthetic group = magnesium

Question 32

what are glycoproteins?

Answer 32

• proteins with a carbohydrate prosthetic group

Question 33

what are the characteristics of glycoproteins?

Answer 33

• harder to break down (the carbohydrate part holds on to a lot of water)
• slippery (due to the water-holding properties)

Question 34

what are some examples of glycoproteins?

Answer 34

• mucus

- synovial fluid in the joints

Question 35

what are lipoproteins?

Answer 35

a protein with a lipid prosthetic group

Question 36

what are lipoproteins important for?

Answer 36

• the transport of cholesterol in the blood

Question 37

what are the two types of lipoproteins found in your blood?

Answer 37

1. Low density lipoproteins (LDLs)
2. High density lipoproteins (HDLs)

• HDLs contain more protein (partly why they are denser as proteins are more compact than lipids)