Proteins - Biological Molecules

Question 1

What elements do proteins contain?

Answer 1

Carbon, hydrogen, oxygen, nitrogen
Sometimes sulphur and other elements

Question 2

Describe the test for proteins

Answer 2

• add sample to test tube
• add biuret’s reagent
• shake to mix
• biurets is blue and turns pink-purple in presence of peptide bonds and hence protein

Question 3

Give examples of protein functions

Answer 3

• enzymes (catalyse reactions)
• carrier proteins carry molecules across membranes
• antibodies defend against disease
• structural proteins support cells and tissues
• hormones transmit information
• transport proteins eg haemoglobin carry oxygen
• contractile proteins allow muscles to contract

Question 4

What are proteins made up of?

Answer 4

Polymer molecules
Made up of amino acids

Question 5

How many amino acids are there?

Answer 5

20 naturally occurring in all living organisms
100,000 combinations

Of the 20, 9 cannot be synthesised by the body and must be obtained from food (known as essential amino acids)
Remaining 11 can be synthesised

Question 6

What do all amino acids contain in their structure?

Answer 6

Amine group - NH2
Carboxyl group - COOH
R group - a side group consisting of many different elements

Question 7

How do amino acids join together?

Answer 7

Condensation reaction
Water removed by OH group from carboxyl group of one amino acid
And H from amine group of another
Bond formed is called a peptide bond

Question 8

How is a polypeptide formed?

Answer 8

2 amino acids form a dipeptide
Many monomers can be joined together to form a polypeptide by polymerisation

Question 9

What is the primary structure of a protein?

Answer 9

The NUMBER and SEQUENCE of amino acids in the polypeptide chain

Determines eventual shape of protein, is therefore responsible for function of that protein

Question 10

What is the secondary structure of a protein?

Answer 10

Amino acids in primary structure don’t lie straight
Hydrogen bonds form between amino acids in chains
Making protein coil into α helix or β pleated sheet

H bonds form between every N-H group and the oxygen of a C=O

Question 11

What is tertiary structure?

Answer 11

Coiled chains often coil further
More bonds form due to interactions between R-groups of the polypeptide chain.
For proteins formed of a single polypeptide chain, this is the final 3D structure

Question 12

What bonds exist in tertiary structure?

Answer 12

Disulphide bonds
- strong, not easily broken, between 2 amino acids containing sulphur

Ionic bonds
- easily broken down in changes of pH (between carboxyl/amine groups not involved in peptide bond)

Hydrogen bonds
- numerous but very easily broken down occur between +ve and -ve charged -NH and -C=O

Question 13

What is quaternary structure?

Answer 13

Some proteins are held together by multiple polypeptide chains held together by bonds. Quaternary structure is how these chains are held together.
Eg haemoglobin, made of 4 polypeptide chains, quaternary structure determined the 3D structure.
There can also be non protein groups (prosthetic) eg haem in haemoglobin

Question 14

What bonds are in primary structure?

Answer 14

Peptide

Question 15

What bonds are in secondary structure?

Answer 15

Peptide and hydrogen

Question 16

What bonds are in tertiary structure?

Answer 16

Peptide, hydrogen, ionic and disulphide bonds

Question 17

What bonds are in quaternary structure?

Answer 17

Peptide, hydrogen, ionic, disulphide
Depends on tertiary structure of polypeptides and so is influenced by all bond types

Question 18

What are globular proteins?

Answer 18

Round, compact and easily soluble so can be transported in fluids.
They have a metabolic function, eg haemoglobin and enzymes

Question 19

What are fibrous proteins?

Answer 19

Tough and rope shaped.
Found in connective tissues such as tendons
Eg collagen

Question 20

What is the structure of collagen?

Answer 20

Primary - unbranched polypeptide chain
Secondary - tightly coiled a helix, many glycine amino acids - can be tightly packed together
Tertiary - chain twisted into a second helix
Quaternary - three polypeptide chains wound together

Question 21

Describe the structure of collagen fibres

Answer 21

Chains interlinked by strong covalent bonds forming cross linkages
Minerals can bind to triple helix increase strength
Molecules make up fibrils which make up fibres increasing collective strength

Question 22

How is structure adapted for function of proteins?

Answer 22

Enzymes- precise tertiary structure for active site
Protein carriers - in membranes specific shape for molecule to attach to them
Antibodies - specific shape to bind to antigens
Fibrous proteins - twisted in fibres to give strength
Tertiary structure affected by any factor eg pH or temperature - affecting bonds between R groups that maintain structure

Question 23

What are the similarities between haemoglobin and collagen?

Answer 23

• have quaternary structure
• proteins
• linked by peptide bonds
• made of amino acids

Question 24

What is unique to haemoglobin?

Answer 24

• globular
• contains (haem) a prosthetic group
• easily soluble
• transported in fluid
• have a metabolic function (carry O2)
• four polypeptide chains (2a, 2B)

Question 25

What is unique to collagen?

Answer 25

• fibrous
• no prosthetic group (has minerals embedded in quaternary structure)
• chains linked with covalent bonds
• insoluble
• rope-shaped
• found in tissues (structural role) eg tendons
• 3 polypeptide chains (all same)