Biological Molecules: Proteins

Question 1

What are proteins made of?

Answer 1

• Amino acids are the monomers that proteins are made from
• A condensation reaction between two amino acids forms a peptide bond
• Dipeptides are formed by the condensation of two amino acids
• Polypeptides are formed by the condensation of many amino acids

• Proteins are made from the elements carbon, hydrogen, oxygen, nitrogen and sometimes sulphur

Question 2

Primary Structure

Answer 2

• The number and order of amino acids

- This sequence will determine the protein’s function in the end

Question 3

Secondary Structure

Answer 3

• The curling or folding of polypeptide chains into alpha-helices and beta pleated sheets due to the formation of hydrogen bonds

• In an α-helix, the polypeptide chain coils with hydrogen bonds keeping the coil stable
• In β-pleated sheets, the chains form a zig-zag and fold over themselves

• These shapes arise due to the structure of amino acids

• Amino acids all contain a carboxyl group (-C=O) and an amine group (-NH) when bonded in a chain
• The hydrogen in -NH is slightly positive
• The oxygen in -C=O is slightly negative
• Results in a hydrogen bond between different amino acids
• Hydrogen bonds are weak, but many hundreds of them keep the secondary structure stable

Question 4

Tertiary Structure

Answer 4

• Further folding of secondary structure to create a unique 3D structure held together by hydrogen and ionic/disulphide bonds
• The structure is maintained by bonds formed between the R groups of amino acids
• Hydrogen bonds form between polar R groups
• Ionic bonds form between positive and negatively charged R groups
• Disulphide links form between sulphur atoms in R groups

Question 5

Quaternary Structure

Answer 5

• The specific 3D shape of a protein that is determined by the multiple polypeptide chains and/or prosthetic groups bonded together

• Some proteins are only composed of one polypeptide chain, but many proteins are made of multiple polypeptide chains e.g haemoglobin
• Non protein groups are known as prosthetic groups
• Conjugated proteins are proteins with a prosthetic group added onto it

Question 6

Globular Proteins

Answer 6

• Globular proteins carry out metabolic, transport and hormonal roles

• Relatively unstable structure
• They are roughly spherical in shape
• Soluble in water
• They have very specific shapes which allow them to carry out specific functions

• Examples

• All enzymes
• All antibodies
• Some hormones e.g. insulin
• Haemoglobin

Question 7

Fibrous Proteins

Answer 7

• Fibrous proteins carry out structural functions

• Polypeptide chains form long twisted strands linked together
• Stable structure
• Insoluble in water
• Strength gives structural function:
• Collagen in bone
• Keratin in hair

Question 8

Structure of Amino Acids

Answer 8

R
                    |
H2N -------- C -------- COOH
                    |
                   H

• NH2 represents amine group
• COOH represents a carboxyl group
• R represents a side chain
• The proteins found in all living things are built from a bank of around 20 amino acids
• These amino acids differ only in their R group

Question 9

Test for Proteins

Answer 9

• The Biuret test can be used to test for proteins by detecting the presence of peptide bonds

• Test solution needs to be alkaline so Biuret A (sodium hydroxide) is added to the test solution
• A few drops of Biuret B (copper (II) sulphate) is then added and the solution is gently mixed
• If protein is present, the colour of the reagent changes from blue to purple