3.1.2 The digestive system provides an interface with the environment. Digestion involves enzymic hydrolysis producing smaller molecules that can be absorbed and assimilated.

Proteins

• Proteins have a variety of functions within all living organisms. The general structure of an amino acid as R(H2N)-C-(H)COOH
• Condensation and the formation of peptide bonds linking together amino acids to form polypeptides. The relationship between primary, secondary, tertiary and quaternary structure, and protein function.
• The biuret test for proteins.

Proteins are a class of biological molecules.

Along with carbohydrates and fats, they are one of the three main macromolecule nutrients that humans consume.

Functions of proteins
For GCE AQA Biology, you should know that proteins have many possible functions in all living organisms. Proteins can be divided into 2 types:

• Fibrous proteins: structural functions 
  • For example, collagen is a protein found in connective tissue like tendons, and joins muscle to bone 
  • Fibrous protein is made of long, parallel chains that are connected by cross-bridges 
  • This makes fibrous protein ideal to help with structural support as it makes the protein very strong 

Globular proteins: metabolic functions

• For example: 
  • Insulin is a hormone that helps to control blood sugar levels 
  • Digestive enzymes speed up the catalytic breakdown of large food molecules 
  • Haemoglobin helps to transport oxygen in red blood cells to respiring body cells 

The structure of proteins
The building blocks (‘monomers’) of proteins are individual amino acids. In GCE AQA Biology, you will be asked to identify the general structure of an amino acid as:

• Central carbon atom attached to
  • Amino group (-NH2): nitrogen-containing group characteristic of amino acids
  • Carboxyl group (-COOH): acidic group that makes it an amino ‘acid’ 
  • Hydrogen atom (-H)
  • R/Side chain: different chemical groups that give the amino acid its identity. Different amino acids have different R groups. 

Formation of proteins

Amino acids are joined together in long chains, forming a polypeptide, which is eventually coiled into a protein.

• The amino acids are joined together via condensation reactions, which form peptide bonds between adjacent amino acids. 
  • -OH group from the carboxyl of an acid + H group from another amino acid → water 
  • The resultant peptide bond is the CONH atom group 

• This process is repeated between amino acids until a long polypeptide chain is formed 
• The long polypeptide chain can then be coiled up and folded to form the specific shape of a protein 

Protein structure
The structure of a protein can be divided up into 4 main parts: primary, secondary, tertiary, and quaternary.

Primary structure: the sequence of amino acids 

• The order of amino acids is coded for by the order of the bases in genes 
  • Ribosomes assemble 20 different amino acids into polypeptides based on mRNA transcripts that are obtained from DNA 
  • Different amino acids have different chemical properties, so the order determines the final shape of the protein, which affects its function 

Secondary structure: alpha-helices formed by hydrogen bonds 

• In a peptide bond (CONH), the hydrogen has a slight positive charge and the oxygen has a slight negative charge. 
• This allows a weak attractive force known as a hydrogen bond to form 
• The repeated formation of these bonds between non-adjacent amino acids in a polypeptide create a repeating 3D shape known as an alpha-helix 
• Such bonds help to stabilise the protein 

Tertiary structure: the 3D shape of the protein 

• The overall 3D shape of the protein is formed by further twisting and folding of the alpha-helices, e.g: 
  • Hydrogen bonds: fairly weak associations 
  • Disulfide bridges: very strong
  • Ionic bonds: often formed between carboxyl and amino groups that are not in any peptide bonds 
• The 3D shape of the protein determines the overall function of the protein 

Quaternary structure: when more than one polypeptide chain are linked, and/or when prosthetic groups are added 

• Many proteins are not just made up of 1 polypeptide chain. Many polypeptides can join to form one protein. 
• Prosthetic groups are non-protein groups associated with the molecule 
• For example, haemoglobin contains 4 polypeptide chains + 1 iron-containing haem group 

Test for protein
The test for proteins is known as a Biuret test. It tests for the characteristic peptide bond of proteins.

Procedure for GCE AQA Biology:

1. Add a sample of the solution to be tested to a test tube. The test should be conducted at room temperature.  
2. Add an equal volume of sodium hydroxide solution, then add a few drops of dilute copper(II) sulfate solution. 
3. Mix gently. 

Positive result (peptide bonds are present, so protein is likely present): Purple colour
Negative result (no peptide bonds present, no protein): Remains blue in colour

Wanna Boost Up Your GCE AQA Biology? Check out our study tips here!

Written by Justine | Biology Specialist @ Tuttee

References

1. Toole, G., & Toole, S. (2015). Aqa biology A level. Oxford: Oxford University Press.
2. Image retrieved from https://bestofbiochemistry.wordpress.com/2013/04/01/qyatenary-structure-of-proteins/ 
3. Image retrieved from https://guidancecorner.com/types-of-amino-acids/
4. Image retrieved fromhttps://www.quora.com/What-is-condensation-reaction
5. Image retrieved fromhttp://cbm.msoe.edu/teachingResources/jmol/proteinStructure/secondary.html
6. Image retrieved from https://www.researchgate.net/figure/Primary-secondary-tertiary-and-quaternary-protein-structures-3_fig2_282790336  
7. Image retrieved fromhttps://www.chemistrylearner.com/biuret-test.html
8. Image retrieved from https://thrivous.com/blogs/views/pulse-87-designer-proteins-snap-together-like-legos