Amino Acids
•Proteins are made of amino acids.
•Amino acids are made of 5 elements CHONS (carbon, hydrogen, oxygen, nitrogen, sulfur)
•All amino acids have the same general structure – a carboxyl group (COOH), and an amino group (NH
amino acids is the variable group (R).
•The amino group is attached by a covalent bond to a central carbon atom called the alpha carbon.
•The R group determines the specific properties of a given amino acid (there are 20 different R groups)
•Amino acids are amphoteric; they have both acidic and basic properties when they dissociate in water.
•The ability to donate or receive protons causes amino acid solutions to behave asbuffers.
Peptide Bonds
•Amino acids are linked together by peptide bonds to form dipeptides and polypeptides.
•It is a condensation reaction, which means a molecule of water is released during the reaction.
•The reverse of this reaction adds a molecule of water to break the peptide bond.
•Dipeptide (2 amino acids join together), Tripeptide (3 amino acids join together), polypeptide (many amino acids join together).
•Proteins are made up of one or more polypeptides.
•Amino acid polymerisation to form polypeptides is part of protein synthesis. It takes place in ribosomes.
Protein Structure
- Proteins are polymers made up of amino acid monomers.
Primary Structure:
•This is the sequence of amino acids in the polypeptide chain.
•For example in myoglobin or any other protein to carry out its specific function, it must contain the correct amino acids arranged in a precise order.
•If only one amino acid is out of place its function may be disrupted. For example; in haemoglobin (the main oxygen-carrying blood protein) a change in a single amino acid, causes sickle-cell anaemia, a serious blood disorder.
Secondary Structure:
•The polypeptide chain doesn’t remain flat and straight.
•Hydrogen bonds form between the amino acids in the chain. This makes it automatically coil into an alpha (α) helix or fold into a beta (β) pleated sheet.
•Both the helix and the sheet are held together by many hydrogen bonds, which makes them very stable and strong structures.
Tertiary Structure:
•This refers to the 3D shape of a polypeptide chain.
•The tertiary structure is held together by bonds between the R group of the amino acids in the protein. ( more bonds form in the tertiary structure)
•The coiled and folded chain of amino acid is often coiled and folded further.
Quaternary Structure:
•Many proteins consist of more than one polypeptide chain bonded together. The quaternary structure is the arrangement of the different chains.
•For example; Haemoglobin is made of 4 polypeptide chains 2 distinct types (alpha and beta) and 4 non-protein haem groups.
Two Main Groups of Proteins
Globular proteins
•They have polypeptide chains that are tightly folded to form a spherical shape.
•Many are folded so that their hydrophobic groups are on the inside of the molecule, and the hydrophilic groups face outwards, making these proteins soluble in water.
•The group includes enzymes, antibiotics and many hormones.
•There shape is maintained by various bonds including ionic bonds, hydrogen bonds, disulfide bonds and hydrophobic interactions.
Fibrous proteins:
•Consists of parallel polypeptide chains, cross-linked at intervals to form long fibres or sheets.
•They are usually insoluble in water and physically tough.
•They include collagen (bone), silk and keratin (hair).
Protein Denaturing
In IBDP Biology, Protein is one of the important building blocks of human.
If the hydrogen bonds holding the protein in shape are broken, a process called denaturing occurs.
•The covalent bonds in the primary structure are not broken, but the polypeptide chains unravel and lose their specific shape.
•It can be caused by changes in pH, salt concentration or temperature.
Protein Test
Biuret test
•The test solution needs to be alkaline, so first you need to add a few drops of sodium hydroxide solution.
•Then you add some copper sulfate solution.
•If protein is present a purple layer forms.
•If there’s no protein, the solution will stay blue.
This is the end of the topic
Drafted by Eva (Biology)
Photo references:
- http://www.astrochem.org/sci/Amino_Acids.php
- https://www.chemistrylearner.com/chemical-bonds/peptide-bond
- https://commons.wikimedia.org/wiki/File:Protein-primary-structure.png
- https://wellnessadvocate.com/?dgl=70872
- https://byjus.com/chemistry/protein-structure-and-levels-of-protein/
- https://schoolworkhelper.net/protein-structures-primary-secondary-tertiary-quaternary/
- https://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/24-proteins/fibrous-vs-globular-protein.html
- http://pressbooks-dev.oer.hawaii.edu/humannutrition/chapter/the-role-of-proteins-in-foods-cooking-and-denaturation/