Back to: Organic Chemistry 400 Level
Welcome to class!
Hello excellent learner, welcome back! I’m really proud of the consistency and passion you’re putting into your learning. Today we shall continue with Amino Acids and Proteins II. In the previous lesson, we introduced amino acids, their structures, classifications and how they join together to form peptide bonds. In this lesson, we will go deeper into protein structure, protein functions, and some important reactions of amino acids and proteins that are essential in biochemistry and medicine.
Amino Acids and Proteins II
Let’s imagine a protein as a carefully built house. Amino acids are the blocks, peptide bonds are the cement, and the way the blocks are arranged and folded determines the strength and function of the building. The same applies to proteins in the body.
Levels of Protein Structure
Proteins have different levels of structural organisation:
Primary Structure – This is the linear sequence of amino acids in a polypeptide chain. The specific order (e.g., Ala–Gly–Val–Lys…) determines everything that will follow. You can compare it to the exact arrangement of bricks in the foundation of a house.
Secondary Structure – This refers to regular local folding patterns due to hydrogen bonding. The two major types are:
α-Helix – a spiral structure (like a coiled rope)
β-Pleated Sheet – a folded sheet structure (like folded fabric)
Tertiary Structure – The overall 3D shape of a protein created by interactions between side chains (hydrophobic interactions, hydrogen bonds, disulfide bridges). This is like the full architectural structure when all rooms, walls and roofs come together.
Quaternary Structure – When two or more polypeptide chains come together to form a functional protein. A good example is haemoglobin, which consists of four different polypeptide chains working together.
Functions of Proteins
Proteins carry out a wide range of functions in the body, including:
Enzymes – Biological catalysts (e.g., amylase, lipase)
Transport Proteins – e.g., haemoglobin transports oxygen
Structural Proteins – e.g., collagen in skin and keratin in hair
Defence Proteins – e.g., antibodies that fight infections
Hormones – e.g., insulin regulates blood sugar
Contractile Proteins – e.g., actin and myosin in muscle contraction
This explains why a protein-deficient diet results in weakness, poor wound healing and muscle loss.
Denaturation of Proteins
Denaturation is the process in which a protein loses its natural 3D structure due to heat, pH changes, alcohol or heavy metals. Once the structure is lost, the protein becomes inactive.
For example, when you fry an egg, the transparent egg white (albumin) becomes solid and white. This is because heat denatures the protein.
Important Reactions of Amino Acids and Proteins
Ninhydrin Reaction: Amino acids react with ninhydrin to produce a purple colour. This reaction is used in the laboratory to detect amino acids in chromatography.
Biuret Test: Proteins react with copper (II) ions in alkaline solution to give a violet colour. This test is used to detect the presence of peptide bonds.
Xanthoproteic Test: Proteins containing aromatic amino acids give a yellow colour when treated with concentrated nitric acid.
These reactions are useful for identifying and studying proteins in food, blood serum and biological samples.
Summary
- Proteins have four structural levels: primary, secondary (α-helix, β-sheet), tertiary and quaternary.
- Proteins perform vital functions such as catalysis (enzymes), transport, structure, defence, hormonal regulation and movement.
- Denaturation is the loss of protein structure due to heat, pH or chemicals, leading to loss of function.
- Amino acids and proteins undergo reactions such as the ninhydrin test, biuret test and xanthoproteic test used for qualitative analysis.
Evaluation
- Differentiate between primary and tertiary protein structure.
- Give two examples of protein functions in the human body.
- What is protein denaturation and what causes it?
- State one laboratory test used to detect proteins and the principle behind it.
Well done for staying focused and committed! You’re doing excellently and Afrilearn is really proud of your growth. Keep going — your next lesson will build your confidence even more!