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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Protein Structure and Amino Acids
Proteins are large, complex molecules that play many critical roles in our bodies. They do most of the work in cells and are required for the structure, function and regulation of the body's tissues and organs. But what are proteins made of and how do they get their unique shapes?
Key Definitions:
All amino acids share a common structure. They have:
There are 20 different amino acids commonly found in proteins, each with a different R group.
Amino acids join together through a condensation reaction between the carboxyl group of one amino acid and the amino group of another. This forms a peptide bond and releases a water molecule.
The resulting chain of amino acids is called a polypeptide. A protein may consist of one or more polypeptide chains.
When two amino acids join, the result is called a dipeptide. Three amino acids form a tripeptide and many amino acids form a polypeptide.
Proteins have four levels of structure, each building upon the previous level to create the final functional protein.
The primary structure is simply the sequence of amino acids in the protein chain. This sequence is determined by the genetic code in DNA.
Even a small change in the primary structure can have significant effects on the protein's function. For example, in sickle cell anaemia, a single amino acid substitution in haemoglobin causes the protein to form abnormal fibres.
The secondary structure refers to regular, repeated patterns formed by hydrogen bonds between the amino and carboxyl groups of the amino acid backbone.
The two main types of secondary structure are:
The tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It forms when the secondary structures fold and twist due to interactions between the R groups of the amino acids.
These interactions include:
Not all proteins have a quaternary structure. This level only exists when a protein consists of more than one polypeptide chain. The quaternary structure describes how these multiple chains are arranged relative to each other.
For example, haemoglobin has four polypeptide chains (two alpha and two beta chains) that work together to carry oxygen in our blood.
The function of a protein is directly related to its three-dimensional shape. This shape allows proteins to:
Denaturation is the process where proteins lose their structure due to external stress, causing them to stop functioning. Proteins can be denatured by:
Denaturation is usually irreversible in biological systems. Once a protein is denatured, it typically cannot regain its original shape and function.
You can observe protein denaturation in your kitchen:
These examples show how changing protein structure affects their properties.
Enzymes are proteins that act as biological catalysts, speeding up chemical reactions in living organisms without being used up themselves. Each enzyme has a specific shape that allows it to bind to its substrate (the molecule it acts upon).
For example, the enzyme amylase in your saliva breaks down starch into smaller sugar molecules. The shape of amylase perfectly fits the shape of starch molecules, like a key fits a lock. This is called the "lock and key" model.
If amylase is exposed to high temperatures (like boiling water), it becomes denatured. Its shape changes and it can no longer fit with starch molecules, making it unable to function. This is why cooking food can make it easier to digest - the heat denatures some of the enzymes that might otherwise break down nutrients before we can absorb them.
Proteins have diverse roles in living organisms. Here are some examples:
Collagen: The most abundant protein in mammals, providing strength and support in skin, bones, tendons and ligaments.
Keratin: Forms the structural component of hair, nails and the outer layer of skin.
Haemoglobin: Carries oxygen in red blood cells.
Insulin: Regulates blood glucose levels.
Antibodies: Help defend against disease by recognising foreign substances.
Digestive enzymes: Break down food (e.g., amylase, protease, lipase).
DNA polymerase: Helps replicate DNA during cell division.
ATP synthase: Produces ATP, the energy currency of cells.
Proteins are essential molecules made up of amino acids joined by peptide bonds. They have up to four levels of structure, from the simple sequence of amino acids (primary) to complex arrangements of multiple polypeptide chains (quaternary).
The specific three-dimensional shape of a protein is crucial for its function. This shape can be disrupted through denaturation, causing the protein to lose its function.
Understanding protein structure helps us comprehend how proteins perform their diverse roles in living organisms, from providing structural support to catalysing chemical reactions and defending against disease.
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