Proteins are the most complex and most diverse group of biological molecules. They have an astonishing range of different structures and functions, for example: -
Structural proteins: - e.g. collagen (bone, cartilage, tendon), keratin (hair), actin (muscle)
Enzymes: - e.g. Amylase, Lipase, Lactase, Pepsin, etc (>10,000 others)
Transport proteins: - e.g. haemoglobin (oxygen), transferrin (iron)
Pumps: - e.g. Na+K+ pump in cell membranes
Motors: - e.g. myosin (muscle), kinesin (cilia)
Hormones: - e.g. insulin, glucagon
Receptors: - e.g. rhodopsin (light receptor in retina)
Antibodies: - e.g. immunoglobulins
Storage: - e.g. albumins in eggs and blood, caesin in milk
Blood clotting: - e.g. thrombin, fibrin
Lubrication: - e.g. glycoproteins in synovial fluid
Toxins: - e.g. diphtheria toxin
Antifreeze proteins: - e.g. antifreeze proteins in arctic spiders!
This list is just a snapshot view of some the structural and functional roles proteins play in living organisms… and the following lessons we’ll take a look at how this super diverse group of biological molecules are made from amino acids, which form polypeptides though a series of stages resulting in the final 3D conformational shape of the protein with a unique structure and function.
A Level Biology: Proteins: -
Amino Acid Structure, dipeptides and polypeptides.
In this A-Level Biology Lesson "Amino acids structure, dipeptides and polypeptides" you’ll learn that Proteins are diverse macromolecules and that ALL proteins are polymers composed of smaller monomers known as amino acids. We’ll take a good look at the structure of a typical amino acid (remember you should be able to label this up under exam conditions, so make sure you’re super familiar with it). Once we’ve illustrated the general structure of an amino acid we’ll go though how amino acids join together via condensation reactions to form dipeptides. Here we can also emphasise that Proteins are polymers made up from Hydrogen, Oxygen and Carbon… and we’ll take a look at a couple of common amino acids (Glycine (GLY)) and Methionine (MET). (If you only remember 1 amino acid make it MET!). We’ll finish here with the key point that Polypeptides are many amino acids joined together by peptide bonds and summarise this lesson.
When you’re happy you know all about amino acids, their structure and can easily describe (and illustrate) how amino acids join together to form dipeptides and therefore polypeptides you’ll be ready to download the knowledge check PDF and test your knowledge. When you’ve answered all the questions, compare your answers to the ones I’ve written - you’ll see exactly how you should write answers in a way that gains maximum marks in the exams.
A-Level Biology "Proteins: Amino Acids - Dipeptides and Polypeptides"
Proteins are extremely diverse, Whether the protein is an enzyme, transport protein, antibody… etc. All are polymers made up from Amino Acids.
Amino Acids join together via condensation reactions to form polypeptides.
You have to be able to recognise the structure of an amino acid, be able to label it and show how they join together to form dipeptides, [and therefore, tripeptides and polypeptides].
Here’s is the structure of a typical amino acid:
When amino acids join together via condensation reactions, the NH2 group from one amino acid joins with the COOH group of another. A H2O molecule is released and a Peptide bond linking the amino acids is formed.
Here’s an example of a dipeptide:
Polypeptides range from a few amino acids to thousands in length.
A functional protein maybe just one of these polypeptides (which has folded and coiled – see next lesson), or could be more than one polypeptide associated together to form a functional protein, e.g. collagen and haemoglobin.
A Level Biology: Proteins - The 4 Levels of Protein Structure.
In this A-Level Biology Lesson “The 4 levels of Protein Structure” you’ll learn that polypeptides are composed of amino acids. Polypeptides go through a series of stages before they arrive at their final 3D conformational shape. Going thorough each stage in order, we’ll start by learning about the primary sequence of amino acids which makes that “primary polypeptide chain”. Continuing to stage 2: the secondary protein structure defined by its alpha-helices and Beta-pleated sheets - which seamlessly transition into the tertiary structure of a protein. Finally we’ll consider what is meant by the quaternary level of protein structure before wrapping up this lesson and getting ready to move on the next lesson where we look at some key example of proteins and their roles in living organisms.
From this lesson you must know the following in detail: -
The primary Structure (polypeptide chain).
The Secondary Structure (alpha-Helices and Beta-Pleated Sheets).
The Tertiary Structure (bonds - Hydrogen bonds, Ionic bonds and Disulfide bridges).
The Quaternary Level of Protein Structure (Haemoglobin, Insulin and Collagen).
When you’re happy you know all about The 4 Levels of Protein Structure and can easily describe (and illustrate) each of the levels of polypeptide (protein) structure you’ll be ready to download the knowledge check PDF and test your knowledge. When you’ve answered all the questions, compare your answers to the ones I’ve written - you’ll see exactly how you should write answers in a way that gains maximum marks in the exams. For example take a look at the typical exam stye question "Describe the structure of proteins" below. Here you can also download the 'exam' key words pdf too.
A-Level Biology: Proteins -
The 4 Levels of Protein Structure Exam Style Q and A
★ NOTE ★
Examiners mark schemes will indicate key words and ideas expected in your answer.
Be aware! Marks can easily be lost if your answer does not include the correct terminology or does not demonstrate an appropriate level of understanding.
Describe the structure of proteins. [5 marks]
Proteins (polypeptides) are polymers made up from amino acids. Proteins are synthesised as amino acids are joined by condensation reactions and the amino acids are ‘arranged’ into their primary sequence during translation. The resulting bond that holds amino acids together is known as a peptide bond.
Once the primary sequence of the polypeptide is complete, folding and coiling occurs – resulting in the secondary structure of the protein being determined. Alpha-helices and beta-pleated sheets held together by hydrogen bonds fold and coil and form the conformational (3D) structure of the protein. This tertiary level of protein formation is held in its 3D shape by means of further hydrogen interactions, additional ionic bonds and disulphide bridges between cysteine residues of amino acids.
Finally the polypeptide may interact with other polypeptides to form the quaternary structure. For example collagen is a quaternary protein determined by 3 polypeptide interactions.
Mark schemes are often quite vague - only including the key points needed to gain a mark... they do not show you how to properly structure your answers!
Typical mark scheme: -
1. Polymer of amino acids/Joined by peptide bonds; formed by condensation;
2, Primary structure is order of amino acids;
3. Secondary structure - folding of polypeptide chain due to hydrogen bonding; [accept alpha helix / pleated sheet].
4. Tertiary structure is 3-D folding due to hydrogen bonding / ionic / disulphide.
5. Quaternary structure is two or more polypeptide chains.
A Level Biology: Proteins - The Shape and Function of Enzymes, Antibodies, Transport and Structural Proteins.
In this A-Level Biology Lesson “A-Level biology: Proteins - The Shape and Function of Enzymes, Antibodies, Transport Proteins and Structural Proteins" You'll learn about the structure and function of some frequently encountered protein during the course of your A-level biology. Following the Learning outcomes and brief introduction you'll learn that proteins are either globular or fibrous. Next we'll take a look at some examples of proteins including the familiar category the enzymes. Moving on next to structural proteins and Collagen. We'll wrap up this lesson with a look at transport proteins and the structure of an antibody.
When you're confident you know all the content covered in this lesson, you’ll be ready to download the knowledge check PDF and test your knowledge. When you’ve answered all the questions, compare your answers to the ones I’ve written - you’ll see exactly how you should write answers in a way that gains maximum marks in the exams.
Check Your Exam Specification
★ AQA Specification Reference: - 220.127.116.11 General properties of proteins. Amino acids are the monomers from which proteins are made. The general structure of an amino acid. where NH2 represents an amine group, COOH represents a carboxyl group and R represents a carbon-containing side chain. The twenty amino acids that are common in all organisms differ only in their side group. 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. A functional protein may contain one or more polypeptides. The role of hydrogen bonds, ionic bonds and disulfide bridges in the structure of proteins. Proteins have a variety of functions within all living organisms. The relationship between primary, secondary, tertiary and quaternary structure, and protein function.
Proteins have a variety of functions within all living organisms. Students should be able to relate the structure of proteins to properties of proteins named throughout the specification.
★ CIE Specification Reference: - 2.3 Proteins: Describe the structure of an amino acid and the formation and breakage of a peptide bond. Explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins and describe the types of bonding (hydrogen, ionic, disulphide). An understanding of protein structure and how it is related to function is central to many aspects of biology, such as enzymes, antibodies and muscle contraction. Globular and fibrous proteins play important roles in biological processes such as the transport of gases and providing support for tissues. Collagen as an example of a fibrous protein and relate these structures to their functions. There should be a distinction between collagen molecules and collagen fibres.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 2: Genes and Health 2.9 i) Know the basic structure of an amino acid ii) Understand the formation of polypeptides and proteins (amino acid monomers linked by peptide bonds in condensation reactions).
Understand the significance of a protein’s primary structure in determining its three-dimensional structure and properties (globular and fibrous proteins and the types of bonds involved in its three-dimensional structure). Globular and fibrous proteins: Know the molecular structure of a globular protein and a fibrous protein and understand how their structures relate to their functions (including haemoglobin and collagen).
★ Edexcel (Biology B) Specification Reference: - Topic 1: Biological Molecules 1.3 i) Know the basic structure of an amino acid ii) Understand the formation of polypeptides and proteins (amino acid monomers linked by peptide bonds in condensation reactions).
Understand the significance of the primary, secondary, tertiary and quaternary structure of a protein in determining the properties of fibrous and globular proteins, including collagen and haemoglobin. Understand the role of ionic, hydrogen and disulfide bonding in the structure of proteins. Structure of a protein in determining the properties of fibrous and globular proteins, including collagen and haemoglobin. Understand how the structure of collagen and haemoglobin are related to their function. (i) Know the structure of enzymes as globular proteins. The structure and function of globular proteins - the properties and functions of fibrous proteins to include collagen and keratin.
★ OCR (Biology A) Specification Reference: - 2.1.2 Biological molecules (k) the general structure of an amino acid (l) the synthesis and breakdown of dipeptides and polypeptides, by the formation and breakage of peptide bonds. The levels of protein structure. To include primary, secondary, tertiary and quaternary structure AND hydrogen bonding, disulfide bonds and ionic bonds.
★ OCR (Biology B) Specification Reference: - 2.1.3 Proteins and enzymes: The basic structure of an amino acid and the formation of peptide bonds. The molecular structure of globular proteins as illustrated by the structure of enzymes and haemoglobin. To include primary, secondary and tertiary structure in relation to enzymes, and quaternary structure and prosthetic groups in relation to haemoglobin. How the structure of globular proteins enable enzyme molecules to catalyse specific metabolic reactions. The molecular structure of globular proteins as illustrated by the structure of enzymes and haemoglobin.
★ WJEC Specification Reference: - 1. Basic Biochemistry and Cell Organisation: The structure and role of amino acids and proteins. the primary, secondary, tertiary and quaternary structure of proteins. The relationship of the fibrous and globular structure of proteins to their function. 4. Biological reactions are regulated by enzymes. the protein nature of enzymes.