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Proteins

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Biology

Proteins are biological macromolecules and one of the four most important in living organisms.

When you think of proteins, the first thing that comes to mind might be protein-rich foods: lean chicken, lean pork, eggs, cheese, nuts, beans, etc. However, proteins are so much more than that. They are one of the most fundamental molecules in all living organisms. They are present in every single cell in living systems, sometimes in numbers larger than a million, where they allow for various essential chemical processes, for instance, DNA replication.

Proteins are complex molecules due to their structure, explained in more detail in the protein structure article.

The structure of proteins

The basic unit in the protein structure is an amino acid. Amino acids join together by covalent peptide bonds to form polymers called polypeptides. Polypeptides are then combined to form proteins. Therefore, you can conclude that proteins are polymers composed of monomers that are amino acids.

Amino acids

Amino acids are organic compounds composed of five parts:

  • the central carbon atom, or the α-carbon (alpha-carbon)
  • amino group -NH2
  • carboxyl group -COOH
  • hydrogen atom -H
  • R side group, which is unique to each amino acid.

There are 20 amino acids naturally found in proteins, and each one has a different R group. Figure 1. shows the general structure of amino acids, and in figure 2. you can see how the R group differs from one amino acid to another. All 20 amino acids are shown here for you to be familiar with their names and structures. It is not necessary to memorize them at this level!

The formation of proteins

Proteins form in a condensation reaction of amino acids. Amino acids join together by covalent bonds called peptide bonds.

A peptide bond forms with the carboxylic group of one amino acid reacting with the amino group of another amino acid. Let's call these two amino acids 1 and 2. The carboxylic group of amino acid 1 loses a hydroxyl -OH, and the amino group of amino acid 2 loses a hydrogen atom -H, creating water that is released. The peptide bond always forms between the carbon atom in the carboxyl group of amino acid 1 and the hydrogen atom in the amino group of amino acid 2. Observe the reaction in figure 3.

When amino acids join together with peptide bonds, we refer to them as peptides. Two amino acids joined together by peptide bonds are called dipeptides, three are called tripeptides, etc. Proteins contain more than 50 amino acids in a chain and are called polypeptides (poly- means 'many').

Proteins can have one very long chain or multiple polypeptide chains combined.

The amino acids that make proteins are sometimes referred to as amino acid residues. When the peptide bond between two amino acids forms, water is removed, and it 'takes away' atoms from the original structure of amino acids. What is left from the structure is called an amino acid residue.

Four types of protein structure

Based on the sequence of amino acids and the complexity of the structures, we can differentiate four structures of proteins: primary, secondary, tertiary and quaternary.

The primary structure is the sequence of amino acids in a polypeptide chain. The secondary structure refers to the polypeptide chain from the primary structure folding in a certain way. When the secondary structure of proteins starts to fold further to create more complex structures, the tertiary structure is formed. The quaternary structure is the most complex of them all. It forms when multiple polypeptide chains, folded in their specific way, are bonded with the same chemical bonds.

You can read more about these structures in the article Protein structure.

The function of proteins

Proteins have a vast array of functions in living organisms. According to their general purposes, we can group them into three groups: fibrous, globular, and membrane proteins.

1. Fibrous proteins

Fibrous proteins are structural proteins that are, as the name suggests, responsible for the firm structures of various parts of cells, tissues and organs. They do not participate in chemical reactions but strictly operate as structural and connective units.

Structurally, these proteins are long polypeptide chains that run parallel and are tightly wound to one another. This structure is stable due to cross-bridges that link them together. It makes them elongated, fiberlike. These proteins are insoluble in water, and that, along with their stability and strength, makes them excellent structural components.

Fibrous proteins include collagen, keratin and elastin.

  • Collagen and elastin are building blocks of skin, bones, and connective tissue. They support the structure of muscles, organs, and arteries as well.

  • Keratin is found in the outer layer of human skin, hair and nails, and feathers, beaks, claws, and hooves in animals.

2. Globular proteins

Globular proteins are functional proteins. They perform a much wider range of roles than fibrous proteins. They act as enzymes, carriers, hormones, receptors, and much more. You can say that globular proteins carry out metabolic functions.

Structurally, these proteins are spherical or globe-like, with polypeptide chains that fold to form the shape.

Globular proteins are haemoglobin, insulin, actin and amylase.

  • Haemoglobin transfers oxygen from the lungs to cells, giving the blood its red colour.

  • Insulin is a hormone that helps to regulate blood glucose levels.

  • Actin is essential in muscle contraction, cell motility, cell division and cell signalling.

  • Amylase is an enzyme that hydrolyses (breaks down) starch into glucose.

Amylase belongs to one of the most significant types of proteins: enzymes. Mostly globular, enzymes are specialized proteins found in all living organisms where they catalyze (accelerate) biochemical reactions. You can find out more about these impressive compounds in our article on enzymes.

We mentioned actin, a globular protein involved in muscle contraction. There is another protein working hand in hand with actin, and that is myosin. Myosin cannot be placed into either of the two groups since it consists of a fibrous "tail" and a globular "head". The globular part of myosin binds actin and binds and hydrolyses ATP. The energy from ATP is then used in the sliding filament mechanism. Myosin and actin are motor proteins, which hydrolysis ATP to use the energy to move along cytoskeletal filaments within the cell's cytoplasm. You can read more about myosin and actin in our articles on muscle contraction and the sliding filament theory.

3. Membrane proteins

Membrane proteins are found in plasma membranes. These membranes are cell surface membranes, meaning they separate the intracellular space with everything extracellular or outside the surface membrane. They are composed of a phospholipid bilayer. You can learn more about this in our article on the cell membrane structure.

Membrane proteins serve as enzymes, facilitate cell recognition, and transport the molecules during active and passive transport.

Integral membrane proteins

Integral membrane proteins are permanent parts of the plasma membrane; they are embedded within it. Integral proteins that span across the entire membrane are called transmembrane proteins. They serve as transport proteins, allowing ions, water and glucose to pass through the membrane. There are two types of transmembrane proteins: channel and carrier proteins. They are essential for the transport across cell membranes, including active transport, diffusion and osmosis.

Peripheral membrane proteins

Peripheral membrane proteins are not permanently attached to the membrane. They can attach and detach either to the integral proteins or either side of the plasma membrane. Their roles include cell signalling, the preservation of the structure and the shape of the cell membrane, protein-protein recognition, and enzymatic activity.

Proteins Peripheral and Integral Channel Proteins in the Plasma Cell Membrane Study SmarterFigure 5. Structure of the cell plasma membrane. Recognize various positions of peripheral and integral (channel and carrier) proteins

It is important to remember that membrane proteins differ according to their position in the phospholipid bilayer. This is especially important when discussing channel and carrier proteins in transports across cell membranes such as diffusion. You might be required to draw the fluid-mosaic model of the phospholipid bilayer, indicating its relevant components, including membrane proteins. To learn more about this model, check out the article on cell membrane structure.

Biuret test for proteins

Proteins are tested using a biuret reagent, a solution that determines the presence of peptide bonds in a sample. That is why the test is called the Biuret test.

To perform the test, you would need:

  • A clean and dry test tube.

  • A liquid test sample.

  • Biuret reagent.

The test is performed as follow:

  1. Pour 1-2 ml of the liquid sample into the test tube.

  2. Add the same amount of Biuret reagent to the tube. It is blue.

  3. Shake well and allow to stand for 5 minutes.

  4. Observe and record the change. A positive result is the colour change from blue to deep purple. The purple colour indicates the presence of peptide bonds.

If you are not using Biuret reagent, you can use sodium hydroxide (NaOH) and dilute (hydrated) copper (II) sulfate. Both solutions are components of the biuret reagent. Add an equal amount of sodium hydroxide to the sample, followed by a few drops of dilute copper (II) sulfate. The rest is the same: shake well, allow to stand and observe the colour change.

Result

Meaning

No change in colour: the solution stays blue.

Negative result: proteins are not present.

Change in colour: solution turns purple.

Positive result: proteins are present.

Proteins Purple color indicates a positive result of the Biuret test for proteins StudySmarterPurple colour indicates a positive result of the Biuret test: proteins are present. Source: Ozone aurora CC-BY-SA-3.0, commons.wikimedia

Proteins - key takeaways

  • Proteins are complex biological macromolecules with amino acids as basic units.
  • Proteins form in condensation reactions of amino acids, which join together by covalent bonds called peptide bonds. Polypeptides are molecules composed of more than 50 amino acids. Proteins are polypeptides.
  • Fibrous proteins are structural proteins responsible for the firm structures of various parts of cells, tissues and organs. Examples include collagen, keratin and elastin.
  • Globular proteins are functional proteins. They act as enzymes, carriers, hormones, receptors, and much more. Examples are haemoglobin, insulin, actin and amylase.
  • Membrane proteins are found in plasma membranes (cell surface membranes). They serve as enzymes, facilitate cell recognition, and transport the molecules during active and passive transport. There are integral and peripheral membrane proteins.
  • Proteins are tested with a biuret test, using a biuret reagent, a solution that determines the presence of peptide bonds in a sample. A positive result is a change in colour from blue to purple.

Proteins

Examples of proteins include haemoglobin, insulin, actin, myosin, amylase, collagen and keratin.

Proteins are one of the most important molecules because they facilitate many vital biological processes, such as cellular respiration, oxygen transport, muscle contraction, and more.

The four protein structures are primary, secondary, tertiary and quaternary.

Proteins can be found in both animal and plant products. The products include lean meats, chicken, fish, seafood, eggs, dairy products (milk, cheese, etc.) and legumes and beans. Proteins are also abundant in nuts.

Proteins are composed of amino acids, which are linked together forming long polypeptide chains. There are four protein structures: primary, secondary, tertiary and quaternary. Proteins function as hormones, enzymes, messengers and carriers, structural and connective units, and provide nutrient transport.

Final Proteins Quiz

Question

What is the definition of proteins?

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Answer

Proteins are biological macromolecules and one of the four most important in living organisms. They are large, complex polypeptides.

Show question

Question

What is the basic unit of proteins called?

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Answer

Amino acid. 

Show question

Question

Amino acids are composed of five parts. Choose the correct answer.

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Answer

A hydrogen atom, an amino group, a central carbon atom, an R group, and a carboxyl group.

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Question

Draw the structure of amino acids and mark relevant groups. Use a piece of paper or your gadget. 

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Answer

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Question

How can you differentiate amino acids?

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Answer

You can tell amino acids apart by looking at their R groups, unique to each amino acid.

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Question

How do proteins form? Choose the correct sentence.

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Answer

Proteins form in a condensation reaction of amino acids. Amino acids join together by peptide bonds into long polypeptide chains.

Show question

Question

Peptide bond forms between two atoms. Which atoms are they?

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Answer

Carbon (C) and nitrogen (N). 

Show question

Question

A peptide bond forms between specific groups in different amino acids. Fill in the gaps so that the sentence is correct:
A peptide bond forms with the ____________of one amino acid reacting with the ___________ of another amino acid. It forms between the _______ atom from the __________ group of one amino acid and the _______ atom from the ______group of another amino acid.

Show answer

Answer

 A peptide bond forms with the carboxylic group of one amino acid reacting with the amino group of another amino acid. It forms between the carbon atom from the carboxyl group of one amino acid and the hydrogen atom from the amino group of another amino acid.

Show question

Question

Draw a condensation reaction between two amino acids and mark the peptide bond. Use a piece of paper or your gadget.

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Answer


Show question

Question

What are polypeptides? Are Proteins polypeptides?

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Answer

Polypeptides are molecules composed of more than 50 amino acids.

Yes, proteins are polypeptides. They can consist of one or more polypeptide chains.

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Question

What are fibrous proteins?

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Answer

Fibrous proteins are structural proteins responsible for the firm structures of various parts of cells, tissues and organs. They do not participate in chemical reactions but are rather strictly operating as structural and connective units.

Show question

Question

What is the structure of fibrous protein like?

Show answer

Answer

Structurally, fibrous proteins are long polypeptide chains that run parallel and are tightly wound to one another. This structure is stable due to cross-bridges that link them together. It makes them elongated, fiberlike.


Show question

Question

Name three examples of fibrous proteins.

Show answer

Answer

Collagen, keratin and elastin.

Show question

Question

What are globular proteins?

Show answer

Answer

Globular proteins are functional proteins. They perform a much wider range of roles than fibrous proteins. They act as enzymes, carriers, hormones, receptors, and much more. You can say that globular proteins carry out metabolic functions.


Show question

Question

What is the structure of globular proteins?

Show answer

Answer

Structurally, these proteins are spherical or globe-like, with polypeptide chains that are folded to form the shape.


Show question

Question

Name at least three examples of globular proteins.

Show answer

Answer

Haemoglobin, insulin, actin and amylase.


Show question

Question

What is insulin important for?

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Answer

Insulin is a hormone that helps to regulate blood glucose levels. 

Show question

Question

Amylase is a protein that functions as an enzyme. What does that mean?

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Answer

Mostly globular, enzymes are specialised proteins found in all living organisms where they catalyse (accelerate) biochemical reactions. Amylase is an enzyme that hydrolyses (breaks down) starch into glucose.

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Question

Where are membrane proteins found in cells?

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Answer

Membrane proteins are found in the phospholipid bilayers of plasma membranes (cell surface membranes).

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Question

Membrane proteins can be separated into two groups: integral and peripheral. What is the difference between the groups?

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Answer

Integral membrane proteins are permanent parts of the plasma membrane; they are embedded within it. Peripheral membrane proteins are not permanently attached to the membrane. They can attach and detach either to the integral proteins or either side of the plasma membrane.

Show question

Question

Integral membrane proteins can serve as transport proteins. What are the two types? What are they essential for?

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Answer

The two types are channel and carrier proteins. They allow molecules such as ions, water and glucose to pass through the membrane.  They are essential for the transport across cell membranes, such as active transport, diffusion, and osmosis.

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Question

What are the roles of peripheral membrane proteins?

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Answer

Their roles include cell signalling, the preservation of the structure and the shape of the cell membrane, protein-protein recognition, and enzymatic activity.

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Question

How does the Biuret test work?

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Answer

The Biuret test works by using a Biuret reagent, a solution that determines the presence of peptide bonds in a sample. 

Show question

Question

How is the Biuret test performed? List all steps.

Show answer

Answer

  1. Pour 1-2 ml of the liquid sample into the test tube.
  2. Add the same amount of Biuret reagent to the tube. It is blue in colour. 
  3. Shake well and allow to stand for 5 minutes.
  4. Observe and record the change.

Show question

Question

What would help you conclude that a result of the Biuret test is positive, meaning peptide bonds are present?

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Answer

The colour change: blue to purple.

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Question

Should you not have a Biuret agent to test for proteins, you could use sodium hydroxide (NaOH) and dilute (hydrated) copper (II) sulfate. Which solution would you use first, and in what amount?

Show answer

Answer

We would use sodium hydroxide first, adding an amount equal to the amount of the sample. Then, we would follow by adding a few drops of dilute copper (II) sulfate.

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Question

In the image below, mark the negative and the positive result of a Biuret test. 

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Answer


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