Proteins

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For educators: ready-to-use lesson slides, revision notes for Proteins (Biology, SL) — use them in your lesson, or run the topic as an interactive class activity your learners play as a live game.

Lesson notes

Proteins: Polymers of Amino Acids

Proteins are polymers (and macromolecules) made of monomers called amino acids.
The sequence, type and number of amino acids in a protein determines its shape and therefore its function.
Proteins are extremely important in cells because they form: enzymes, cell membrane proteins (e.g. carriers), hormones, immunoproteins (e.g. immunoglobulins), transport proteins (e.g. haemoglobin), structural proteins (e.g. keratin, collagen) and contractile proteins (e.g. myosin).
Because all genes code for proteins, all reactions necessary for life depend on the function of proteins.

Amino Acid Structure

Amino acids are the monomers of polypeptides; there are 20 amino acids found in polypeptides common to all living organisms.
The general structure of all amino acids is a central carbon atom (the α carbon) bonded to: an amine/amino group (-NH2), a carboxylic acid/carboxyl group (-COOH), a hydrogen atom, and an R group.
The R group is how each amino acid differs and why amino acid properties differ (e.g. whether they are acidic or basic, or polar or non-polar).
The R group can be as simple as another hydrogen atom (glycine), right through to complex aromatic ring structures (e.g. phenylalanine).

General structure of an amino acid

The Peptide Bond

To form a peptide bond, a hydroxyl group (-OH) is lost from the carboxylic group (-COOH) of one amino acid and a hydrogen atom is lost from the amine group (-NH2) of another amino acid.
The remaining carbon atom (with the double-bonded oxygen) from the first amino acid bonds to the nitrogen atom of the second amino acid.
This is a condensation reaction, so water is released.
Dipeptides are formed by the condensation of two amino acids: amino acid + amino acid → dipeptide.
Polypeptides are formed by the condensation of many (3 or more) amino acids.
A protein may have only one polypeptide chain or it may have multiple chains interacting with each other.
During hydrolysis reactions, the addition of water breaks peptide bonds, resulting in polypeptides being broken down into amino acids.

Forming a polypeptide

Dietary Requirements for Amino Acids

There are 20 naturally occurring amino acids.
Our cells can synthesise 11 of these from other amino acids; these are termed non-essential amino acids.
The remaining nine we need to consume via our diets; these are called essential amino acids.
A healthy, varied, well balanced diet will contain all nine essential amino acids required.
Diets that restrict certain foods may require supplementation; meat contains all nine essential amino acids, so a vegetarian or vegan diet needs to be well balanced and varied to ensure all essential amino acids are consumed regularly.

The Variety of Proteins

There is a large variety of proteins because: there are 20 naturally occurring amino acids, polypeptides can vary in length from a few to thousands, and the structure and amino acid sequence can also vary.
The genetic code (DNA base sequence) codes for the number and order of amino acids in a polypeptide, and there is a huge variety of options for DNA base sequence.
20 amino acids can give an almost infinite number of polypeptides.
Polypeptides are assembled at a ribosome by condensing individual amino acids onto a growing chain, one by one; the mRNA codon determines which amino acid is added.
For a polypeptide chain of 50 amino acids, there would be 20⁵⁰ possible combinations, giving 1.13 x 10⁶⁵ combinations.
Given that the average length of a protein is 300 amino acids, the number of possible combinations is so large it can be considered infinite.

Roles of Proteins

Speeding up cellular reactions (catalysis) is performed by enzymes.
Blood clotting, where blood proteins interact with oxygen to form a gel-like scab across a wound.
Strengthening fibres in skin, hair, tendons, blood vessels e.g. collagen, keratin.
Transport of vital metabolites e.g. oxygen carried by haemoglobin.
Formation of the cytoskeleton, a network of tubules within a cell that cause chromosomes to move during the cell cycle.
Cell adhesion, where cells in the same tissue stick together.
Hormones, chemical messengers secreted in one part of the body to have an effect elsewhere.
Compaction of DNA in chromosomes for storage, caused by histone proteins; the immune response produces antibodies, the most diverse group of proteins.
Membrane transport channel and carrier proteins determine which substances can pass across a membrane; cell receptors are binding sites for hormones, chemical stimuli such as tastes, and for other stimuli such as light and sound.

Examples of Polypeptides

Rubisco (Ribulose Bisphosphate Carboxylase): an enzyme that catalyses the fixing of CO2 from the atmosphere during photosynthesis; composed of 16 polypeptide chains as a globular protein; the most abundant enzyme on Earth and arguably the most important enzyme in nature.
Insulin: a hormone produced and secreted by β-cells in the pancreas; binds to insulin receptors (on liver, fat and muscle cells) reversibly, causing absorption of glucose from the blood; composed of 2 polypeptide chains as a short, globular protein.
Immunoglobulins (antibodies): have a generic 'Y' shape, with specific binding sites at the two tips of the 'Y'; bind to specific antigens; binding areas are highly variable, meaning antibodies can be produced against millions of different antigens; globular and the most diverse range of proteins.
Rhodopsin: a pigment in the retina of the eye; a membrane protein expressed in rod cells; contains a light-sensitive part, retinal, derived from Vitamin A; a photon of light causes a conformational change, sending a nerve impulse along the optic nerve to the central nervous system.
Collagen: a fibrous protein made of three separate polypeptide chains; the most abundant protein in the human body (approximately 25%); fibres form a network in skin, blood vessel walls and connective tissue that can resist tearing forces; plays a role in teeth and bones, helping to reduce their brittleness.
Spider Silk: as strong as steel wire though considerably lighter; contains rope-like, fibrous parts but also coiled parts that stretch under tension; does not denature easily at extremes of temperature; can be genetically engineered to be expressed in goats' milk.

Protein Structure: Effect of pH & Temperature

Protein structure is sensitive to changes in the environment, particularly temperature and pH changes.
The precise structure of a protein depends on ionic interactions, hydrogen bonds and other intermolecular forces between polypeptide chains being intact.
Denaturation is the irreversible change of protein conformation; it may occur by temperature and pH extremes that interfere with these bonds.
The bonds that form between different R groups are relatively weak (compared to peptide bonds); they can be broken easily, causing the conformation to change and denaturation.
The altered protein shape may affect its function, physical state and general usefulness in its original role.
A certain pH is considered an optimum for a particular protein because at that pH, the protein's 3D structure is not denatured.
Denaturation is almost always irreversible; the protein cannot be re-formed in its original conformation by reversing the change in conditions. However, small denaturations and renaturations are possible in certain proteins to respond to small fluctuations in pH e.g. haemoglobin.

Denaturation in Action

Denaturation can be seen most easily by looking at the changes in an egg white as the egg is fried or poached; egg white is mainly the protein albumin.
The hydrophobic amino acids in albumin are at the centre of the molecule in its normal state, so albumin is soluble.
Heating causes the hydrophobic amino acids to appear at the edges, where they cause the protein to become insoluble; a harder, solid layer forms, which is the cooked white.
Denaturation also occurs in the stomach, where the low pH (pH 2) causes proteins in the diet to become denatured on their way to being fully hydrolysed further down the digestive system.
The stomach enzyme pepsin, a protein-digesting enzyme, has an optimum pH of 2 for this reason.
Certain extremophiles have evolved to have proteins that are stable even at extreme pH or temperature, e.g. Thermus aquaticus, a bacterium that lives in hot springs at 80°C.
Denaturation of enzymes can be used as part of experiments to measure enzyme activity, e.g. to establish the optimum pH or temperature of an enzyme such as pepsin or lipase.
Many drugs are proteins that cannot be taken by mouth because the protein will be denatured by stomach acid; these drugs should be delivered in another way e.g. by direct injection into the blood.

Denaturation of an enzyme

Slides

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

Practice questions

Free preview — 8 of 59 questions. Sign up to see them all.

1.Which of the following is not a function of proteins?
Easy
- AAct as a store of chemical potential energy in cells
- BAct as chemical messengers that are secreted by glands and act on different parts of the body
- CSpeed up chemical reactions within a cell
- DForm the cytoskeleton within a cell that facilitates chromosome movement
2.A simple tetrapeptide consists of four amino acids. How many different combinations of amino acids would be possible for this peptide?
Easy
- A160
- B1 600
- C160 000
- D16 000 000
3.Which of the following are all examples of proteins?
Easy
- ARubisco, collagen, amylopectin
- BAmylopectin, collagen, guanine
- CInsulin, amylose, spider silk
- DRhodopsin, immunoglobulins, rubisco
4.Which of the following would represent all the elements that are present in proteins?
Easy
- ACarbon, hydrogen, oxygen, nitrogen
- BCarbon, hydrogen, oxygen, nitrogen, sulfur
- CCarbon, hydrogen, oxygen, nitrogen, phosphorus
- DCarbon, hydrogen, oxygen
5.
Amino acids consist of oxygen, hydrogen, carbon and nitrogen atoms. The diameter of each atom when bonded to another atom is shown in the table below.
atom single bond / nm double bond / nm
O 0.13 -
H 0.06 0.110
C 0.154 0.120
N 0.14 0.134
Using the figures in the table, the approximate length of one amino acid is 0.7 nm. What would be the approximate length of a dipeptide of this amino acid after a condensation reaction has occurred?
Medium
- A1.0 nm
- B1.2 nm
- C1.4 nm
- D1.6 nm
6.Which of the following chemical groups does not bond directly with the central carbon of an amino acid?
Medium
- A-OH
- B-NH2
- C-COOH
- D-H
7.Which of the following causes fibrous polypeptides to be insoluble?
Medium
- AThey are very long.
- BTheir surface has nonpolar amino acids.
- CThey are usually structural.
- DThey have more than one polypeptide chain.
8.
Which row of the table best classifies common proteins with differing numbers of polypeptide chains?
One polypeptide chain Two polypeptide chains Three polypeptide chains
A Collagen Insulin Haemoglobin
B Lysozyme Insulin Collagen
C Lysozyme Haemoglobin Insulin
D Haemoglobin Lysozyme Collagen
Medium
- AA
- BB
- CC
- DD

More questions

9.Which of the following diagrams correctly shows the structure of a dipeptide?
Medium
- ADiagram A (shows two amino acids joined by a peptide bond between the amine group of one and the carboxyl group of the other, with the R groups on the outside)
- BDiagram B (shows two amino acids joined by a peptide bond between the R groups)
- CDiagram C (shows two amino acids joined by a peptide bond between the carboxyl groups)
- DDiagram D (shows two amino acids joined by a peptide bond between the amine groups)
10.Trypsin is a digestive enzyme found in pancreatic juices that breaks down proteins into polypeptides in the small intestine. The image below represents the three dimensional structure of trypsin. Which of the following would be the most accurate description of the conformation of trypsin?
Hard
- AIt is a functional protein that is folded into a specific shape which is held in position by bonds between the R-groups of neighbouring amino acids, with hydrophilic R-groups facing towards the outside of the molecule
- BIt is a functional protein that is folded into a specific shape which is held in position by bonds between the R-groups of neighbouring amino acids, with hydrophobic R-groups facing towards the outside of the molecule
- CIt is a structural protein that is folded into a specific shape which is held in position by bonds between the R-groups of repeating amino acids
- DIt is a functional protein that is folded into a specific shape which is held in position by bonds between the R-groups of neighbouring amino acids, with hydrophilic R-groups facing towards the inside of the molecule
11.Rituximab is an example of a therapeutic protein that is used to treat certain types of cancers and autoimmune diseases. It is very sensitive to temperature and pH changes and is typically injected into the bloodstream of patients. Which of the following statements would explain why therapeutic proteins, such as rituximab, cannot be taken orally? I. The molecule would vibrate so fast once in the stomach that the intermolecular bonds would break II. The conformation of the protein will change once in the stomach and it may become non-functional III. The protein may become insoluble once in the stomach and form a precipitate due to the breakage of ionic bonds IV. Conditions in the stomach will cause the hydrophobic R-groups of the protein to be exposed to the outside of the molecule
Hard
- AI and IV
- BI, II and III
- CII and III
- DII, III and IV
12.The denaturation of three different proteins (P, Q and R) at different temperatures were investigated. The more denatured a protein is, the less stable the molecule becomes. The following graph shows the results of this investigation. Which of the following would be the most valid conclusion that the scientists can draw from these results?
Challenge
- AProtein R would be less stable and had a lower rate of denaturation above 70°C compared to protein Q, while protein P was fully denatured by 80°C
- BProtein R would be more stable and had a lower rate of denaturation above 70°C compared to protein Q, while protein P was the least heat tolerant of all the proteins
- CProtein Q would be more stable and had a lower rate of denaturation below 70°C compared to protein R, while protein P was fully denatured by 80°C
- DProtein Q would be less stable and had a higher rate of denaturation below 70°C compared to protein R, while protein P was the least heat tolerant of all the proteins
13.Match each protein with its correct description.
Medium
- Rubisco
- Insulin
- Collagen
- Rhodopsin
- Globular enzyme made of 16 polypeptide chains that fixes CO2 in photosynthesis
- Short globular hormone of 2 polypeptide chains secreted by pancreatic β-cells
- Fibrous protein of 3 polypeptide chains, the most abundant protein in the human body
- Membrane pigment in rod cells containing retinal derived from Vitamin A
14.Place the following events of protein synthesis into the correct chronological order, starting with the earliest.
Medium
- The two strands of a DNA molecule separate
- mRNA nucleotides join with exposed DNA bases and form a molecule of mRNA
- The introns are spliced from the pre-mRNA to produce mRNA
- The mRNA molecule leaves the nucleus
- A ribosome attaches to the mRNA molecule
- tRNA molecules bring specific amino acids to the mRNA molecule
- Peptide bonds form between the amino acids
15.Which of the following statements about protein denaturation are correct? (select all that apply)
Medium
- ADenaturation is the irreversible change of protein conformation
- BDenaturation breaks the peptide bonds that hold amino acids in sequence
- CThe bonds between R groups are relatively weak and can be broken easily
- DDenaturation can be caused by extremes of temperature and pH
- EA denatured protein always regains its original conformation when conditions return to normal
16.Which of the following are roles of proteins in organisms? (select all that apply)
Medium
- ACatalysing cellular reactions as enzymes
- BTransporting oxygen as haemoglobin
- CStoring genetic information in the nucleus
- DForming the cytoskeleton that moves chromosomes during the cell cycle
- EActing as hormones that are secreted in one part of the body to have an effect elsewhere
17.Which of the following statements about amino acids are correct? (select all that apply)
Hard
- AThere are 20 amino acids found in polypeptides common to all living organisms
- BHuman cells can synthesise all 20 amino acids
- CThe R group is the part of the amino acid that differs between amino acids
- DThe central carbon of an amino acid is bonded to an amine group, a carboxyl group, a hydrogen atom and an R group
- EEssential amino acids are those that must be consumed in the diet
18.Which of the following contribute to the large variety of proteins available to living organisms? (select all that apply)
Medium
- AThere are 20 naturally occurring amino acids
- BPolypeptides can vary in length from a few to thousands of amino acids
- CThe amino acid sequence of a polypeptide can vary
- DAll proteins have the same number of polypeptide chains
- EThe DNA base sequence coding for a polypeptide can vary enormously
19.Which of the following are fibrous proteins? (select all that apply)
Medium
- ACollagen
- BKeratin
- CInsulin
- DSpider silk
- EHaemoglobin
20.A peptide bond forms between the amine group of one amino acid and the carboxyl group of another amino acid.
Easy
True or false?
21.The R group of an amino acid is directly involved in forming the peptide bond.
Easy
True or false?
22.Denaturation of a protein is almost always irreversible.
Easy
True or false?
23.When albumin in egg white is heated, its hydrophobic amino acids move to the outside of the molecule, making the protein insoluble.
Medium
True or false?
24.The stomach enzyme pepsin has an optimum pH of 2.
Easy
True or false?
25.Which of the following correctly describes the monomers that make up proteins?
Easy
- AAmino acids
- BMonosaccharides
- CNucleotides
- DFatty acids
26.Which of the following is the correct word equation for the formation of a dipeptide?
Medium
- Aamino acid + amino acid → dipeptide + water
- Bamino acid + amino acid + water → dipeptide
- Cdipeptide + water → amino acid + amino acid
- Damino acid + amino acid → dipeptide + carbon dioxide
27.Which of the following best explains why proteins are described as versatile molecules?
Medium
- AThey can be made from 20 different amino acids in almost infinite combinations, giving many shapes and functions
- BThey are all enzymes that speed up cellular reactions
- CThey are all structural and provide support only
- DThey can store genetic information for the cell
28.A polypeptide chain is 50 amino acids long. Approximately how many possible combinations of amino acids are there for this chain?
Medium
- A1.13 × 10⁶⁵
- B1.13 × 10⁵⁰
- C50 × 20
- D20⁵⁰
29.In a polypeptide chain containing 100 amino acids, how many peptide bonds are present?
Medium
- A99
- B100
- C101
- D200
30.Which part of an amino acid determines whether it is acidic, basic, polar or non-polar?
Medium
- AThe R group
- BThe amine group
- CThe carboxyl group
- DThe hydrogen atom
31.A protein has a glutamic acid (negatively charged) at position 6 and a lysine (positively charged) at position 23. Which interaction most likely helps maintain the tertiary structure?
Medium
- AAn ionic bond between the two R groups
- BA peptide bond between the two R groups
- CA hydrogen bond between the two R groups
- DA covalent bond between the two R groups
32.A scientist heats a solution of a globular enzyme and observes that it loses its catalytic activity. Which of the following best explains this observation?
Medium
- AThe weak bonds maintaining the enzyme's 3D shape are broken, altering the active site
- BThe peptide bonds holding the amino acid sequence are broken
- CThe amino acids are hydrolysed into individual molecules
- DThe enzyme's primary structure is changed
33.Which of the following is NOT a role of proteins?
Medium
- AStoring genetic information in the nucleus
- BCatalysing cellular reactions as enzymes
- CTransporting oxygen as haemoglobin
- DForming the cytoskeleton that moves chromosomes
34.An extremophile bacterium lives in hot springs at 80°C. Which of the following best explains how its proteins remain functional at this temperature?
Hard
- AIts proteins have evolved stronger intermolecular bonds that resist denaturation at high temperatures
- BIts proteins have no tertiary structure
- CIts proteins are all fibrous and insoluble
- DIts proteins contain no R groups
35.A rare metabolic disorder prevents the normal folding of a specific enzyme, although its amino acid sequence is unchanged. Which of the following best explains why the enzyme is non-functional?
Medium
- AThe peptide bonds holding the amino acids in sequence have broken
- BThe intermolecular bonds between R groups that maintain the 3D conformation are not forming correctly
- CThe amino acids have been hydrolysed into individual monomers
- DThe enzyme has been permanently denatured by high temperature
36.A patient is prescribed a therapeutic protein drug. Which of the following statements explain why this drug cannot be taken orally? (select all that apply)
Hard
- AThe low pH in the stomach will interfere with the bonds holding the protein's 3D shape
- BThe protein will be broken down into amino acids by stomach acid
- CThe protein may become insoluble and form a precipitate as ionic bonds break
- DThe drug will be absorbed too quickly through the stomach lining
- EThe protein will spontaneously refold into its original conformation after leaving the stomach
37.Extremophile proteins that function at 80°C are permanently denatured when returned to 25°C.
Medium
True or false?
38.A scientist measures the activity of an enzyme at different pH values. The activity is highest at pH 2, drops sharply at pH 4, and is zero at pH 7. Which of the following is the most valid conclusion?
Hard
- AThe enzyme is denatured at pH 2 and has an optimum pH of 7
- BThe enzyme has an optimum pH of 2 and is denatured at pH 7
- CThe enzyme is active across all pH values but works fastest at pH 2
- DThe enzyme requires a cofactor that is only present at pH 2
39.Match each protein with its correct structural description.
Medium
- Collagen
- Haemoglobin
- Insulin
- Rubisco
- Fibrous protein made of three polypeptide chains
- Globular protein with four polypeptide chains that transports oxygen
- Short globular hormone composed of two polypeptide chains
- Globular enzyme composed of 16 polypeptide chains that fixes CO2
40.A student plans an experiment to determine the optimum temperature of the enzyme pepsin. Which of the following steps would improve the validity and reliability of the results? (select all that apply)
Medium
- AUse the same volume and concentration of protein suspension in every tube
- BIncubate tubes at a range of temperatures and measure the rate of protein breakdown at each
- CMeasure the rate of reaction only once at a single temperature
- DUse a water bath to maintain each temperature constant during the investigation
- EAdd the enzyme to the substrate before adjusting the temperature of the tubes
41.Name the type of reaction that occurs when two amino acids bond with each other.
Easy
42.Name the bond that forms between the amine group of one amino acid and the carboxyl group of another.
Easy
43.Name the protein that is the most abundant in the human body and is made of three polypeptide chains.
Medium
44.Name the protein found in rod cells that contains retinal derived from Vitamin A.
Medium
45.A gene codes for a polypeptide of 351 amino acids. 40% of the gene is non-coding. Calculate the total length of the gene in pairs of nucleotides.
Medium
46.A polypeptide chain contains 250 amino acids. How many peptide bonds are present in the chain?
Medium
47.A polypeptide is made of 120 amino acids. How many nucleotides are required in the coding region of the mRNA to specify this polypeptide?
Medium
48.Complete the sentence about the general structure of an amino acid.
Easy
The central carbon of an amino acid is bonded to an amine group, a carboxyl group, a hydrogen atom and a variable ____ group.
49.Complete the sentence about peptide bond formation.
Medium
In a condensation reaction, a hydroxyl group is lost from the ____ group of one amino acid and a hydrogen atom is lost from the ____ group of another amino acid.
50.Name the three broad categories of protein function listed below.
Medium
Proteins that speed up cellular reactions are ____, proteins that carry substances such as oxygen are ____, and proteins such as keratin and collagen that provide support are ____.
51.Complete the sentence about the effect of heat on egg white.
Medium
Egg white is mainly the protein ____. Heating causes its ____ amino acids to appear at the edges, making the protein insoluble.
52.State the type of reaction that occurs when two amino acids bond with each other.
Easy
53.Describe the process of protein denaturation.
Hard
54.Use your knowledge of protein structure to explain the term 'specificity' in the context of immunoglobulins and their mode of action.
Hard
55.Outline, with examples, the main roles that proteins play in organisms.
Hard
56.Explain how the structure of collagen relates to its function in the body.
Hard
57.A gene codes for a polypeptide of 351 amino acids. 40% of the gene is non-coding. Calculate the total length of the gene in pairs of nucleotides.
Medium
- A1755 base pairs
- B1931 base pairs
- C1579 base pairs
- D2107 base pairs
58.A polypeptide chain contains 250 amino acids. How many peptide bonds are present in the chain?
Medium
- A274 peptide bonds
- B299 peptide bonds
- C224 peptide bonds
- D249 peptide bonds
59.A polypeptide is made of 120 amino acids. How many nucleotides are required in the coding region of the mRNA to specify this polypeptide?
Medium
- A324 nucleotides
- B360 nucleotides
- C396 nucleotides
- D432 nucleotides

Unlock all 59 questions & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

Coming soon

	One polypeptide chain	Two polypeptide chains	Three polypeptide chains
A	Collagen	Insulin	Haemoglobin
B	Lysozyme	Insulin	Collagen
C	Lysozyme	Haemoglobin	Insulin
D	Haemoglobin	Lysozyme	Collagen

atom	single bond / nm	double bond / nm
O	0.13	-
H	0.06	0.110
C	0.154	0.120
N	0.14	0.134