Proteins: Amino Acids and Peptide Bonds

This lesson covers the structure of amino acids, the formation of peptide bonds, and the assembly of polypeptides as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand the general structure of amino acids, how they join together, and the significance of the R-group in determining the properties of different amino acids.

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Introduction to Proteins

Proteins are the most structurally and functionally diverse group of biological molecules. They are involved in virtually every biological process:

• Enzymes — biological catalysts (e.g. amylase, DNA polymerase)
• Structural proteins — collagen (tendons, skin), keratin (hair, nails), actin and myosin (muscle)
• Transport proteins — haemoglobin (oxygen transport), channel proteins (ion transport across membranes)
• Hormones — insulin, glucagon (blood glucose regulation)
• Antibodies — immunoglobulins (immune defence)
• Receptors — membrane proteins that bind signalling molecules
• Motor proteins — myosin (muscle contraction), kinesin (intracellular transport)

All proteins are polymers of amino acids. The sequence of amino acids in a protein is determined by the sequence of nucleotides in DNA (the genetic code).

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Amino Acid Structure

There are 20 different amino acids commonly found in proteins. All amino acids share the same basic structure:

The General Structure

Every amino acid has four groups attached to a central carbon atom (the α-carbon):

1. An amino group (–NH₂) — basic (can accept H⁺)
2. A carboxyl group (–COOH) — acidic (can donate H⁺)
3. A hydrogen atom (–H)
4. An R-group (side chain) — variable; determines the unique properties of each amino acid

Key Definition: The R-group (or side chain) is the variable part of an amino acid that distinguishes one amino acid from another. The identity, chemical properties and size of the R-group determine the amino acid's characteristics.

The Importance of the R-Group

The 20 amino acids differ only in their R-group. The R-group determines:

• Whether the amino acid is polar (hydrophilic), non-polar (hydrophobic), positively charged or negatively charged at physiological pH.
• How the amino acid interacts with other amino acids and with the surrounding environment.
• The three-dimensional shape of the protein (folding is driven by interactions between R-groups).

R-Group Category	Properties	Examples	Role in Protein Structure
Non-polar (hydrophobic)	Hydrocarbon chains; repel water	Alanine (Ala), Valine (Val), Leucine (Leu), Isoleucine (Ile), Phenylalanine (Phe)	Found in the interior of globular proteins, away from water; interact via van der Waals forces
Polar uncharged (hydrophilic)	Contain –OH, –SH or –NH₂ groups; form hydrogen bonds with water	Serine (Ser), Threonine (Thr), Asparagine (Asn), Glutamine (Gln)	Found on the surface of globular proteins; form hydrogen bonds
Positively charged (basic)	Carry a positive charge at pH 7	Lysine (Lys), Arginine (Arg), Histidine (His)	Form ionic bonds with negatively charged R-groups
Negatively charged (acidic)	Carry a negative charge at pH 7	Aspartic acid (Asp), Glutamic acid (Glu)	Form ionic bonds with positively charged R-groups
Sulfur-containing	Contain –SH (thiol) group	Cysteine (Cys), Methionine (Met)	Cysteine forms disulfide bonds (–S–S–) between two cysteine residues

Exam Tip: You do not need to memorise the structures of all 20 amino acids, but you must be able to draw the general structure of an amino acid and explain how R-groups differ. In particular, know that cysteine can form disulfide bonds — this is frequently examined.

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Zwitterions and Ionisation

At physiological pH (approximately 7.4), amino acids exist as zwitterions — molecules that carry both a positive and a negative charge simultaneously:

• The amino group is protonated: –NH₃⁺ (positive)
• The carboxyl group is deprotonated: –COO⁻ (negative)

The overall molecule has no net charge at its isoelectric point (the pH at which the positive and negative charges are equal).

In acidic solutions (low pH), amino acids gain an extra H⁺ and carry a net positive charge. In alkaline solutions (high pH), amino acids lose an H⁺ and carry a net negative charge.

This amphoteric property (ability to act as both an acid and a base) allows amino acids to act as buffers in biological systems.

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The Peptide Bond

Amino acids are linked together by peptide bonds to form chains called polypeptides.

Formation of a Peptide Bond (Condensation Reaction)

A peptide bond forms between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of another:

1. The –OH from the carboxyl group of amino acid 1 and the –H from the amino group of amino acid 2 are removed.
2. A molecule of water (H₂O) is released.
3. A peptide bond (–CO–NH–) is formed between the two amino acids.

This is a condensation reaction catalysed by the ribosome during translation (protein synthesis).

Key Definition: A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of water. It has the structure –CO–NH–.

Hydrolysis of Peptide Bonds

The reverse reaction is hydrolysis: a water molecule is added across the peptide bond, breaking it and releasing the individual amino acids.

Hydrolysis of peptide bonds occurs:

• During digestion — proteases (e.g. pepsin, trypsin) hydrolyse peptide bonds in dietary proteins.
• In lysosomes — intracellular digestion of damaged or redundant proteins.
• In the laboratory — by boiling with concentrated acid (e.g. 6 mol dm⁻³ HCl) for several hours.

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Dipeptides, Polypeptides and Proteins

Term	Description
Dipeptide	Two amino acids joined by one peptide bond
Tripeptide	Three amino acids joined by two peptide bonds
Oligopeptide	A short chain of amino acids (typically 2–20)
Polypeptide	A long chain of amino acids (typically 50 or more) joined by peptide bonds
Protein	One or more polypeptide chains folded into a specific 3D shape, often with additional non-protein groups

A polypeptide chain has two distinct ends:

• The N-terminus (amino terminus) — the end with a free –NH₂ group.
• The C-terminus (carboxyl terminus) — the end with a free –COOH group.

By convention, amino acid sequences are always written from the N-terminus to the C-terminus (this is the order in which they are synthesised on the ribosome).

Exam Tip: Be precise with terminology. A polypeptide is a chain of amino acids linked by peptide bonds. A protein is a functional molecule that may consist of one or more polypeptides folded into a specific 3D conformation, sometimes with prosthetic groups. Not all polypeptides are proteins (e.g. some are still being translated or have not yet folded).

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The Biuret Test for Proteins

The biuret test is used to detect the presence of peptide bonds (and therefore proteins and polypeptides) in a sample.

Method:

1. Add the food sample to a test tube and dissolve in water.
2. Add an equal volume of sodium hydroxide solution (NaOH) to make the solution alkaline.
3. Add a few drops of dilute copper(II) sulfate solution (CuSO₄).
4. Mix gently (do not heat).
5. Observe the colour change.

Results:

Observation	Conclusion
Solution turns purple/violet	Protein (peptide bonds) present
Solution remains blue	Protein absent

The purple colour forms because Cu²⁺ ions form a complex with the peptide bonds (specifically with the nitrogen atoms in the peptide bonds) in alkaline conditions. The more peptide bonds present, the more intense the purple colour.

Exam Tip: A common mistake is heating the biuret test — you must not heat this test. Also, remember that the biuret test detects peptide bonds, not individual amino acids (a solution of free amino acids gives a negative result unless they are dipeptides or longer).

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Amino Acid Diversity and the Genetic Code

There are 20 standard amino acids encoded by the genetic code. The sequence of amino acids in a polypeptide is determined by the sequence of codons (triplets of nucleotide bases) in the messenger RNA (mRNA), which is itself determined by the DNA sequence of the gene.

Because there are 20 different amino acids and each can appear at any position in the chain, the number of possible polypeptides is astronomically large. For a polypeptide of length n:

Number of possible sequences = 20ⁿ

For example, a polypeptide just 100 amino acids long has 20¹⁰⁰ possible sequences — a number far greater than the number of atoms in the observable universe. This explains the enormous diversity of proteins found in living organisms.

Essential and Non-Essential Amino Acids

• Essential amino acids — cannot be synthesised by the organism and must be obtained from the diet (e.g. lysine, tryptophan, valine in humans).
• Non-essential amino acids — can be synthesised by the organism from other metabolic intermediates (e.g. alanine, aspartic acid).

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Summary

• Proteins are polymers of amino acids joined by peptide bonds.
• All amino acids have the same basic structure: an amino group, a carboxyl group, a hydrogen atom and an R-group attached to a central α-carbon.
• The R-group varies between the 20 amino acids and determines the properties of each amino acid (hydrophobic, hydrophilic, charged, sulfur-containing).
• Peptide bonds form by condensation reactions and are broken by hydrolysis.
• Polypeptide chains have an N-terminus and a C-terminus.
• At physiological pH, amino acids exist as zwitterions.
• The biuret test detects proteins (purple colour with Cu²⁺ in alkaline conditions).

Exam Tip: In extended response questions about proteins, always start with amino acids and peptide bonds before discussing higher levels of structure. A logical progression from primary → secondary → tertiary → quaternary structure shows the examiner clear biological understanding.

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A-Level Deep Dive: Amino Acids and Peptide Bonds

Spec mapping

This lesson sits in Edexcel 9BI0 Topic 1 — Biological Molecules, on amino-acid structure and peptide-bond formation. Content statements paraphrase to: describe the general structure of an amino acid (central α-carbon bearing –NH₂, –COOH, –H and an R-group); describe peptide-bond formation by condensation between the carboxyl of one amino acid and the amino of another with elimination of water, and the reverse hydrolysis; understand that amino acids vary only in their R-group, whose chemistry (hydrophobic, hydrophilic, charged, sulfur-containing) drives higher-order folding; describe the biuret test as a qualitative detection method for peptide bonds (refer to the official Pearson Edexcel 9BI0 specification for exact wording). The material is examined directly on Paper 1 and reactivated synoptically on the next lesson (protein structure), Topic 2 (membrane proteins), Topic 3 (translation at the ribosome), Topic 6 (antibody quaternary structure) and Topic 7 (haemoglobin cooperativity). Synoptic reach extends into folding pathology (sickle-cell, prion disease) and clinical immunology (antibody hyper-variable Fab regions).

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Worked example with full mark scheme

Question (8 marks): A patient with sickle-cell disease has a single nucleotide substitution in the gene encoding β-globin, which changes codon 6 from GAG to GTG. The resulting amino acid change converts glutamic acid (Glu, –CH₂CH₂COO⁻) to valine (Val, –CH(CH₃)₂) at position 6 of the β-chain.

(a) Identify the level of protein structure directly altered by the substitution and explain how this single change propagates to altered quaternary structure. (4)

(b) A dipeptide (Gly–Ala) is hydrolysed in 6 mol dm⁻³ HCl. Write a word-and-skeletal account of the hydrolysis, identifying which atoms in the products come from water. (4)

Solution with mark scheme:

(a) M1 (AO1) — the substitution alters primary structure (the linear sequence). A common pitfall is to write "tertiary" because the protein folds wrongly; the direct change is to sequence, with tertiary/quaternary changes downstream.

A1 (AO2) — Glu's R-group is negatively charged and hydrophilic; Val's is non-polar and hydrophobic. The substitution introduces a hydrophobic patch on the protein surface.

A1 (AO2) — propagation: the exposed hydrophobic patch on deoxyhaemoglobin S binds a complementary pocket on a neighbouring tetramer, polymerising tetramers into rigid fibres. This is altered quaternary assembly driven by altered tertiary surface chemistry — a primary → tertiary surface → quaternary polymerisation chain.

A1 (AO3) — clinical synthesis: polymerised HbS distorts erythrocytes into sickles, occluding capillaries (haemolytic anaemia, vaso-occlusive crisis). One R-group change propagates to organ-level pathology — the AO3 discriminator.

(b) M1 (AO1) — equation: Gly–Ala + H₂O → Gly + Ala; the peptide bond –CO–NH– is cleaved.

A1 (AO1) — locate the bond: between the carboxyl C of glycine and the amino N of alanine. Hydrolysis cleaves the C–N bond.

A1 (AO2) — assign the water atoms: the –OH of H₂O attaches to the carboxyl carbon of glycine, regenerating its free –COOH; the –H of H₂O attaches to the amino nitrogen of alanine, regenerating its free –NH₂. A common pitfall is to write "water is added" without saying which atom goes where — a free 1-mark loss.

A1 (AO2) — conditions: boiling 6 mol dm⁻³ HCl for several hours, or proteases (pepsin, trypsin) physiologically. The peptide bond is kinetically stable at neutral pH despite being thermodynamically hydrolysable.

Total: 8 marks (a: M1 A1 A1 A1; b: M1 A1 A1 A1). A clean A* response keeps levels-of-structure language precise and assigns water atoms explicitly to products.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Explain how the chemistry of the R-group determines whether a particular amino acid is more likely to be found on the surface or in the interior of a soluble globular protein, with reference to two contrasting examples.

Mark scheme decomposition by AO:

Mark	AO	Awarded for
1	AO1	Stating that all amino acids share the same backbone (–NH₂, –COOH, –H, α-carbon) and differ only in their R-group.
2	AO1	Classifying R-groups as hydrophobic (non-polar hydrocarbon, e.g. valine, leucine), hydrophilic-polar (e.g. serine, asparagine), charged (e.g. lysine⁺, glutamate⁻) or sulfur-containing (cysteine).
3	AO2	Linking hydrophobic R-groups to the interior of a soluble globular protein — sequestered away from water to minimise unfavourable hydration ordering.
4	AO2	Linking polar/charged R-groups to the surface — interacting with the aqueous environment via H-bonds and ion-dipole interactions, conferring solubility.
5	AO2	Worked contrast: valine (hydrophobic, –CH(CH₃)₂) buries; lysine (basic, –(CH₂)₄–NH₃⁺) projects to the surface and forms ionic interactions with negatively charged R-groups or solvent.
6	AO3	Evaluative synthesis: the hydrophobic effect (entropy of surrounding water) is the dominant thermodynamic driver of folding; primary sequence therefore encodes tertiary fold via a statistical bias on R-group placement, with ~30% of residues typically buried and ~70% surface-exposed in a typical globular protein.

Total: 6 marks split AO1 = 2, AO2 = 3, AO3 = 1. The AO3 mark rewards the candidate who frames folding as a thermodynamic optimisation of R-group placement, not just "hydrophobic in, hydrophilic out."

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Synoptic links

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