01 / 20
AA AA PEPTIDE

Peptide Bonds &
Primary Structure

Discover how amino acids link together through covalent peptide bonds to form the linear sequence that defines every protein's identity.

Biochemistry

What Are Amino Acids?

Amino acids are the monomers (building blocks) of proteins. Each amino acid has a common backbone structure: a central \(\alpha\)-carbon bonded to four groups.

Amino Group
\(\text{-NH}_2\) : acts as a base, can accept a proton
Carboxyl Group
\(\text{-COOH}\) : acts as an acid, can donate a proton
R Group
Variable side chain: determines the identity and properties of each amino acid
H Atom
A single hydrogen completes the four bonds of the \(\alpha\)-carbon

The General Structure of an Amino Acid

\(\alpha\)C
\(\text{NH}_2\)
 Amino Group
\(\text{COOH}\)
 Carboxyl Group R Side Chain H Hydrogen

Why Does Primary Structure Matter?

The primary structure of a protein is its unique, linear sequence of amino acids. This sequence is the foundation of all higher-level protein organization.

1
Dictates 3D shape: The amino acid sequence determines how the protein folds into secondary, tertiary, and quaternary structures.
2
Determines function: Even a single amino acid change can alter protein activity (e.g., sickle cell disease).
3
Encoded by DNA: The gene sequence in DNA is transcribed and translated to produce the specific amino acid order.

The Peptide Bond: A Covalent Link

A peptide bond forms when the amino group of one amino acid reacts with the carboxyl group of another. This is a dehydration synthesis (condensation) reaction: one molecule of \(\text{H}_2\text{O}\) is released.

\(\text{NH}_2\)
\(\text{CHR}_1\)
\(\text{COOH}\)
 Amino Acid 1 +
\(\text{NH}_2\)
\(\text{CHR}_2\)
\(\text{COOH}\)
 Amino Acid 2
\(\text{NH}_2\)
\(\text{CHR}_1\)
\(\text{CO-NH}\)
\(\text{CHR}_2\text{COOH}\)
 Dipeptide
+ \(\text{H}_2\text{O}\)
 Water released Peptide Bond (C-N)

Key Properties of the Peptide Bond

Covalent
A strong bond formed by sharing electrons between the C and N atoms
Planar & Rigid
Partial double-bond character prevents free rotation around C-N
Trans Configuration
The \(\alpha\)-carbons on either side are almost always in trans orientation

Resonance in the Peptide Bond

Form 1
\(\text{C}\alpha\)
 C O N H ↔ Form 2
\(\text{C}\alpha\)
 C
\(\text{O}^-\)
\(\text{N}^+\)
 H Resonance gives partial double-bond character to C-N Result: Rigid, planar peptide bond

N-terminus to C-terminus: Directionality

A polypeptide chain has inherent directionality. By convention, the sequence is always read from the N-terminus (free amino group) to the C-terminus (free carboxyl group).

\(\text{H}_2\text{N}-\)
 N-terminus Ala Gly Ser Val
-\(\text{COOH}\)
 C-terminus Reading direction: N → C bond bond bond

Peptide Bonds vs. Other Protein Bonds

Peptide Bond Hydrogen Bond Disulfide Bond
Type Covalent Non-covalent Covalent
Strength Strong Weak (individually) Strong
Structure Level Primary Secondary / Tertiary Tertiary
Atoms Involved C and N N-H...O=C S-S (Cys-Cys)
Broken By Hydrolysis / proteases Heat / pH change Reducing agents

Formation vs. Breakdown of Peptide Bonds

Dehydration Synthesis
Two amino acids join, releasing one \(\text{H}_2\text{O}\) molecule. Catalyzed by the ribosome during translation. Requires energy (GTP).
Hydrolysis
Water is added to break the peptide bond, separating amino acids. Performed by proteases (enzymes) during digestion or protein turnover.

Four Levels of Protein Structure

Primary Linear sequence Peptide bonds THIS LESSON Secondary
\(\alpha\)-helix &
\(\beta\)-sheet
 H-bonds Tertiary 3D folding R-group interactions Quaternary Multiple subunits Subunit assembly Primary structure is the foundation: it determines all higher levels

How to Determine Primary Structure

1
Identify each amino acid in the chain by its R group (side chain). Use the three-letter or one-letter abbreviation code.
2
Locate the N-terminus (free \(\text{-NH}_2\) or \(\text{-NH}_3^+\)) and the C-terminus (free \(\text{-COOH}\) or \(\text{-COO}^-\)).
3
Read the sequence from N-terminus to C-terminus. This is the primary structure. Example: Ala-Gly-Ser-Val.
4
Count peptide bonds: For \(n\) residues, there are \(n - 1\) peptide bonds and \(n - 1\) water molecules released during synthesis.

Example: Counting Peptide Bonds

Problem: A polypeptide chain consists of 150 amino acids. How many peptide bonds does it contain, and how many water molecules were released during its synthesis?

Example: Sickle Cell Disease

Problem: Hemoglobin's \(\beta\)-chain has 146 amino acids. In sickle cell disease, the 6th amino acid changes from glutamic acid (Glu) to valine (Val). Why is this single change so devastating?

Normal (HbA): Val His Leu Thr Pro Glu Glu ... Sickle (HbS): Val His Leu Thr Pro Val Glu ... Position 6

Example: Molecular Weight of a Peptide

Problem: Estimate the molecular weight of a tripeptide (Ala-Gly-Ser) given that the average molecular weight of an amino acid is approximately 128 Da (daltons) and water has a molecular weight of 18 Da.

Common Mistakes to Avoid

✕ Reading Direction
Writing sequence from C-terminus to N-terminus. Always read N to C.
✕ Bond Count
Saying \(n\) amino acids have \(n\) peptide bonds. The correct count is \(n - 1\).
✕ Bond Type Confusion
Confusing peptide bonds (covalent, primary structure) with hydrogen bonds (non-covalent, secondary structure).

Question 1 of 5

A peptide bond forms between which two functional groups on amino acids?

Question 2 of 5

How many peptide bonds are present in a polypeptide composed of 200 amino acids?

Question 3 of 5

The peptide bond has partial double-bond character because of:

Question 4 of 5

In sickle cell disease, the substitution of glutamic acid with valine at position 6 of the hemoglobin \(\beta\)-chain illustrates the importance of:

Question 5 of 5

Which reaction breaks a peptide bond?