Lesson 4 of 7 / Biomolecules: structure explains function
Join amino acids in a particular order
Which part varies and which part forms the link?
I can describe amino acid structure and peptide-bond formation and hydrolysis.
I can describe alpha/beta-glucose, glycerol, fatty acids and amino acids.Syllabus
Syllabus 8876, Core 1(e). Describe alpha/beta-glucose, glycerol, fatty acids and amino acids.
I can explain formation and hydrolysis of glycosidic, ester and peptide bonds.Syllabus
Syllabus 8876, Core 1(f). Explain formation and hydrolysis of glycosidic, ester and peptide bonds.
About 5 min
Make a guess. You are not marked.
Which parts of two amino acids are joined by a peptide bond?
Show the answer
One amino group and one carboxyl group.
Condensation between these groups forms the backbone. R groups stick out and are not part of the bond.
The key idea
An amino acid has an amino group, a carboxyl group, hydrogen and an R group around its central carbon. Peptide bonds join the amino and carboxyl groups of adjacent units.
Explore the idea
Condensation joins the backbone
Condensation removes the elements of water while joining neighbouring units; the R groups are not the joining groups.
Explanation
The central carbon of an amino acid is bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group. The R group determines differences such as charge, polarity and hydrophobicity. You need not memorise chemical formulae of particular R groups for H1.
A peptide bond forms by condensation between the carboxyl group of one amino acid and the amino group of another. Water is released and a -CO-NH- link forms. Repeating this process produces a polypeptide with an amino end and a carboxyl end.
Peptide bonds form the covalent backbone; R groups project from it. A chain of n amino acids has n - 1 peptide bonds if it is one unbranched, non-cyclic chain. Hydrolysis uses water to break the bonds and release shorter peptides or amino acids.
The order of amino acids is the primary structure. Two chains with the same overall amino-acid composition can have different sequences and therefore different patterns of folding and function.
Step by step
- 1
Find the constant groups
Locate amino and carboxyl groups around each central carbon.
- 2
Join neighbouring units
Condensation creates the peptide link and releases water.
- 3
Retain the side chains
R groups remain attached to the backbone and influence later folding.
Worked example
Build a short peptide
How many peptide bonds and released water molecules result from joining six free amino acids into one linear chain?
One way to explain it
Five peptide bonds form and five water molecules are released. Each new connection links two previously separate parts of the growing chain.
Why this answer works
- A linear sequence of six units has five joins.
- Each join is one condensation reaction.
Watch out for this
A student says: "Peptide bonds join the R groups together." What is wrong with this?
Show the answer
Peptide bonds join amino and carboxyl groups to form the backbone. R-group interactions contribute to higher levels of structure.
Check your understanding
Which link is broken when a polypeptide backbone is hydrolysed?
Show the answer
Peptide bond.
Hydrolysis uses water to break the covalent -CO-NH- link.
Exam-style questions
Check your understanding
A polypeptide of 150 amino acids is completely hydrolysed. How many water molecules are used?
Show the answer
149
A linear chain of 150 amino acids has 149 peptide bonds, and each hydrolysis uses one water molecule.
Quick recall
Which part varies and which part forms the link?