Lesson 4 of 7 / Biomolecules: structure explains function
Join amino acids in a particular order
Which part varies and which part forms the link?
In this lesson: Describe amino acid structure and peptide-bond formation and hydrolysis.
About 5 min
The key ideaAn 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.
Is this true? "Peptide bonds join the R groups together."
Peptide bonds join amino and carboxyl groups to form the backbone. R-group interactions contribute to higher levels of structure.