Chapter 7 · Macromolecules
Proteins and Denaturing
Identify the parts of an amino acid, explain how peptide bonds build a chain, and predict what denaturing does and does not change.
The monomer: amino acids
Proteins are the cell's molecular tool kit. They are CHON molecules (carbon, hydrogen, oxygen, nitrogen) built from 20 different amino acids.
Every amino acid has a central carbon bonded to an amino group, a carboxyl group and a hydrogen atom. The fourth bond holds the R group (side group).
Key concept
The only difference
The R group is the only part that differs from one amino acid to another. Some R groups are polar and some are not, so polarity is one result of having a different R group. If a question asks what makes amino acids different, the answer is the R group.
Building the chain
Amino acids link by a dehydration reaction between the amino group of one and the carboxyl group of the next. The link is a peptide bond. The chain is a polypeptide, which is the polymer.
Quick check
A polypeptide has 50 amino acids. How many peptide bonds hold it together?
Shape is everything
A chain that is just strung out cannot do the job. A working protein is one or more polypeptide chains precisely twisted, folded and coiled. Different structure causes different function.
Temperature and pH influence shape. An unfavorable environment makes a protein denature, which means unfold.
| After denaturing | |
|---|---|
Still a protein (polypeptide chain)? | Yes |
Peptide bonds broken? | No |
Original folded shape? | Lost |
Still functional? | No |
Worked example
The fried egg
Raw egg white is clear and runny. Fried, it is white and firm. What happened to its proteins, and are they still proteins?
Frying is heat. Heat is one of the two factors that change protein shape.
The proteins denatured: they unfolded and tangled, which is why the texture changed.
Denaturing does not break peptide bonds, so each chain is still a connected polypeptide.
Answer
Still proteins, still chains, no longer folded the original way, no longer functional. Cooling the egg does not fold them back.
Four proteins to know
| Protein | Where | Job |
|---|---|---|
Hemoglobin | Blood | Oxygen transport |
Collagen | Skin, bone, muscle | Strength |
Keratin | Nails, hair | Strength |
Myosin | Muscle cells | Muscle contraction |
Explore deeper: Four levels of structure
Optional. This goes beyond what most courses ask for, and mastery here does not depend on it.
Not every course asks about the folding types. They still explain what denaturing really changes, so they are worth ten minutes.
| Level | What it is | Held by or set by |
|---|---|---|
Primary | The order of amino acids in the chain | Peptide bonds |
Secondary | Parts of the chain form coils and folded sheets | Hydrogen bonds along the backbone |
Tertiary | One whole chain folds into a 3-D shape | Interactions between R groups |
Quaternary | Two or more chains fit together (hemoglobin has 4) | Interactions between chains |
Denaturing undoes the secondary, tertiary and quaternary levels. The primary level survives, because it is held by peptide bonds. Not every protein has a quaternary level, because some are made of a single chain.