BIOB 160 Principles of Living Systems · Unit 1: The chemistry of life · on Exam 1
1.6 Proteins: structure and function
Study notes for Proteins: structure and function in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- Proteins do most of the cell’s work: enzymes, structure, transport, storage, hormones, receptors, movement and defense.
- An amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen and a variable R group. The 20 amino acids are grouped by R group: nonpolar (hydrophobic), polar and electrically charged (acidic or basic).
- Amino acids join by peptide bonds (dehydration synthesis) into a polypeptide with an N-terminus and a C-terminus.
- Four levels of structure: primary (the amino acid sequence), secondary (α helices and β pleated sheets held by hydrogen bonds between backbone atoms), tertiary (the 3-D shape from R-group interactions: hydrophobic interactions, hydrogen bonds, ionic bonds and covalent disulfide bridges) and quaternary (two or more polypeptides, such as hemoglobin).
- Shape determines function. A single amino acid change can matter: in sickle-cell disease, valine replaces glutamic acid in hemoglobin. Heat, pH or salt can denature a protein by disrupting its shape; chaperonins help proteins fold.
Worked example
A mutation replaces a hydrophobic amino acid buried in the core of an enzyme with a charged one. Which level of structure is changed first, and why might the enzyme stop working?
Show the solution
The primary structure changes first (the sequence). A charged R group in the hydrophobic core disrupts the tertiary fold, which can distort the active site so the enzyme no longer binds its substrate.
Common mistakes
- Saying secondary structure comes from R groups. It comes from hydrogen bonds between backbone atoms.
- Thinking every protein has quaternary structure. Only those with more than one polypeptide do.
- Believing denaturation breaks peptide bonds. It disrupts weaker interactions; the sequence stays intact.
Exam tipReading: Campbell 5.4 (pp. 78–83). Link each level to the bond that holds it: peptide (primary), backbone H bonds (secondary), R-group interactions (tertiary).
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