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Proteins
Structure and Function

Explore how amino acids build into complex macromolecules that drive nearly every process in living cells: from catalysis to structural support.

AP Biology

What Are Proteins?

Proteins are polymers made of amino acid monomers linked by peptide bonds. They are the most structurally and functionally diverse macromolecules in living systems, making up over 50% of the dry mass of most cells.

Anatomy of an Amino Acid

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

Peptide Bonds and Dehydration Synthesis

Amino Acid 1
\(\text{H}_2\text{N-CH(R)-COOH}\)
 + Amino Acid 2
\(\text{H}_2\text{N-CH(R)-COOH}\)
\(\text{H}_2\text{O}\)
 released Dipeptide
\(\text{H}_2\text{N-CH(R)-CO-NH-CH(R)-COOH}\)
 peptide bond

The Four Levels of Protein Structure

Primary
Linear sequence of amino acids in the polypeptide chain, determined by DNA.
Secondary
Local folding into alpha helices and beta pleated sheets via hydrogen bonds.
Tertiary
3D shape of one polypeptide from R-group interactions: hydrophobic, ionic, disulfide.
Quaternary
Multiple polypeptide subunits assembled into one functional protein (e.g., hemoglobin).

Visualizing Protein Folding

Primary amino acid chain Secondary alpha helix Tertiary 3D fold Quaternary subunits Key Bonds and Interactions Hydrogen 2° and 3° structure Hydrophobic 3° structure (core) Ionic charged R groups Disulfide covalent S-S bond Each level of structure depends on different chemical interactions

Sequence folds into local, global and multi-chain structure

Main sculpture has four heme-bearing folded lobes, two teal and two coral. Extending teal ribbon belongs to one illustrative chain; anatomy is stylized and not atom resolved. Separate exact panel: Sequence and backbone-bond schematic accompany a four-node hemoglobin assembly with two alpha and two beta subunits.Protein organization spans amino-acid sequence, local secondary structure, a folded chain and multi-chain assemblies. An α-helix uses backbone hydrogen bonds from residue i to i+4; hemoglobin contains two α and two β chains.

Protein structure spans four levels

Protein structure spans four levels. Protein organization spans amino-acid sequence, local secondary structure, a folded chain and multi-chain assemblies. An α-helix uses backbone hydrogen bonds from residue i to i+4; hemoglobin contains two α and two β chains.Protein organization spans amino-acid sequence, local secondary structure, a folded chain and multi-chain assemblies. An α-helix uses backbone hydrogen bonds from residue i to i+4; hemoglobin contains two α and two β chains.

Fibrous vs Globular Proteins

Fibrous Globular
Shape Long, rope-like strands Compact, spherical
Solubility Insoluble in water Usually water-soluble
Function Structural support Catalysis, transport, signaling
Examples Collagen, keratin Hemoglobin, enzymes
Structure level Mostly 2° (repetitive) Complex 3° and 4°

Major Functions of Proteins

Enzymes
Catalyze reactions by lowering activation energy (e.g., DNA polymerase)
Structural
Provide physical support and shape (e.g., collagen, actin)
Transport
Move molecules across membranes or through blood (e.g., hemoglobin)
Defense
Antibodies recognize and neutralize pathogens
Signaling
Hormones and receptors coordinate cellular communication (e.g., insulin)
Motor
Generate movement through conformational changes (e.g., myosin)
IgG binding and enzyme catalysis
IgG binding and enzyme catalysis
A generic antibody and enzyme provide material context. A separately authored IgG schematic shows two heavy chains, two light chains and antigen-binding tips; a generic hydrolysis converts substrate and water to two products.
An ordinary noncatalytic IgG recognizes antigen at its Fab tips without changing it; a generic enzyme catalyzes a reaction. The selected schematic has two heavy and two light chains; protein forms are illustrative, not atomic structures. Catalytic antibodies exist and are outside this example.

Enzymes: Lock and Key Model

Enzyme Active site S E-S S Enzyme-Substrate Complex Substrate binds to the active site complementary shape allows specificity

Denaturation: Losing Shape

Native (Functional) folded Heat, pH, chemicals Denatured (Non-functional) unfolded Disruption of weak bonds unfolds the protein Primary structure (peptide bonds) stays intact

Cold, heated and recovered: an enzyme experiment with a visible control story

A researcher transfers tubes between thermal pretreatment stations and an assay area beside an exact protocol and synthetic reaction-rate comparison.To distinguish prior temperature exposure from immediate reaction conditions, return all enzyme treatments to the same assay temperature and pH. Recovery after cooling differs from persistent activity loss after heating.

Separate treatment from the assay

Separate treatment from the assay. To distinguish prior temperature exposure from immediate reaction conditions, return all enzyme treatments to the same assay temperature and pH. Recovery after cooling differs from persistent activity loss after heating.To distinguish prior temperature exposure from immediate reaction conditions, return all enzyme treatments to the same assay temperature and pH. Recovery after cooling differs from persistent activity loss after heating.

How to Identify Structure Levels

1
Primary: Look for references to amino acid sequence, gene mutations, or the order of residues.
2
Secondary: Look for alpha helices or beta sheets, and hydrogen bonds between backbone atoms (\(\text{N-H}\) to \(\text{C=O}\)).
3
Tertiary: Look for R-group interactions (hydrophobic, ionic, disulfide bonds) that create the overall 3D shape of a single polypeptide.
4
Quaternary: Look for multiple polypeptide subunits assembling together (e.g., hemoglobin has 4 subunits).

Example: Sickle Cell Disease

Normal Hemoglobin HbA round shape Position 6: Glu (hydrophilic) Single amino acid substitution Sickle Hemoglobin HbS sickle shape Position 6: Val (hydrophobic) Primary structure change → Tertiary shape change → Loss of function Demonstrates that even one amino acid substitution matters

Mutation can expose a hydrophobic interaction surface

A sickle-shaped red-cell cutaway exposes long bundled fibers beside a normal biconcave red cell. The exact companion contrasts Glu6 and Val6 chemistry and low-oxygen HbS assembly.The mature β-globin Glu6→Val6 substitution replaces a charged surface residue with a nonpolar one. Under low oxygen, HbS can assemble into fibers that deform red cells.

One residue changes interactions

One residue changes interactions. The mature β-globin Glu6→Val6 substitution replaces a charged surface residue with a nonpolar one. Under low oxygen, HbS can assemble into fibers that deform red cells.The mature β-globin Glu6→Val6 substitution replaces a charged surface residue with a nonpolar one. Under low oxygen, HbS can assemble into fibers that deform red cells.

Example: Enzyme Specificity

Question: Why can lactase break down lactose but not sucrose, even though both are disaccharides?

1
Identify the enzyme: Lactase is a globular protein with a specific 3D active site shape.
2
Apply the induced fit model: The active site is complementary only to the shape of lactose.
3
Explain specificity: Sucrose has a different 3D shape, so it cannot bind to lactase's active site. No binding = no catalysis.
4
Connect to structure: The R-group interactions (tertiary structure) of lactase create the unique active site geometry.

Example: Collagen Triple Helix

Collagen: Fibrous Protein with Quaternary Structure Chain 1 Chain 2 Chain 3 H-bonds Properties ★ Glycine every 3rd residue ★ High in proline, hydroxyproline ★ Insoluble in water ★ Provides tensile strength ★ Found in skin, bone, tendons

Common Mistakes on the AP Exam

✕ Denaturation = Destroyed
Denaturation unfolds the 3D structure but does not break peptide bonds. The primary sequence remains intact.
✕ Confusing Levels
Hydrogen bonds between backbone atoms are secondary; R-group interactions are tertiary. Don't mix them up.
✕ "All Proteins are Enzymes"
Enzymes are proteins, but not all proteins are enzymes. Proteins also serve structural, transport, defense, and signaling roles.

Summary: Protein Structure at a Glance

Level What It Describes Key Bonds Example
Primary Amino acid sequence Peptide (covalent) Insulin chain
Secondary Local folds (helix, sheet) Hydrogen (backbone) Alpha helix in keratin
Tertiary 3D shape of polypeptide R-group interactions Myoglobin
Quaternary Multi-subunit assembly Same as tertiary Hemoglobin (4 subunits)

Question 1 of 5

Which level of protein structure is directly determined by the gene sequence encoding the protein?

Question 2 of 5

A researcher heats a protein to 95°C and observes it loses its function. Which statement best describes what happened?

Question 3 of 5

Hemoglobin consists of four polypeptide subunits working together. This arrangement represents which level of protein structure?

Question 4 of 5

In sickle cell disease, the substitution of valine for glutamic acid at position 6 of the beta-globin chain causes the protein to aggregate. This demonstrates that:

Question 5 of 5

Which bond type is responsible for the formation of alpha helices and beta pleated sheets in secondary structure?