Proteins

🧬 Proteins: The Cell's Workhorses

Proteins carry out most active jobs in cells: catalyzing reactions, transporting cargo, signaling between cells, fighting pathogens, and providing structure. There are roughly 20,000 distinct proteins in a human cell, all built from just 20 amino acid monomers.

Where you'll meet proteins

Element composition

Proteins always contain C, H, O, N (nitrogen makes proteins distinctive), and often S (sulfur, in cysteine and methionine).

Concept Check 🎯

Amino Acid Anatomy

Every amino acid has the same backbone:

• Amino group ($-NH_2$)
• Central α-carbon with an H
• Carboxyl group ($-COOH$)
• R group (side chain) — this is what makes each amino acid unique

R-group classes (high yield!)

Concept Check 🎯

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Proteins: Structure & Function

Amino Acids

All amino acids have:

• A central carbon (α-carbon)
• An amino group ($-NH_2$)
• A carboxyl group ($-COOH$)
• A hydrogen atom
• A variable R group (side chain) — determines the amino acid's properties

20 different amino acids exist, differing only in their R groups.

Peptide Bonds

Amino acids link via (dehydration synthesis between and ).

Functions Driven by Shape

Protein function depends on 3D shape, which depends on amino acid sequence. Even one wrong amino acid can change the shape and break function — as in sickle cell anemia, where a single Glu→Val swap in hemoglobin's β-chain causes red blood cells to deform.

Major functional categories

🎯 AP Review: Proteins

Must-know synthesis points

1. 20 amino acids → ~20,000 distinct proteins in the human cell — diversity comes from sequence and length.
2. Four levels of structure — primary (sequence) → secondary (α-helix / β-sheet, H-bonds) → tertiary (R-group interactions) → quaternary (multiple polypeptides).
3. Sequence determines shape; shape determines function. Sickle cell and prion diseases both demonstrate this.
4. Denaturation disrupts higher-order structure (heat, pH, salts) and usually destroys function.
5. Enzymes lower activation energy through a complementary active site (induced fit).

Common AP traps

• Don't say "proteins are made of nucleotides." Nucleotides → nucleic acids; amino acids → proteins.
• Quaternary structure exists only for proteins with two or more polypeptide chains.
• Hydrogen bonds dominate secondary structure; R-group interactions dominate tertiary.

Workshop Problem 📐

Workshop Problem 📐

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