Biology by Bradford
Biology by Bradford · AP Biology · Unit 1.4

Proteins

One core, twenty R groups, four levels of folding — and a function that collapses the moment the shape does.

AP · U1.4CED 1.7Interactive lesson
Concept 01 · CED 1.7

Twenty variations on one molecule

Every amino acid is the same molecule with one part swapped out. That one part is why a protein can be an enzyme, a hormone, a hair or an antibody.

Constant

The core

An amino group, a carboxyl group, a hydrogen and a central carbon — identical in all twenty.

Variable

The R group

Nonpolar, polar, acidic or basic. It decides how each residue behaves in water and what it can bond to.

The join

Peptide bond

Carboxyl meets amino, water leaves, a covalent bond forms. Same condensation reaction as everywhere else.

H₂NCOOHCHRSAME IN ALL 20ONLY THIS VARIES—CH₃NONPOLARburied in the core—CH₂OHPOLARhydrogen-bonds—CH₂COO−ACIDICionic bonds—CH₂SHCYSTEINEdisulfide bridges
The core is drawn once because it never changes. Everything that distinguishes one amino acid from another sits in the pink box — and the four examples below show the four behaviours you need: hides from water, hydrogen-bonds with it, carries a charge, or forms a covalent bridge to another cysteine.
Diagram · interactive

Anatomy of an amino acid

Tap each group. The inset on the right shows what happens when two of them meet.

CαH₂NAMINOCOOHCARBOXYLHRVARIABLEAA 1AA 2PEPTIDE BONDWATER LEAVESTAP A PART
Diagram

Four groups, one carbon

Tap the amino group, the carboxyl group, the central carbon, the hydrogen, the R group, or the peptide bond inset.

Signature interactive · CED 1.7

Four levels, then failure

Step through the folding of a protein — and then watch what heat does to it.

Concept 02 · CED 1.7

Shape is the whole point

A protein does not work because of what it is made of. It works because of the shape that assembly produces — and shape is fragile.

Specificity

Complementary shapes

Active sites, receptors and antibody binding sites are all shapes that fit one partner and not others.

Fragility

Denaturation

Heat and pH break weak interactions. Shape goes, function goes — but the sequence stays.

Consequence

One residue matters

Change a single amino acid and you may change nothing at all, or you may change the whole molecule's behaviour.

NORMAL β-CHAINVALHISLEUTHRPROGLUPOSITION 6GLUcharged, stays solubleSICKLE β-CHAINVALHISLEUTHRPROVALPOSITION 6GLUnonpolar patch, chains stick
Six amino acids into the β-chain of haemoglobin, one charged glutamate becomes a nonpolar valine. Nothing else in the 146-residue chain changes. That single hydrophobic patch lets deoxygenated haemoglobin molecules stick to one another and polymerise into fibres, which distort the red blood cell into the shape on the right.
Concept map · interactive

How it all hangs together

Tap a node to light up its connections. The pink dashed link is the shortcut from a single base change to a clinical phenotype.

folds intothen foldssometimesC=O to N-HR groups bondgive itdeterminesdestroysone swap can change itPrimary sequenceSecondaryTertiaryQuaternaryBackbone H-bondsR group bondsFunctionPrecise 3-D shapeDenaturation
Concept map

Tap a node

Read it downwards on the left for the four levels, then across to the right for why any of it matters.

Retrieval · drag and drop

Fill the gaps

Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.

aminocarboxylR grouppeptideprimaryhydrogen bondsdisulfidedenatured glycosidicphosphodiester

Every amino acid shares the same core: an group at one end, a group at the other, and a variable attached to the central carbon. Two amino acids join by condensation to form a bond, and the order of amino acids along the finished chain is its structure. The α-helix and β-pleated sheet are held together by between atoms of the backbone. Among the R group interactions that create tertiary structure, only the bridge is covalent. When heat or extreme pH breaks those weak interactions, the protein is and can no longer do its job.

Check yourself · AP-style

Six questions

Single best answer. You get the reasoning as soon as you commit.