3.1 Protein Structure

3.1 Protein Structure

Proteins do most of the specific chemical work in a cell. Enzymes speed reactions, hemoglobin carries oxygen, and structural proteins shape hair, tendon, and the cytoskeleton. CLEP Biology asks for the chemistry of that polymer: the shared parts of an amino acid, the bond that joins amino acids, the four levels of folding, and what denaturation does and does not break.

The Amino Acid and the Peptide Bond

A protein is a polymer of amino acids. Each amino acid is built on an alpha carbon bonded to four partners: an amino group, a carboxyl group, a hydrogen atom, and a variable R group, also called a side chain. The R group is the piece that changes from one amino acid to another. About 20 standard amino acids are used to build proteins. Side chains may be nonpolar, polar uncharged, acidic, or basic. Cysteine matters in particular because its side chain contains sulfur and can form a covalent cross-link with a second cysteine.

Dehydration builds a directional chain

Two amino acids join by a dehydration reaction. The carboxyl group of one and the amino group of the next release the atoms of a water molecule, $H_2O$. The bond left behind is a peptide bond, a covalent link between a carbon and a nitrogen. A short chain is a peptide. A long chain is a polypeptide. Hydrolysis is the reverse reaction: water is added and the peptide bond is cut. A fever does not perform hydrolysis.

The chain has direction. The N-terminus carries the free amino group. The C-terminus carries the free carboxyl group. Biochemists write the sequence from the N-terminus toward the C-terminus, and ribosomes assemble polypeptides in that same direction. On a drawing, identify those two ends before you decide which level of structure the question is asking about.

Keep the bonds distinct. The peptide bond is the covalent backbone connection that defines the sequence. It is not a hydrogen bond. Hydrogen bonds, ionic bonds, and disulfide bridges describe folding, which begins with the next levels.

Four Levels of Protein Structure

Every protein has a sequence, and a typical working chain also has local folds and an overall shape. A fourth level appears only when the functional protein contains more than one polypeptide.

Primary structure is the order of amino acids. Change that order and you change the protein. Sickle-cell hemoglobin shows how specific the sequence is: one substitution in the beta chain replaces a glutamate with valine, and that single change is enough to alter how the protein behaves.

Secondary structure is local folding held by hydrogen bonds between backbone atoms, not between R groups. The usual partners are a carbonyl oxygen and a hydrogen attached to nitrogen along the chain. An alpha helix is a coil stabilized by hydrogen bonds that run with the coil. A beta pleated sheet is an array of strands linked sideways by hydrogen bonds. A single polypeptide often contains several helices and sheets joined by loops. Either pattern is secondary structure.

Tertiary structure is the three-dimensional shape of one whole polypeptide. It comes from interactions among R groups:

• Hydrophobic clustering packs nonpolar side chains away from water, commonly into the interior.
• Additional hydrogen bonds can form between polar side chains. Those contacts are separate from the backbone hydrogen bonds of an alpha helix or a beta pleated sheet.
• Ionic bonds, also called salt bridges, attract a negatively charged side chain to a positively charged side chain. A large pH change can alter those charges and weaken the attraction.
• A disulfide bridge is a covalent bond between two cysteine side chains. It is a cross-link, stronger than a hydrogen bond or an ionic bond, and it is still not a peptide bond.

Quaternary structure means two or more polypeptides assemble into one functional protein. Hemoglobin has four polypeptide subunits: two alpha chains and two beta chains. That fact is worth memorizing, and so is the limit: plenty of proteins work as a single chain. A single-chain protein has primary, secondary, and tertiary structure and does not have quaternary structure.

Denaturation Without Cutting the Chain

Denaturation is the loss of functional shape. Heat, extreme pH, and some solvents disrupt hydrophobic clustering, hydrogen bonds, and ionic bonds. The active site of an enzyme disappears because the fold disappears. The amino acid sequence can remain intact, because those stresses do not add water across peptide bonds.

Worked example: fever and extreme pH

Picture catalase, the enzyme that breaks down hydrogen peroxide, folded so that its active site fits the substrate. A very high fever, or heat well above the temperature the enzyme normally experiences, increases motion in the polypeptide. Hydrogen bonds and ionic bonds let go, and nonpolar groups that were buried become exposed to water. The chain unfolds. Catalase loses activity. The covalent peptide bonds are still present, so the primary structure has not been chopped into free amino acids.

Now change the pH far outside the range the enzyme is built for. Acid or base changes the charges on ionizable R groups, so ionic bonds fail and some hydrogen bonds fail with them. The three-dimensional shape collapses for a charge reason rather than a heat reason. The sequence still need not break. Extreme pH can denature a protein that is not built for it, while pepsin is a separate enzyme suited to stomach acid. Extreme pH is a classic denaturing stress. It is not, by itself, proof that peptide bonds were hydrolyzed.

Digestive hydrolysis is the contrasting case. There, enzymes add water and cut peptide bonds, so primary structure is destroyed. If the stem says heat, fever, or the wrong pH, and it never mentions cutting the chain, the event is denaturation.