1.7: PROTEINS

Amino Acids and Peptide Bond Formation

Every protein begins as a linear chain of amino acids linked end to end by covalent bonds. Each amino acid has a central carbon bonded to four groups: a hydrogen atom, a carboxyl group (−COOH), an amine group (−NH₂), and a variable R group (also called a side chain). The R group is what makes each of the 20 amino acids chemically unique.

R groups fall into three categories based on their chemical properties:

The interactions among these R groups ultimately determine both the shape and function of every region of the protein.

To build a chain, the carboxyl group of one amino acid reacts with the amine group of the next amino acid. This dehydration synthesis reaction releases one water molecule and forms a covalent peptide bond, producing a growing peptide chain (also called a polypeptide). Because each bond links the same two functional groups, the backbone of every protein is identical — only the sequence of R groups hanging off the backbone differs.

Out of ScopeThe detailed molecular structure of individual amino acids is beyond the scope of the AP Exam. Do not memorize this for the exam.

MisconceptionStudents often confuse the peptide bond with hydrogen bonds that appear later in folding. The peptide bond is a strong *covalent* bond in the backbone, not a weak interaction between R groups.

Exam TipIf a question asks what holds amino acids together in the primary chain, the answer is always the peptide (covalent) bond.

Exam TipAnnotate the diagram to show that the bond forms specifically between −NH₂ and −COOH, not between R groups.

Primary and Secondary Structure

The primary structure of a protein is its specific, linear sequence of amino acids. Even a single change in this sequence — such as a substitution of one amino acid for another — can alter the protein's overall shape and, therefore, its function. The primary structure acts as the instruction set: it dictates every level of folding that follows.

Secondary structure results from local folding within the polypeptide backbone. Atoms in the backbone (not the R groups) form hydrogen bonds with one another at regular intervals. These repeating interactions produce two common shapes:

A single protein can contain multiple alpha-helices and beta-pleated sheets in different regions, connected by loops of less regular structure.

Examiner InsightAP free-response questions frequently ask students to distinguish the *bonds* responsible for each level of structure. For secondary structure, emphasize that hydrogen bonds form between backbone atoms — not between R groups.

Exam TipAlways specify *where* the hydrogen bonds occur (backbone vs. R groups) when comparing structural levels.

Tertiary and Quaternary Structure

The tertiary structure is the complete three-dimensional shape of a single polypeptide chain. While secondary structure involves only backbone interactions, tertiary structure arises from interactions among the R groups distributed along the entire chain. Four types of interaction drive tertiary folding:

Together, these interactions pull the polypeptide into a precise shape. Because shape determines function, any factor that disrupts these interactions — such as extreme pH, high temperature, or heavy metal ions — can denature [unfold] the protein, destroying its ability to function even though the peptide bonds of the primary structure remain intact.

Some functional proteins consist of two or more polypeptide subunits assembled together. The quaternary structure describes the arrangement of these multiple polypeptides into one functional complex. Hemoglobin, for example, consists of four polypeptide subunits that must associate correctly to carry oxygen. The same types of interactions that stabilize tertiary structure — hydrogen bonds, hydrophobic interactions, ionic interactions, and disulfide bridges — also hold the subunits together in quaternary structure.

All four levels of structure collectively determine a protein's function. A protein cannot carry out its biological role unless it achieves its correct final shape.

MisconceptionStudents sometimes think quaternary structure is "more important" than the other levels. In reality, all four levels contribute to function. A single amino acid change at the primary level can disrupt every higher level of structure — as seen in sickle cell disease, where one amino acid substitution alters hemoglobin's quaternary assembly.

Exam TipWhen asked how structure relates to function, connect all four levels, starting from the amino acid sequence.

Exam TipLabel the specific bond or interaction type responsible for each level of structure.