Your body contains tens of thousands of different proteins, each with a specific structure and function. The unique three-dimensional shape of each of these diverse proteins is based on several superimposed levels of structure.

Question

Which of the following statements is an accurate description of proteins?

The primary structure of a protein is the order of amino acids in a polypeptide, as coded for in the DNA of a gene.
The 20 amino acids found in proteins differ in the composition of their R groups, which may be either polar or charged.
Secondary structures, which include the alpha helix or beta pleated sheet, are held together by interactions between R groups.
Tertiary structure is the overall shape of a polypeptide, which may be stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and peptide bonds.
Quaternary structure is only found in proteins that have four identical polypeptide subunits.

Answer 1

Let's analyze each statement for accuracy regarding protein structure.

1. The primary structure of a protein is the order of amino acids in a polypeptide, as coded for in the DNA of a gene.

   • Accurate. The primary structure refers to the linear sequence of amino acids in the polypeptide chain that is directly determined by the sequence of nucleotides in the DNA.

2. The 20 amino acids found in proteins differ in the composition of their R groups, which may be either polar or charged.

   • Partially accurate. While it is true that the 20 amino acids have different R groups (side chains), it's important to note that their R groups can also be nonpolar or hydrophobic. Some amino acids have polar, charged, or nonpolar properties.

3. Secondary structures, which include the alpha helix or beta pleated sheet, are held together by interactions between R groups.

   • Inaccurate. Secondary structures (alpha helices and beta sheets) are primarily held together by hydrogen bonds between the backbone atoms of the polypeptide chain, not the R groups. R group interactions are more relevant in tertiary structure.

4. Tertiary structure is the overall shape of a polypeptide, which may be stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and peptide bonds.

   • Partially accurate. Tertiary structure refers to the overall three-dimensional shape of a polypeptide and is indeed stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges. However, peptide bonds are already part of the primary structure as they link amino acids together, so they are not considered stabilizing forces of tertiary structure.

5. Quaternary structure is only found in proteins that have four identical polypeptide subunits.

   • Inaccurate. Quaternary structure refers to the arrangement and interaction of multiple polypeptide chains (subunits) in a protein. It does not have to consist of four identical subunits; it can involve any combination of identical and different subunits.

In summary, the accurate statement is the first one regarding primary structure. The other statements contain inaccuracies or misleading information regarding protein structure.