Answer:
D
Explanation:
There are four types of protein structures:
1. Primary Structure - linear order of amino acids in a polypeptide
2. Secondary Structure - folding of the amino acids into a repeated pattern due to hydrogen bonding of the polypeptide backbone
3. Tertiary Structure - the three dimensional structure created by a single polypeptide
4. Quaternary Structure - the arrangement of a number of polypeptide (two or more) folded into a larger complex.
The figure shows the Quaternary structure found in proteins, as is evident from the two different polypeptide chains (identified as light grey and a darker grey) that fold into a complex. This structure is often held together by van der waals forces, ionic bonds, hydrogen bonds, and in some cases, covalent bonds.
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