Problem 4 - Biochemistry: Concepts and Connections

Question

In the protein adenylate kinase, the C-terminal region has the sequence $$ \begin{gathered} \text { Val-Asp-Asp-Val-Phe-Ser-Gln-Val-Cys-Thr-His- } \\ \text { Leu-Asp-Thr-Leu-Lys- } \end{gathered} $$ The hydrophobic residues in this sequence are presented in boldface type. Suggest a possible reason for the periodicity in their spacing.

Step-by-Step Solution

Answer

Periodicity in hydrophobic residue spacing suggests an alpha-helical structure, exposing these residues on one face of the helix.

Step 1: Identify the Hydrophobic Residues

In the given sequence of adenylate kinase's C-terminal region, the hydrophobic amino acids are often highlighted differently. Typically, they include Valine (Val), Phenylalanine (Phe), Leucine (Leu), and Isoleucine (Ile). In this sequence, boldface indicates these hydrophobic residues: Val, Val, Phe, Val, Leu, Leu.

Step 2: Observe Periodicity

Examine the sequence pattern. Note that hydrophobic residues are spaced in a pattern: Val-2-2-Val-3-Phe-1-Val-4-Leu-2-4-Leu-K. This suggests a rhythmic pattern, albeit not perfectly periodic, with recurrent intervals between hydrophobic residues, approximately every 3 to 4 positions.

Step 3: Possible Reason for Periodicity

The observed periodicity in hydrophobic residue spacing can suggest an alpha-helical structure within the protein. In an alpha helix, amino acids are spaced such that those on the same face of the helix are about 3 to 4 positions apart, thus periodically exposing hydrophobic residues to form a hydrophobic face.

Key Concepts

Hydrophobic Amino Acids

Hydrophobic amino acids are essential building blocks in proteins. They have side chains that are resistant to water, hence preferring to reside in the interior of proteins. This helps stabilize protein structure by avoiding contact with water. Common hydrophobic amino acids include:

• Valine (Val)
• Leucine (Leu)
• Isoleucine (Ile)
• Phenylalanine (Phe)
• Methionine (Met)

In the specific sequence of adenylate kinase, these residues likely play a crucial role in maintaining the structure, specifically by contributing to the protein's burying of hydrophobic side chains. Moreover, their strategic periodic positioning aids in forming secondary structures like alpha helices to stabilize the protein complex further. By hiding from water, they help the protein maintain its functional configuration.

Alpha Helix

The alpha helix is a common structural motif in proteins, characterized by its right-handed coil shape. In this structure, every backbone $N-H$ group forms a hydrogen bond with the backbone $C=O$ group of the amino acid located three to four residues earlier, stabilizing the helix. A notable feature of the alpha helix is its ability to present hydrophobic amino acids on one face of the helix, which can align with similar faces on other helices or protein surfaces, forming compact regions that avoid water. This alignment can be understood by considering the regular periodicity in the spacing of hydrophobic residues, like in adenylate kinase, where every 3-4 residues, a hydrophobic side chain appears on the same helical face. Such configurations are crucial in functionalities where the protein may need to interface with lipid membranes or other protein components, maintaining functionality and structural integrity.

Adenylate Kinase

Adenylate kinase is an enzyme that plays a vital role in cellular energy homeostasis. It catalyzes the conversion of two molecules of ADP into ATP and AMP, a crucial reaction in maintaining the adenine nucleotide pool within cells. The protein's structure supports its function, with its hydrophobic core often assisting in stabilizing the folded state necessary for enzymatic activity. The C-terminal region sequence, for instance, includes various hydrophobic amino acids. These amino acids not only participate in forming alpha helices but also in creating hydrophobic interactions that bring distant parts of the protein together, ensuring the enzyme’s configuration remains conducive for catalysis. Understanding these sequences allows researchers to gain insights into how enzyme function relates to its intricate structural design.