Question

During SDS-Poly Acrylamide Gel Electrophoresis (SDS-PAGE), how many molecules of SDS are expected to bind to a 44 kD protein?

• A. 200
• B. 400
• C. 22
• D. 44

Hint:

Standard SDS-PAGE Constant: 1.4g SDS per 1g Protein.
Shortcut for Number of SDS molecules:
Number of SDS molecules $\approx$ Molecular Weight in Da 220.
Calculation: $44,000 / 220 = 200$.
This allows proteins to be separated solely by size.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to estimate the number of Sodium Dodecyl Sulfate (SDS) molecules that associate with a protein of a specific molecular weight ($44$ kiloDaltons) during electrophoresis.
Key Formula or Approach:
SDS-PAGE relies on the principle that SDS binds to proteins at a nearly constant weight ratio, conferring a uniform negative charge-to-mass ratio.
The standard binding ratio is approximately 1.4 grams of SDS per 1.0 gram of protein.

Step 2: Detailed Explanation:

• Average Amino Acid Weight: The average molecular weight of an amino acid residue in a protein is approximately $110$ Da.

• Number of Amino Acids: For a $44$ kDa ($44,000$ Da) protein:
\[ \text{Number of residues} = \frac{44,000}{110} = 400 \text{ amino acids} \]

• SDS Binding Stoichiometry: Scientific studies (like those by Reynolds and Tanford) have established that, on average, one SDS molecule binds for every two amino acid residues.

• Calculation of SDS Molecules:
\[ \text{SDS molecules} = \frac{\text{Total Amino Acids}}{2} \]
\[ \text{SDS molecules} = \frac{400}{2} = 200 \]

• Alternative Weight-Based Calculation:
Mass of protein $= 44,000$ Da.
Mass of bound SDS $= 44,000 \times 1.4 = 61,600$ Da.
Molecular weight of SDS ($C_{12}H_{25}SO_4Na$) is $\approx 288$ Da.
Number of SDS molecules $= \frac{61,600}{288} \approx 213$.
The value $200$ is the standard theoretical approximation used in textbooks based on the 1:2 residue ratio.

Step 3: Final Answer:
A $44$ kDa protein contains roughly $400$ amino acids. Since one SDS molecule binds to every two amino acids, a total of $200$ molecules of SDS are expected to bind.