Escherichia coli biochemistry – Acyl carrier protein (ACP): Approximately what is the molecular weight of E. coli ACP?

Introduction / Context:
Acyl carrier protein (ACP) is a central, highly conserved component of the fatty acid synthase (FAS) system in bacteria such as Escherichia coli. Knowing ACP’s size is useful for interpreting SDS-PAGE bands, mass spectrometry data, and for understanding how this small protein shuttles growing acyl chains via its phosphopantetheine arm during fatty acid biosynthesis.

Given Data / Assumptions:

• The question asks for the approximate molecular weight of E. coli ACP.
• ACP is a small, acidic protein that becomes functional upon post-translational attachment of a 4′-phosphopantetheine group to a conserved serine.
• We are choosing among plausible protein sizes measured in daltons (Da).

Concept / Approach:
Bacterial ACPs are typically small proteins in the range of 8–10 kDa before modification. Escherichia coli ACP specifically is widely cited as ~9 kDa (about 77–88 amino acids depending on the reference sequence form). The 4′-phosphopantetheine arm adds a small mass but does not change the classification as a ~9 kDa protein. Hence, among the provided options, ~9000 Da is the most accurate approximation.

Step-by-Step Solution:
Identify ACP as a small carrier protein in FAS.Recall canonical literature values placing bacterial ACP near 9 kDa.Compare with options: 19 kDa, 39 kDa, and 90 kDa are typical of larger enzymes and structural proteins, not ACP.Select the closest correct choice: 9000 Da.

Verification / Alternative check:
Standard protein ladders and proteomic databases consistently annotate E. coli ACP near 9 kDa. Experimental SDS-PAGE also places ACP at ~9–10 kDa, confirming the widely accepted value.

Why Other Options Are Wrong:

• 19000 / 39000 / 90,000: Far larger than the known ACP size; values of this magnitude would correspond to different enzymes or multiprotein domains.
• 12,500: Closer than the others but still appreciably above the well-established ~9 kDa estimate.

Common Pitfalls:
Confusing ACP with the full multifunctional FAS complex or with acyl-CoA-binding proteins in eukaryotes, which are different entities and may have different sizes.

Final Answer:
9000.