Which of the following is responsible for the negative charge in fibrinopeptide A?

Correct Answer: A. Aspartate and glutamate

Fibrinopeptide A is a small peptide released from fibrinogen during the coagulation cascade when thrombin cleaves fibrinogen. The negative charge in fibrinopeptide A arises from acidic amino acids — specifically aspartate (Asp, D) and glutamate (Glu, E). Both these amino acids possess carboxyl groups (-COOH) in their side chains with pKa values around 3.9 and 4.2 respectively. At physiological pH (7.4), these carboxyl groups are fully ionized to carboxylate (-COO⁻), conferring a negative charge. Fibrinopeptide A contains multiple acidic residues that contribute to its overall negative charge, which is functionally important for its solubility and interaction with fibrin molecules during clot formation. The release of these negatively charged peptides from fibrinogen is a hallmark of the coagulation cascade and is exploited clinically in India through thrombin time measurements to assess fibrinogen quality and quantity in bleeding disorders. Understanding the charge properties of fibrinopeptides is essential for comprehending fibrinogen structure and the biochemistry of hemostasis.

Why the other options are wrong

B. Serine and threonine — Serine and threonine are polar uncharged amino acids with hydroxyl (-OH) groups in their side chains. While these groups can form hydrogen bonds, they do not carry a net negative charge at physiological pH. They contribute to hydrophilicity but not to the negative charge of fibrinopeptide A. This is a common trap for students who confuse polarity with charge. C. Histidine and lysine — Histidine and lysine are basic amino acids with positively charged side chains (pKa _{6 and} 10.5 respectively). At physiological pH, their imidazole and amino groups are protonated, making them positively charged. They contribute positive charge, not negative charge, to proteins. This option represents a classic NBE trap pairing charged amino acids but with the wrong polarity. D. Glutamate and valine — While glutamate is indeed an acidic amino acid contributing negative charge, valine is a nonpolar hydrophobic amino acid with a branched aliphatic side chain. Valine carries no charge at any physiological pH. This option mixes one correct component (glutamate) with an incorrect one (valine), making it a distractor designed to catch students who partially recall the answer.

High-Yield Facts

Mnemonics

ACIDIC = Aspartate, Glutamate (A-G) Acidic amino acids = Asp (D) + Glu (E). Both have carboxyl side chains → negative charge at physiological pH. Remember: Asp has 4 carbons, Glu has 5 carbons in the side chain. Charge Rule: pH >> pKa = Deprotonated (−) When physiological pH (7.4) >> pKa of side chain (~3.9 for Asp/Glu), the group loses its proton and becomes negatively charged. Use this for any amino acid charge question.

NBE Trap

NBE pairs charged amino acids (option C: histidine + lysine) to trap students who recognize that charged residues matter in fibrinopeptide A but confuse the polarity — basic amino acids are positively charged, not negative. Option D mixes one correct (glutamate) with one incorrect (valine) to catch partial recall.

Clinical Pearl

In Indian coagulation labs, thrombin time is prolonged when fibrinogen is abnormal or deficient — the test directly measures how quickly thrombin releases fibrinopeptides (including fibrinopeptide A) from fibrinogen. Patients with dysfibrinogenemia or hypofibrinogenemia present with bleeding tendency, and understanding fibrinopeptide charge helps explain why abnormal fibrinogen fails to polymerize properly into stable clots.

_Reference: Harper Biochemistry Ch. 3 (Amino Acids); KD Tripathi Biochemistry Ch. 5 (Protein Structure and Function)_