= Fibrinopeptide A
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<Fibrinopeptide A> is the 16-<amino acid> <peptide> released from the <N-terminus> of each A-alpha chain of human <fibrinogen> by <thrombin>. Removing <Fibrinopeptide A> exposes the A knob on the remaining molecule, enabling a <fibrin knob-hole interaction>. The released fragment and the exposed binding site are different objects.<Fibrinopeptides A and B release in the process of surface fibrin formation by Tomas Riedel and others>[Introduction: chain origin, peptide lengths and exposure of polymerization sites]{ref}
Release of <Fibrinopeptide A> provides a measurable record of a particular <peptide bond> cleavage. A kinetic study measured <Fibrinopeptide A> and <Fibrinopeptide B> separately: under its solution conditions, at least 97% of <Fibrinopeptide B> release followed <Fibrinopeptide A> release. The study also found that a change at the cleavage-site <arginine> strongly reduced catalytic efficiency, linking <amino acid> identity to reaction rate.<Steady state kinetic parameters for the thrombin-catalyzed conversion of human fibrinogen to fibrin by D. L. Higgins and others>[Abstract: sequential release and the Arg16-to-His variant]{ref}
For a counting example, $1\,\mathrm{nmol}$ of fully cleaved <fibrinogen> yields $2\,\mathrm{nmol}$ of <Fibrinopeptide A>, because each molecule has two A-alpha chains. Partial cleavage gives less; this stoichiometric upper limit does not specify the final <fibrin> network's geometry.
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