binding protein
Summary
A binding protein is a protein that reversibly and specifically binds a ligand without chemically changing it, as hemoglobin does with O₂. Affinity is measured by Kd: a lower Kd means tighter binding, and Kd equals the ligand concentration at half-saturation for a single, non-cooperative site.
Detail
Meaning: A binding protein holds a specific ligand through noncovalent forces (hydrogen bonds, ionic interactions, hydrophobic contacts) at a site whose shape and charge complement the ligand. The ligand is released unchanged. Examples include myoglobin and hemoglobin (O₂), antibodies (antigens), receptors (signal molecules), transport proteins such as albumin, and DNA-binding proteins. Binding can change the protein's shape, which is how cooperativity and signaling work.
Equation: Kd = [P][L]/[PL], where [P] is free protein, [L] is free ligand and [PL] is the complex, all in mol/L. Fraction of sites occupied: θ = [L]/(Kd + [L]), which is unitless. When [L] = Kd, θ = 0.5 for a single site with no cooperativity.
On the MCAT: Passages give binding curves of fraction saturated against ligand concentration (or O₂ partial pressure). Myoglobin gives a hyperbola. Hemoglobin gives a sigmoid curve because of cooperativity. You compare curves: a curve shifted left means higher affinity and lower Kd (or lower P50). You also predict how a binding-site mutation, a competitor, or a change in pH, CO₂ or 2,3-BPG shifts the curve.
Common confusion: A binding protein is not judged by Vmax or turnover, because it does not convert ligand to product the way an enzyme does. Also, a lower Kd means higher affinity, not lower.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Biochemistry (Voet and Voet)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.