HOAT1: Difference between revisions

From Proteopedia
Jump to navigationJump to search
← Older editNewer edit →
No edit summary
No edit summary
Line 149: Line 149:




===Full Mechanism of Binding and Inhibition in hOAT1===
===Mechanistic Insights into hOAT1 Function and Inhibition===


[[Image:HOAT1mechanism.png | frame |300px| upright= 1.5 |none | alt= | Fig 2. Mechanism of olmesartan binding and conformational inhibition by probenecid. A) When the transporter is in its outward-facing conformation, substrates or inhibitors enter the central binding pocket and undergo structural rearrangement to
[[Image:HOAT1mechanism.png | frame |300px| upright= 1.5 |none | alt= | Fig 2. Mechanism of olmesartan binding and conformational inhibition by probenecid. A) When the transporter is in its outward-facing conformation, substrates or inhibitors enter the central binding pocket and undergo structural rearrangement to
Line 156: Line 156:
conformation change for inhibition (apo-like conformation).]]
conformation change for inhibition (apo-like conformation).]]


'''Overall Transport Cycle & Substrate Binding (e.g., Olmesartan)'''
'''1. A Dual-Mechanism for Potent Inhibition by Probenecid'''


'''1. Outward-Facing State (Hypothesized):''' The transport cycle begins with the transporter in an outward-facing conformation, open to the extracellular space. Substrates and inhibitors from the blood enter the central binding pocket at this stage.
The study reveals that the classic inhibitor probenecid employs a sophisticated, dual-mechanism to arrest OAT1 function, moving beyond simple competition.


'''2. Transition to Inward-Facing State:''' Upon binding a substrate like olmesartan, the transporter undergoes a conformational change to the inward-facing state, which is the conformation captured in this study.
'''Direct Competition:''' Probenecid occupies the central binding pocket, and its interaction with K382 in Site 1 directly competes with the binding of the counter-substrate α-ketoglutarate (α-KG). This disrupts the exchange cycle that drives substrate transport.


'''3. Substrate Binding and Chloride Coordination in the Inward-Open State:'''
'''Conformational Arrest:''' More significantly, probenecid binding induces subtle conformational changes in the cytoplasmic ends of transmembrane helices (TM5, TM8, TM10, TM11). This leads to a constriction of the cytosolic opening, completely blocking one access path (Path B) and narrowing the other (Path A). This physically prevents substrates from entering or exiting the binding site from the cytoplasm, effectively "locking" the transporter in an inactive, inward-facing state. This mechanism is reminiscent of inhibition seen in other transporters like hURAT1, suggesting it may be a general strategy for effective transport arrest.


*Olmesartan docks into Site 3, the polyspecific substrate-binding site, engaging a cage of hydrophobic and aromatic residues (e.g., F438, Y354).
'''2. Structural Basis for Species-Specific Drug Transport'''


*Its binding induces specific structural rearrangements, most notably a vertical rotation of the Y230 side chain.
A major advancement of this work is the structural explanation for long-observed differences in drug handling between human OAT1 and animal orthologs.


*Crucially, olmesartan binding creates a favorable environment for chloride ion coordination. The chloride ion is stabilized by a network involving S203, the rotated Y230, and R466.
'''The Critical Role of S203:''' The residue S203 in hOAT1 (which is an alanine, A203, in rat OAT1) is identified as a key species-specific determinant. It does not contact substrates like olmesartan directly. Instead, its hydroxyl group is crucial for coordinating a chloride ion along with residues Y230 and R466.


*This chloride coordination, facilitated by the species-specific residue S203, is essential for high-affinity binding and efficient translocation of olmesartan. The bottom-gate residues M207 and F442 also interact with the drug, potentially playing a role in its final release into the cytoplasm.
'''Chloride Coordination Enhances Substrate Affinity:''' This chloride coordination network stabilizes the binding of certain substrates. Functional data confirms that the S203A mutation drastically reduces olmesartan affinity specifically in the presence of chloride. This explains why drugs like olmesartan and tenofovir show different transport kinetics between species; the human transporter, with its S203, has a enhanced, chloride-dependent mechanism for high-affinity binding that the rat ortholog lacks.


'''4. Substrate Release:''' The inward-facing conformation with its open paths (Path A and Path B) allows the substrate to dissociate into the cytoplasm. The transporter then likely resets to the outward-facing state, driven by the exchange with intracellular α-ketoglutarate (α-KG).


'''Inhibition Mechanism (e.g., Probenecid)'''
The inhibitor probenecid exploits the transport cycle but arrests it through a dual mechanism:
'''1. Binding and Competition:'''
*Probenecid enters the binding pocket from the extracellular side and binds in the inward-facing conformation.
*It occupies Site 3 and partially extends into Site 1. In Site 1, it directly competes with the counter-substrate α-KG by forming a key hydrogen bond with K382, a residue critical for α-KG binding.
'''2. Conformational Arrest and Cytoplasmic Blockade:'''
*This is the primary inhibitory mechanism. Probenecid binding induces subtle but critical conformational changes in the cytoplasmic regions of TM5, TM8, TM10, and TM11.
*These helices shift inward, causing a constriction of the entire cytoplasmic opening of the binding pocket.
*This constriction completely blocks Path B and severely narrows Path A.
*By physically obstructing these cytosolic paths, probenecid achieves two things:
:*It prevents intracellular substrates from entering the binding pocket.
:*It traps the transporter in a locked, inward-facing, apo-like conformation, preventing the conformational changes needed to complete the transport cycle.


===Author===
===Author===
Retrieved from "https://proteopedia.org/HOAT1"