HOAT1 - Revision history

Kaushki Sharma at 17:56, 30 November 2025

2025-11-30T17:56:12Z

Kaushki Sharma at 17:51, 30 November 2025

2025-11-30T17:51:16Z

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Kaushki Sharma at 17:12, 30 November 2025

2025-11-30T17:12:07Z

← Older revision		Revision as of 17:12, 30 November 2025
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	~~‘’’Experimental~~ Data ~~Snapshot’’’~~		'''Experimental Data Snapshot'''

	Method: ELECTRON MICROSCOPY		Method: ELECTRON MICROSCOPY

Kaushki Sharma at 17:11, 30 November 2025

2025-11-30T17:11:00Z

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	~~'''Experimental~~ Data ~~Snapshot'''~~		‘’’Experimental Data Snapshot’’’

	Method: ELECTRON MICROSCOPY		Method: ELECTRON MICROSCOPY

Kaushki Sharma at 17:09, 30 November 2025

2025-11-30T17:09:26Z

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 ==Structure Tour==
 <StructureSection load='9kkk' size='340' side='right'caption='Cryo-EM structure of human SLC22A6 (OAT1) in the apo-state, [[Resolution|resolution]] 3.85&Aring;' scene=''>
 ===Introduction===

Kaushki Sharma at 16:38, 30 November 2025

2025-11-30T16:38:44Z

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	===~~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
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	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===

Kaushki Sharma at 12:04, 30 November 2025

2025-11-30T12:04:56Z

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	*Its binding site overlaps with both Site 1 (partially) and Site 3.		*Its binding site overlaps with both Site 1 (partially) and Site 3.

			*In the binding pocket of Site 1, surrounded by 16 residues located within a 5 A ˚ (M31, N35, M142, V145, G227, Y230, W346, Y353, Y354, K382, D378, F438, S462, A465, R466, and S469).

	*It engages in specific, high-affinity interactions with key residues:		*It engages in specific, high-affinity interactions with key residues:
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	:*'''Path A''' (between TM2 and TM11) is narrowed from ~5 Å to ~4 Å.		:*'''Path A''' (between TM2 and TM11) is narrowed from ~5 Å to ~4 Å.

	:*'''Path B''' (between TM5 and TM8) is completely blocked.		:*'''Path B''' (between TM5 and TM8) is completely blocked. Restriction of the access route to path B likely limits the entry of substrates to Site 1 and the exit of substrates from the binding pocket.

	This structural rearrangement is caused by a slight inward movement of the cytoplasmic ends of TM5, TM8, TM10, and TM11 toward the binding pocket.		This structural rearrangement is caused by a slight inward movement of the cytoplasmic ends of TM5, TM8, TM10, and TM11 toward the binding pocket.

	'''3. Locked Conformation'''		'''3. Locked Conformation'''

			By constricting the cytoplasmic access routes, probenecid does not just compete for the substrate-binding site; it stabilizes the transporter in an apo-like, inward-facing conformation that is inaccessible to cytosolic substrates. This prevents the entry of new substrates and likely traps the transporter in this non-functional state, effectively "locking" it and preventing the conformational changes necessary for the transport cycle.

Kaushki Sharma at 11:43, 30 November 2025

2025-11-30T11:43:45Z

← Older revision		Revision as of 11:43, 30 November 2025
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	===Full Mechanism of Binding and Inhibition in hOAT1===		===Full Mechanism of Binding and Inhibition in hOAT1===

	[[Image:HOAT1mechanism.png \| frame \|300px\| upright= 1.5 \|none \| alt= \| Fig.1. 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
	the inward-facing conformation. When olmesartan interacts with the bottom gating residues M207 and F442, the side chains S203, Y230 (not shown here), and		the inward-facing conformation. When olmesartan interacts with the bottom gating residues M207 and F442, the side chains S203, Y230 (not shown here), and
	R466 appear to rearrange to coordinate with a chloride ion and drug compared to the apo structure. Whereas probenecid binding induces an additional		R466 appear to rearrange to coordinate with a chloride ion and drug compared to the apo structure. Whereas probenecid binding induces an additional

Kaushki Sharma at 11:41, 30 November 2025

2025-11-30T11:41:38Z

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	===Cryo-EM structure of hOAT1===		===Cryo-EM structure of hOAT1===

	[[Image:Hoat1domain.png \|200 px\|frame\| right\| Fig 1. (A) Schematic diagram of human OAT1 topology		[[Image:Hoat1domain.png \|HEIGHT1=200\|WIDTH1=200\|frame\| right\| Fig 1. (A) Schematic diagram of human OAT1 topology
	and the overall transport process.]]		and the overall transport process.]]
	The apo state structure of human Organic Anion Transporter 1 (hOAT1), determined by cryo-EM, reveals the transporter in an inward-facing conformation. This means the central substrate-binding cavity is open toward the intracellular side of the membrane, ready to release a substrate or accept one from the cytoplasm.		The apo state structure of human Organic Anion Transporter 1 (hOAT1), determined by cryo-EM, reveals the transporter in an inward-facing conformation. This means the central substrate-binding cavity is open toward the intracellular side of the membrane, ready to release a substrate or accept one from the cytoplasm.

Kaushki Sharma at 11:38, 30 November 2025

2025-11-30T11:38:26Z

← Older revision		Revision as of 11:38, 30 November 2025
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	===Cryo-EM structure of hOAT1===		===Cryo-EM structure of hOAT1===

	[[Image:Hoat1domain.png \|frame\| right\| ~~Fig1~~.]]		[[Image:Hoat1domain.png \|200 px\|frame\| right\| Fig 1. (A) Schematic diagram of human OAT1 topology
			and the overall transport process.]]
	The apo state structure of human Organic Anion Transporter 1 (hOAT1), determined by cryo-EM, reveals the transporter in an inward-facing conformation. This means the central substrate-binding cavity is open toward the intracellular side of the membrane, ready to release a substrate or accept one from the cytoplasm.		The apo state structure of human Organic Anion Transporter 1 (hOAT1), determined by cryo-EM, reveals the transporter in an inward-facing conformation. This means the central substrate-binding cavity is open toward the intracellular side of the membrane, ready to release a substrate or accept one from the cytoplasm.

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	'''Key Structural Characteristics:'''		'''Key Structural Characteristics:'''
	*'''Overall Fold:'''		*'''Overall Fold:'''
	::*Adopts the classic Major Facilitator Superfamily (MFS) fold.		:*Adopts the classic Major Facilitator Superfamily (MFS) fold.

	::*Comprises 12 transmembrane helices (TMs 1-12).		:*Comprises 12 transmembrane helices (TMs 1-12).

	::*Exhibits pseudo-two-fold symmetry, divided into an N-lobe (TMs 1-6) and a C-lobe (TMs 7-12).		:*Exhibits pseudo-two-fold symmetry, divided into an N-lobe (TMs 1-6) and a C-lobe (TMs 7-12).

	*'''Central Binding Cavity:'''		*'''Central Binding Cavity:'''

	::*The cavity is located between the N-lobe (formed by TM1, TM2, TM4, TM5) and the C-lobe (formed by TM7, TM8, TM10, TM11).		:*The cavity is located between the N-lobe (formed by TM1, TM2, TM4, TM5) and the C-lobe (formed by TM7, TM8, TM10, TM11).

	::*It possesses a positively charged electrostatic environment, which explains its strong preference for transporting anionic substrates.		:*It possesses a positively charged electrostatic environment, which explains its strong preference for transporting anionic substrates.

	::*The cavity is lined by 29 residues, forming a hydrophobic and aromatic-rich environment.		:*The cavity is lined by 29 residues, forming a hydrophobic and aromatic-rich environment.

	*'''Cavity Borders and Cytosolic Gate:'''		*'''Cavity Borders and Cytosolic Gate:'''

	::*The top border (extracellular side) of the cavity is formed by residues including N35, Y230, Y353, and Y354 and are involved in substrate recognition		:*The top border (extracellular side) of the cavity is formed by residues including N35, Y230, Y353, and Y354 and are involved in substrate recognition

	::*The bottom border (cytosolic side) features a narrow "thin bottom gate" formed by residues M207 and F442. The interaction between these two residues splits the cytosolic entrance into two distinct pathways:		:*The bottom border (cytosolic side) features a narrow "thin bottom gate" formed by residues M207 and F442. The interaction between these two residues splits the cytosolic entrance into two distinct pathways:

	:::*Path A: Located between TM2 and TM11.		::*Path A: Located between TM2 and TM11.

	:::*Path B: Located between TM5 and TM8.		::*Path B: Located between TM5 and TM8.

	::*This suggests that aromatic residues located at the top border are important for extracellular anion binding, while residues at the bottom play a role in exporting extracellular anions to the cytoplasmic side.		:*This suggests that aromatic residues located at the top border are important for extracellular anion binding, while residues at the bottom play a role in exporting extracellular anions to the cytoplasmic side.

	*'''Conformational State:'''		*'''Conformational State:'''

	::*In the apo state, the transporter is in a relaxed, inward-open conformation, providing access for substrates from the cytoplasm.		:*In the apo state, the transporter is in a relaxed, inward-open conformation, providing access for substrates from the cytoplasm.

	===Olmesartan recognition by hOAT1===		===Olmesartan recognition by hOAT1===