# Determining the Optimal Heparin Binding Domain Distance in VEGF165 Using Umbrella Sampling Simulations for Optimal Dimeric Aptamer Design

## TL;DR

This study uses simulations to determine the ideal distance between heparin-binding domains in VEGF165 to guide the design of effective dimeric aptamers.

## Contribution

The study introduces a computational approach using umbrella sampling to determine optimal HBD distances for dimeric aptamer design.

## Key findings

- VEGF165 prefers compact conformations with HBD-HBD distances below 3 nm.
- The optimal HBD-HBD distance is 1.1 nm, where key hydrogen bonds with VEGFR-2 are formed.
- Aptamer homodimers with specific linker lengths were computationally designed to target VEGF165.

## Abstract

Vascular endothelial growth factor 165 (VEGF165) stands out as a pivotal isoform of the VEGF-A protein and is critically involved in various angiogenesis-related diseases. Consequently, it has emerged as a promising target for diagnosing and treating such conditions. Structurally, VEGF165 forms a homodimer, and each of its constituting monomers comprises a receptor-binding domain (RBD) and a heparin-binding domain (HBD). These two domains are linked by a flexible linker, and thus the overall structure of VEGF165 remains incompletely understood. Aptamers are known as potent drugs that interact with VEGF165, and dimeric aptamers that can simultaneously interact with two distant domains are frequently adopted to improve the potency. However, designing such aptamer dimers faces challenges in regard to determining the appropriate length of the linker connecting the two aptamer fragments. To gain insight into this distance information, we here employ biased molecular dynamics (MD) simulations with the umbrella sampling method, with the distance between the two HBDs serving as a reaction coordinate. Our simulations reveal an overall preference for compact conformations with HBD-HBD distances below 3 nm, with the minimum of the potential of mean force located at 1.1 nm. We find that VEGF165 with the optimal HBD-HBD distance forms hydrogen bonds with its receptor VEGFR-2 that well match experimentally known key hydrogen bonds. We then try to computationally design aptamer homodimers consisting of two del5-1 aptamers connected by various linker lengths to target VEGF165. Collectively, our findings may provide quantitative guidelines for rationally designing high-affinity aptamers for targeting VEGF165.