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Introduction

Mycobacterium tuberculosis very-long-chain fatty acyl-CoA synthetase

Mycobacterium tuberculosis very-long-chain fatty acyl-CoA synthetase, also known as FadD13, is unique within its class in regards to its ability to house lipid substrates longer than itself. Most FadD class proteins exist as integral membrane proteins involved in lipid transport into the cell. FadD13 is unique structurally in that it exists as a peripheral protein on the inside of the cell membrane. This feature is key in the mechanistic basis for FadD13's transport of fatty acids of length C24-C26.

Mechanism

Structural basis for housing lipid substrates longer than the enzyme

The ability for FadD13 to transport and activate fatty acids of the maximum tested length C26, lies in it being a peripheral membrane protein. FadD13 is attached to the membrane via electrostatic interactions in the N-terminal domain. Of importance in the region is the arginine rich lid-loop which serves to block the transport of fatty acids into the enzyme. Once the lid-loop is opened, fatty acids may be pulled from the membrane into a hydrophobic tunnel, which is the main structural component by which fatty acids are transported from the membrane into the cell. Negatively charged residues at the active site of FadD13 are the driving factor in the attraction of the fatty acid from the membrane through the hydrophobic tunnel of the enzyme.

Structure

General overview

FadD13 is composed of 503 amino acid residues divided into three main regions: The N-terminal domain (residues 1-395) and C-terminal domain (residues 402-503) which are connected via a flexible linker (residues 396-401).

Electrostatics

The electrostatics of FadD13 as seen in (figure...) illustrate the hydrophobic and positively charged regions that compose this protein. Experimental results from (Paper) revealed that the peripheral FadD13 attached to the membrane via positively charges interacting with the negatively charged cell membrane (image) located on the top portion of the N-terminal region. Of key importance in this N-terminal domain region attached to the membrane is an area of notable arginine rich residues, known as the arginine rich lid-loop.

Arginine Rich Lid-loop

Hydrophobic Tunnel

The hydrophobic tunnel of FadD13 is essential to the transport of very long fatty acids from the membrane into the cell.

Active Site

Disease

Relevance

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