The key difference between halorhodopsin and bacteriorhodopsin is that halorhodopsin is a light-driven chloride pump found in archaea while bacteriorhodopsin is a light-driven proton pump found in archaea.
Halorhodopsin and bacteriorhodopsin are heptahelical membrane proteins. They are also known as archaea rhodopsins. Generally, both of these are found in the purple membrane, a part of the Halobacterium salinarum cell membrane. Halorhodopsin is a light-driven chloride pump that allows ions to flow from the extracellular side to the cytoplasmic side. On the other hand, bacteriorhodopsin is a light-driven proton pump that allows ions to flow from the cytoplasmic side to the extracellular side. Therefore, halorhodopsin and bacteriorhodopsin are two light-driven ion bumps found in archaea, most notably in halobacteria.
CONTENTS
1. Overview and Key Difference
2. What is Halorhodopsin
3. What is Bacteriorhodopsin
4. Similarities – Halorhodopsin and Bacteriorhodopsin
5. Halorhodopsin vs Bacteriorhodopsin in Tabular Form
6. Summary – Halorhodopsin vs Bacteriorhodopsin
What is Halorhodopsin?
Halorhodopsin is a retinal protein from archaeon Halobacterium salinarum, which uses the energy of green light (500 to 650nm) to transport chloride ions into the cell against the membrane potential. The uptake of potassium chloride by these cells by ions pumps like halorhodopsin is necessary to maintain osmotic balance during cell growth. Moreover, a light-driven anion pump saves a substantial amount of metabolic energy. Halorhodopsin folds into a seven-transmembrane helix topology with short interconnecting loops. The helices (named A to G) are arranged in an arc-like structure and tightly surround a retinal molecule that is covalently bound via a Schiff base to a conserved lysine amino acid (Lys-242) on helix G. The cross section of halorhodopsin with residues is important for chloride transfer. It is the probable path of the anion.
Figure 01: Halorhodopsin
The absorption of a photon by halorhodopsin initiates a catalytic cycle, which leads to the transport of an anion into the cell. The cycle can be described in terms of six steps of isomerisation (I), ion transport (T), and accessibility change (switch S). Furthermore, an important method for analysis of halorhodopsin structure and function is the possibility of producing specifically modified protein by site-directed mutagenesis and homologous overexpression.
What is Bacteriorhodopsin?
Bacteriorhodopsin is known as a light-driven proton pump in archaea such as Halobacterium salinarum. Bacteriorhodopsin is a protein used by archaea, most notably by halobacteria, a class of Euryarchaeota. It acts as a proton pump that captures light energy and uses this energy to move protons across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.
Figure 02: Bacteriorhodopsin
Bacteriorhodopsin is a 27 kDa integral membrane protein. The repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120 degrees relative to the others. Moreover, each monomer has seven transmembrane alpha-helices and an extracellular facing two-stranded beta-sheet. Furthermore, the protein motive force generated by this retinal protein is used by ATP synthase to generate ATP. Therefore, by expressing bacteriorhodopsin, the archaea cells are able to synthesize ATP in the absence of a carbon source.
What are the Similarities Between Halorhodopsin and Bacteriorhodopsin?
- Halorhodopsin and bacteriorhodopsin belong to a subfamily of heptahelical membrane proteins.
- Both are retinal proteins.
- They are also known as archaea rhodopsins.
- They are light-driven ion bumps.
- Both are present in the purple membrane, a part of the Halobacterium salinarum cell membrane.
- They have particular functions that are very important for halobacteria survival.
What is the Difference Between Halorhodopsin and Bacteriorhodopsin?
Halorhodopsin is a light-driven chloride pump found in archaea, while bacteriorhodopsin is a light-driven proton pump found in archaea. Thus, this is the key difference between halorhodopsin and bacteriorhodopsin. Furthermore, halorhodopsin is a light-driven chloride pump that allows ions to flow from the extracellular to the cytoplasmic side. On the other hand, bacteriorhodopsin is a light-driven proton pump that allows ions to flow from the cytoplasmic to the extracellular side.
The below infographic presents the differences between halorhodopsin and bacteriorhodopsin in tabular form for side by side comparison.
Summary – Halorhodopsin vs Bacteriorhodopsin
Halorhodopsin and bacteriorhodopsin are two light-driven ion bumps found in archaea, most notably halobacteria. Halorhodopsin is a light-driven chloride pump, while bacteriorhodopsin is a light-driven proton pump. Halorhodopsin allows ion flow from the extracellular to the cytoplasmic side. In contrast, bacteriorhodopsin allows ion flow from the cytoplasmic to the extracellular side. So, this summarizes the difference between halorhodopsin and bacteriorhodopsin.
Reference:
1. “Halorhodopsin.” An Overview | ScienceDirect Topics.
2. “Bacteriorhodopsin.” An Overview | ScienceDirect Topics.
Image Courtesy:
1. “Halorhodopsin PS” By Asw-hamburg – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Bacteriorhodopsin retinal” By Darekk2 – created using Corel Draw and Adobe Photoshop Elements. (CC BY-SA 3.0) via Commons Wikimedia