Difference Between Phycocyanin and Allophycocyanin

The key difference between phycocyanin and allophycocyanin is that phycocyanin absorbs and emits at shorter wavelengths than allophycocyanin.

Phycobiliproteins are a family of water-soluble proteins that are present in cyanobacteria and certain algae species. Phycocyanin and allophycocyanin are two major members of this family.

CONTENTS

1. Overview and Key Difference
2. What is Phycocyanin 
3. What is Allophycocyanin 
4. Similarities Between Phycocyanin and Allophycocyanin
5. Side by Side Comparison – Phycocyanin vs Allophycocyanin in Tabular Form
6. Summary

What is Phycocyanin?

Phycocyanin is a pigment-protein complex from the family of light-harvesting phycobiliprotein. Other important members of this family include allophycocyanin and phycoerythrin. This pigment is an accessory pigment to chlorophyll. Generally, all phycobiliproteins are water-soluble complexes that cannot exist within the membranes like carotenoids. Instead of existing in membranes, these pigments tend to aggregate, forming clusters that can adhere to the membranes known as phycobilisomes.

We can observe that Phycocyanin has a characteristic light blue colour that can absorb orange and red light (near 620 nm) and can emit fluorescence (around 650 nm). We can find this colour pigment in cyanobacteria and the name “Phycocyanin” comes from the Greek meaning for “Phyco” which refers to “algae” and the suffix “cyanin” from Greek meaning for “Kyanos” which refers to “dark blue”.

Typically, Phycocyanin molecules share a common structure with all phycobiliproteins. When considering the structure of this pigment, it begins with the assembly of phycobiliprotein monomers. These monomers are heterodimers consisting of alpha and beta subunits with their respective chromophores. The chromophores and the subunits are joined together through a thioether chemical bond.

The subunits of Phycocyanin structure typically contain eight alpha-helices. The monomer structures tend to spontaneously aggregate, forming ring-shaped trimers having rotational symmetry and a central channel. Moreover, the trimers tend to aggregate in pairs, forming hexamers that are assisted with additional linker proteins. Therefore, each phycobilisome rod contains two or more Phycocyanin hexamers.

What is Allophycocyanin?

Allophycocyanin is a protein molecule that comes from the phycobiliprotein family, and it is an accessory pigment to chlorophyll. The other members of this light-harvesting phycobiliprotein family include Phycocyanin, phycoerythrin, and phycoerythrocyanin. Allophycocyanin pigments can absorb and emit red light, and we can readily find this pigment in cyanobacteria and red algae.

We can isolate allophycocyanin from various species of red or blue-green algae. These algae produce slightly different forms of the molecule. Generally, the allophycocyanin molecule contains two different subunits named as alpha and beta subunit. Each subunit has one phycocyanobilin chromophore.

There are different applications of allophycocyanin; many instruments have been developed specifically for allophycocyanin. For example, this component is commonly used in immunoassays, including FACS, flow cytometry, etc.

What are the Similarities Between Phycocyanin and Allophycocyanin?

• Phycocyanin and allophycocyanin are water-soluble proteins
• Both are included in the phycobiliprotein family.

What is the Difference Between Phycocyanin and Allophycocyanin?

Phycobiliproteins are a family of water-soluble proteins that are present in cyanobacteria and certain algae species. Phycocyanin and allophycocyanin are two major members of this family. The key difference between phycocyanin and allophycocyanin is that Phycocyanin absorbs and emits at shorter wavelengths than allophycocyanin.

Below infographic lists more differences between phycocyanin and allophycocyanin in tabular form for side by side comparison.

Summary – Phycocyanin vs Allophycocyanin

Phycobiliproteins are a family of water-soluble proteins that are present in cyanobacteria and certain algae species. Phycocyanin and allophycocyanin are two major members of this family. The key difference between Phycocyanin and allophycocyanin is that Phycocyanin absorbs and emits at shorter wavelengths than allophycocyanin.

Reference:

1. “Phycobiliprotein.” Wikipedia, Wikimedia Foundation, 19 July 2020, Available here.

Image Courtesy:

1. “Extracted phycocyanin” By CyanoLakes – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “2vjt” By Deposition authors: Murray, J.W., Benson, S., Nield, J., Barber, J.;visualization author: User:Astrojan – (CC BY 3.0) via Commons Wikimedia