Phycobiliproteins

R. MacColl

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315896465
• eBook ISBN: 9781351092463
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This volume provides a detailed examination of phycobiliproteins, the photosynthetic antenna pigments found in algae.

Chapter 1
Introduction

Biliproteins (also called phycobiliproteins) are photosynthetic antenna pigments found in cyanobacteria (blue-green algae), red algae, and the cryptomonads. These chromoproteins harvest solar energy in regions of the visible spectrum having low chlorophyll absorption and then transfer this excitation energy to chlorophyll in the photosynthetic membrane. The biliproteins obtain their colors from linear tetrapyrrole chromophores which are covalently attached to the apoproteins. The tetrapyrroles are not complexed with metal ions, but being noncyclic, can be readily manipulated by the apoprotein to produce the biologically relevant characteristics.

The several types of biliproteins are distributed in various ways in different organisms. In cyanobacteria and red algae, several types of biliproteins can occur in each organism. The question arises of how the different biliproteins can be organized to achieve efficient energy transfer to chlorophyll a. The answer to this is found in the organization of all these biliproteins into discrete organelles that very effectively control the flow of energy. These organelles, the phycobilisomes, were shown by Gantt et al.¹ to be highly ordered protein structures designed to function with high efficiency. A different situation is the case for the cryptomonad biliproteins, since apparently only a single biliprotein occurs in each organism (Table 1). Here phycobilisomes have not been discovered and the question remains to be answered: how are the pigments organized for efficient energy absorption and distribution? In the cryptomonads, chlorophyll c is present in addition to chlorophyll a which occurs in all three groups.

Biliproteins are named primarily for their colors. Phycoerythrins have absorption maxima between 498 and 568 nm and are red, the blue phycocyanins absorb at lower energies, and allophycocyanins absorb at still lower energies (Table 2). The letter prefixes in their names refer to the types of organisms in which they were discovered. For the cryptomonad biliproteins, the numbers after the names refer to approximate absorption maxima. One of the names used here is not found in the literature: CU-phycoerythrin. CU-Phycoerythrin is the most recently discovered group of phycoerythrins and is prefixed "CU" to indicate its cyanobacterial origins and the presence of a urobilin-like chromophore not found in C-phycoerythrin ("C" indicates cyanobacteria). Allophycocyanin is divided into two functional types: the predominant form which absorbs with a 650-nm maximum and fluoresces at 660 nm and the less abundant form which fluoresces at about 680 nm. This latter type, allo-phycocyanin 680, is the usual ultimate transfer agent of apparently all the excitation energy from the phycobilisome to the chlorophyll. Allophycocyanin 680 activity is found in two proteins, allophycocyanins I and B.

These absorption and fluorescence characteristics are produced by the chemical nature of the chromophores and by the interactions between chromophores and their surroundings. All biliproteins have either phycocyanobilin or phycoerythrobilin, and one, R-phycocyanin ("R" indicates red alga), has both (Table 3). In addition there are perhaps three minor bilins — phycourobilin, cryptoviolin, and the 697-nm bilin — which occur together with a major type on the same protein. Much less is known about these three chromophores, but all five have unique spectral properties. Even a single type of chromophore can be spectrally flexible, and phycoerythrobilin occurs with a maxim...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Chapter 1 Introduction
• Chapter 2 Phycobilisomes: A Structure-Function Model
• Chapter 3 Biliproteins: Covalent Structures
• Chapter 4 Biliproteins: Some Physical Properties
• Chapter 5 Phycobilisome: Linker and Core Structures
• Chapter 6 Excitation Energy Transfer
• Chapter 7 Physiology
• Chapter 8 Biliprotein-Thylakoid Interaction
• Chapter 9 Cryptomonads
• Chapter 10 Biophysical and Immunochemical Techniques
• Index