floridean starch
{{Short description|Type of storage glucan}}
Floridean starch is a type of storage glucan found in glaucophytes and in red algae (or rhodophytes), in which it is usually the primary sink for fixed carbon from photosynthesis. It is found in grains or granules in the cell's cytoplasm and is composed of an α-linked glucose polymer with a degree of branching intermediate between amylopectin and glycogen, though more similar to the former. The polymers that make up floridean starch are sometimes referred to as "semi-amylopectin".{{cite journal|last1=Ball|first1=S.|last2=Colleoni|first2=C.|last3=Cenci|first3=U.|last4=Raj|first4=J. N.|last5=Tirtiaux|first5=C.|title=The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understand the establishment of plastid endosymbiosis|journal=Journal of Experimental Botany|date=10 January 2011|volume=62|issue=6|pages=1775–1801|doi=10.1093/jxb/erq411|pmid=21220783|doi-access=free}}
Properties
Floridean starch consists of a polymer of glucose molecules connected primarily by α(1,4) linkages, with occasional branch points using α(1,6) linkages. It differs from other common α-linked glucose polymers in the frequency and position of the branches, which gives rise to different physical properties. The structure of floridean starch polymers is most similar to amylopectin and is sometimes described as "semi-amylopectin". Floridean starch is often described in contrast to starch (a mixture of amylopectin and amylose) and glycogen:
| class="wikitable"
! !Floridean starch |
| Organisms
|Green algae, plants |
| Composition
|Semi-amylopectin; classically without amylose, though some examples exist with amylose present |Amylopectin and amylose |Glycogen |
| Storage location
|In the cytosol |Inside plastids |In the cytosol |
| Building block
|ADP-glucose |Eukaryotes: UDP-glucose Bacteria: ADP-glucose |
| Branching
|Intermediate level of branching |Amylopectin: Branches are relatively rare and occur in clusters Amylose: Almost entirely linear |Branches are relatively frequent and evenly distributed |
| Genes required for maintenance
|Fewer than 12 |30–40 |6–12 |
Historically, floridean starch has been described as lacking amylose. However, amylose has been identified as a component of floridean starch granules in some cases, particularly in unicellular red algae.{{cite journal|last1=McCracken|first1=D. A.|last2=Cain|first2=J. R.|title=Amylose in Floridean Starch|journal=New Phytologist|date=May 1981|volume=88|issue=1|pages=67–71|doi=10.1111/j.1469-8137.1981.tb04568.x|doi-access=free}}
Evolution
Features such as UDP-glucose building blocks and cytosolic storage differentiate the Archaeplastida into two groups: the rhodophytes and glaucophytes, which use floridean starch, and the green algae and plants (Chloroplastida), which use amylopectin and amylose. There is strong phylogenomic evidence that the Archaeplastida are monophyletic and originate from a single primary endosymbiosis event involving a heterotrophic eukaryote and a photosynthetic cyanobacterium.{{cite journal|last1=Viola|first1=R.|last2=Nyvall|first2=P.|last3=Pedersen|first3=M.|title=The unique features of starch metabolism in red algae|journal=Proceedings of the Royal Society B: Biological Sciences|date=7 July 2001|volume=268|issue=1474|pages=1417–1422|doi=10.1098/rspb.2001.1644|pmid=11429143|pmc=1088757}}
Evidence indicates that both ancestors would have had established mechanisms for carbon storage. Based on review of the genetic complement of modern plastid genomes, the last common ancestor of the Archaeplastida is hypothesized to have possessed a cytosolic storage mechanism and to have lost most of the endosymbiotic cyanobacterium's corresponding genes.{{cite journal|last1=Dauvillée|first1=David|last2=Deschamps|first2=Philippe|last3=Ral|first3=Jean-Philippe|last4=Plancke|first4=Charlotte|last5=Putaux|first5=Jean-Luc|last6=Devassine|first6=Jimi|last7=Durand-Terrasson|first7=Amandine|last8=Devin|first8=Aline|last9=Ball|first9=Steven G.|title=Genetic dissection of floridean starch synthesis in the cytosol of the model dinoflagellate|journal=Proceedings of the National Academy of Sciences|date=15 December 2009|volume=106|issue=50|pages=21126–21130|doi=10.1073/pnas.0907424106|pmid=19940244|pmc=2795531|url=https://hal.archives-ouvertes.fr/hal-00436589/document|doi-access=free}} According to this hypothesis, the rhodophytes and glaucophytes retained the ancestral eukaryote's cytosolic starch deposition. Starch synthesis and degradation in green algae and plants is much more complex – but significantly, many of the enzymes that perform these metabolic functions in the interior of modern plastids are identifiably of eukaryotic rather than bacterial origin.{{cite book|editor1-last=Nakamura|editor1-first=Yasunori|last1=Ball|first1=Stephen|last2=Colleoni|first2=Christophe|last3=Arias|first3=Maria Cecilia|title=Starch: Metabolism and Structure|date=2015|publisher=Springer Japan|isbn=978-4-431-55494-3|pages=93–158|chapter=The Transition from Glycogen to Starch Metabolism in Cyanobacteria and Eukaryotes|doi=10.1007/978-4-431-55495-0_4}}
In a few cases, red algae have been found to use cytosolic glycogen rather than floridean starch as a storage polymer; examples such as Galdieria sulphuraria are found in the Cyanidiales, which are unicellular extremophiles.{{cite journal|last1=Martinez-Garcia|first1=Marta|last2=Stuart|first2=Marc C.A.|last3=van der Maarel|first3=Marc J.E.C.|title=Characterization of the highly branched glycogen from the thermoacidophilic red microalga Galdieria sulphuraria and comparison with other glycogens|journal=International Journal of Biological Macromolecules|date=August 2016|volume=89|pages=12–18|doi=10.1016/j.ijbiomac.2016.04.051|pmid=27107958|url=https://research.rug.nl/en/publications/9436d7ea-84a3-4aad-aea2-6001518e3313 }}{{cite journal|last1=Deschamps|first1=Philippe|last2=Haferkamp|first2=Ilka|last3=d’Hulst|first3=Christophe|last4=Neuhaus|first4=H. Ekkehard|last5=Ball|first5=Steven G.|title=The relocation of starch metabolism to chloroplasts: when, why and how|journal=Trends in Plant Science|date=November 2008|volume=13|issue=11|pages=574–582|doi=10.1016/j.tplants.2008.08.009|pmid=18824400}}
Other organisms whose evolutionary history suggests secondary endosymbiosis of a red alga also use storage polymers similar to floridean starch, for example, dinoflagellates and cryptophytes. The presence of floridean starch-like storage in some apicomplexan parasites is one piece of evidence supporting a red alga ancestry for the apicoplast, a non-photosynthetic organelle.{{cite journal|last1=Coppin|first1=Alexandra|last2=Varré|first2=Jean-Stéphane|last3=Lienard|first3=Luc|last4=Dauvillée|first4=David|last5=Guérardel|first5=Yann|last6=Soyer-Gobillard|first6=Marie-Odile|last7=Buléon|first7=Alain|last8=Ball|first8=Steven|last9=Tomavo|first9=Stanislas|title=Evolution of Plant-Like Crystalline Storage Polysaccharide in the Protozoan Parasite Toxoplasma gondii Argues for a Red Alga Ancestry|journal=Journal of Molecular Evolution|date=February 2005|volume=60|issue=2|pages=257–267|doi=10.1007/s00239-004-0185-6|pmid=15785854|citeseerx=10.1.1.140.4390|s2cid=17216620}}
History
Floridean starch is named for a class of red algae, the Florideae (now usually termed Florideophyceae).{{cite journal|last1=Barry|first1=V. C.|last2=Halsall|first2=T. G.|last3=Hirst|first3=E. L.|last4=Jones|first4=J. K. N.|title=313. The polysaccharides of the florideœ. Floridean starch|journal=Journal of the Chemical Society|date=1949|pages=1468–1470|doi=10.1039/JR9490001468}} It was first identified in the mid-19th century and extensively studied by biochemists in the mid-20th century.{{cite journal|last1=Meeuse|first1=B. J. D.|last2=Andries|first2=M.|last3=Wood|first3=J. A.|title=Floridean Starch|journal=Journal of Experimental Botany|date=1960|volume=11|issue=2|pages=129–140|doi=10.1093/jxb/11.2.129}}