Floridean starch
Floridean starch is a type of a storage
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:[1]
Floridean starch | Starch | Glycogen | |
---|---|---|---|
Organisms | Red algae, glaucophytes | Green algae, plants | Some bacteria, some archaea, fungi, animals |
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 | UDP-glucose
|
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.[2][3]
Evolution
Features such as UDP-glucose building blocks and cytosolic storage differentiate the
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.[1][5] 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.[1][2]
In a few cases, red algae have been found to use cytosolic glycogen rather than floridean starch as a storage polymer; examples such as
Other organisms whose evolutionary history suggests secondary endosymbiosis of a red alga also use storage polymers similar to floridean starch, for example,
History
Floridean starch is named for a class of red algae, the