Floridean starch

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Illustration of glucose polymer branching.

Floridean starch is a type of a storage

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".[1]

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

cyanobacterium.[1][4]

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

Cyanidiales, which are unicellular extremophiles.[6][7]

Other organisms whose evolutionary history suggests secondary endosymbiosis of a red alga also use storage polymers similar to floridean starch, for example,

apicomplexan parasites is one piece of evidence supporting a red alga ancestry for the apicoplast, a non-photosynthetic organelle.[8]

History

Floridean starch is named for a class of red algae, the

Florideae (now usually termed Florideophyceae).[9] It was first identified in the mid-19th century and extensively studied by biochemists in the mid-20th century.[10]

References