The first quantitatively accurate model of the structure of glycogen α-particles was constructed using Random Branching Theory. It shows that they are assembled from β-particles at random, and describes biologically unique size-mass dependence rg ~ log(M), where rg is the polymer’s radius of gyration and M its mass. The previous assumption of a fractal architecture for glycogen is not supported by the experimental data. Instead the structure is more compact, and optimises storage and availability of glucose against the thermodynamic cost of assembly.
Commercially available samples of glycogen have been subjected to extreme conditions that destroy their native structure. To study the biological function of glycogen, samples that retain the in vivo structure are needed. A gentle extraction method was recently developed in David Stapleton's laboratory, and makes available the glycogen samples with structure close to that of the particles in liver cells that mammals use to control blood glucose.
Both muscle and liver glycogens are made up of β-particles, about 10 to 20 nm across, but only in the liver are these aggregated into α-particles, up to 200nm across. The figure is an electron microscope image of α-particles from rat liver.
We recently published the number and mass distributions of rat liver glycogen particles extracted using the Stapleton method, and showed that β-particles are randomly assembled into α-particles, or at least that the sizes of α-particles are accurately described by Random Branching Theory.