2

Nonreducing

ends

glycogen

phosphorylaso

OOOOO OOOO) OOOOO CKKXXKX>-

Glucose 1-phosphato molecules

(al->6) glucusidaso activity of

debranching

enzyme

(3 Glucose

(XXXKKXXK>{>-

Unbranched (al-*4) polymer; substrate for further phosphorylase action

FIGURĘ 15-4 Glycogen breakdown near an («l-*6) branch point.

following sequential removal of terminal glucose residues by glycogen phosphor>'lase (sec Fig. 15-3), glucose residues near a branch are removed in a two-step process that requires a bifunctional "de-branching enzyme." First, the transferase activity of the enzyme shifts a błock of three glucose residues from the branch to a nearby nonre-ducing end, to which they are reattached in (al—»4) linkage. The single glucose residue remaining at the branch point, in (crl-*6) linkage, is then released as free glucose by the enzyme's (a1—»6) glucosidase activity. The glucose residues are shown in shorthand form, which omits the —11, —Ol I, and —Cl l₂OI I groups from the pyranose rings.