Glycogen metabolism

Glycogen is a very large, branched polymer of glucose residues. Within skeletal muscle and liver glucose is stored as glycogen. In the liver, glycogen synthesis and degradation are regulated to maintain blood-glucose levels as required to meet the needs of the organism as a whole. In contrast, in muscle, these processes are regulated to meet the energy needs of the muscle itself. '''Glycogen synthesis''' is, unlike its breakdown, endergonic. This means that glycogen synthesis requires the input of energy. Energy for glycogen synthesis comes from UTP, which reacts with glucose-1-phosphate, forming UDP-glucose, in reaction catalyzed by UDP-glucose pyrophosphorylase. Glycogen is synthesized from monomers of UDP-glucose by the enzyme glycogen synthase, which progressively lengthens the glycogen chain with (a1->4) bonded glucose. As glycogen synthase can only lengthen an existing chain, the protein glycogenin is needed to initiate the synthesis of glycogen. The glycogen-branching enzyme, amylo (a1->4) to (a1->6) transglycosylase, catalyzes the transfer of a terminal fragment of 6-7 glucose residues from a nonreducing end to the C-6 hydroxyl group of a glucose residue deeper into the interior of the glycogen molecule. The branching enzyme can act upon only a branch having at least 11 residues, and the enzyme may transfer to the same glucose chain or adjacent glucose chains. '''Glycogen degradation''' consists of three steps: (1) the release of glucose 1-phosphate from glycogen, (2) the remodeling of the glycogen substrate to permit further degradation, and (3) the conversion of glucose 1-phosphate into glucose 6-phosphate for further metabolism. Information partly derived from "Biochemistry" by Stryer and [http://en.wikipedia.org/wiki/Glycogen Wikipedia].

糖原系由葡萄糖残基构成的一种庞大、分支状的聚合物。在骨骼肌和肝脏中，葡萄糖以糖原的形式储存。在肝脏中，糖原的合成与降解受到调节，以维持血液葡萄糖水平，满足整个机体的需求。相对而言，在肌肉中，这些过程受到调节，以满足肌肉自身的能量需求。糖原的合成过程（与分解过程不同）为吸能反应。这意味着糖原的合成需要能量的输入。糖原合成的能量来源于尿苷三磷酸（UTP），它与葡萄糖-1-磷酸反应，形成尿苷二磷酸葡萄糖（UDP-glucose），此反应由UDP-葡萄糖焦磷酸化酶催化。糖原通过UDP-葡萄糖的单体合成，由糖原合成酶催化，逐渐延长糖原链，形成（a1->4）键合的葡萄糖。由于糖原合成酶只能延长现有链，因此需要糖原蛋白（glycogenin）来启动糖原的合成。糖原分支酶（amylo，a1->4）到（a1->6）转糖基化酶，催化将6-7个葡萄糖残基的末端片段从非还原端转移到糖原分子内部较深处的葡萄糖残基的C-6羟基上。分支酶只能作用于具有至少11个残基的分支，并且酶可以转移到相同的葡萄糖链或相邻的葡萄糖链。糖原降解过程包括三个步骤：（1）从糖原中释放葡萄糖-1-磷酸，（2）重塑糖原底物以允许进一步降解，（3）将葡萄糖-1-磷酸转化为葡萄糖-6-磷酸，以进行进一步的代谢。部分信息来源于Stryer所著《生物化学》及[http://en.wikipedia.org/wiki/Glycogen 维基百科]。