근육의 에너지 사용에 대한 탐구가 계속되고 있다.
골격근(kg당 14-18 g, 70kg 성인 남자. 30kg이 근육량이라면 420g~ 540g)과 간(80-110g) 저장되어 있음.
이것은 약 5 mmol-2(0.9g/liter)의 혈당농도를 유지하기 위해 혈관으로 방출.
지방은?
단백질은 저정되지 않음.
탄수화물 - glycogen - glucose - glycolysis - pyruvate, lactate - 활성아세트산(acetyl CoA) - TCA 회로
다당류 glycogen이 분해되는 과정
glycogen - glucose 6 phosphate
phosphorolysis를 통해 ....
글리코겐 분해과정은 근육에 저장되어 있는 글리코겐을 glucose-1-phosphate로 가수분해하는 과정이고, 해당과정은 glucose나 glucose-1-phosphate를 pyruvate나 lactate로 분해하는 과정과 연관되는 일련의 과정이다. 격렬한 운동이 계속되어진다면 glycogenolysis와 glycolysis로부터 ATP 재합성이 상당히 증가하여야 한다.
에너지 대사과정을 통해 Ca2+와 ATP와 PCr 가수분해로 인한 생성물(ADP, AMP, IMP, NH3, Pi)들의 축적에 의한 근수축 활성화가 glycogenolysis의 자극제로 작용하고, 이러한 방법으로 적어도 얼마동안 혐기성 ATP 생산이 유지된다.
Glycogenolysis is the breakdown of glycogen (n) to glucose-6-phosphate and glycogen (n-1). Glycogen branches are catabolized by the sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.[1]
Contents
[hide]Mechanism of Glycogen[edit]
The overall reaction for the breakdown of glycogen to glucose-1-phosphate is:
- glycogen(n residues) + Pi ⇌ glycogen(n-1 residues) + glucose-1-phosphate
Here, glycogen phosphorylase cleaves the bond linking a terminal glucose residue to a glycogen branch by substitution of a phosphoryl group for the α[1→4] linkage. Glucose-1-phosphate is converted to glucose-6-phosphate by the enzyme phosphoglucomutase. Glucose residues are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with a α[1→6] linkage. Glycogen debranching enzyme then transfers three of the remaining four glucose units to the end of another glycogen branch. This exposes the α[1→6] branching point, which is hydrolysed by α[1→6] glucosidase, removing the final glucose residue of the branch as a molecule of glucose and eliminating the branch. This is the only case in which a glycogen metabolite is not glucose-1-phosphate. The glucose is subsequently phosphorylated to glucose-6-phosphate by hexokinase.
Function[edit]
Glycogenolysis takes place in the cells of the muscle and liver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in the fight-or-flight response and the regulation of glucose levels in the blood.
In myocytes (muscle cells), glycogen degradation serves to provide an immediate source of glucose-6-phosphate for glycolysis, to provide energy for muscle contraction.
In hepatocytes (liver cells), the main purpose of the breakdown of glycogen is for the release of glucose into the bloodstream for uptake by other cells. The phosphate group of glucose-6-phosphate is removed by the enzyme glucose-6-phosphatase, which is not present in myocytes, and the free glucose exits the cell via GLUT2 facilitated diffusion channels in the hepatocyte cell membrane.
Regulation[edit]
Glycogenolysis is regulated hormonally in response to blood sugar levels by glucagon and insulin, and stimulated by epinephrine during the fight-or-flight response. In myocytes, glycogen degradation may also be stimulated by neural signals.[2]
Clinical significance[edit]
Parenteral (intravenous) administration of glucagon is a common human medical intervention in diabetic emergencies when sugar cannot be given orally. It can also be administered intramuscularly.
Pathology[edit]
Mutations at the liver enzyme glycogen phosphorylase (GPLL, 847 aa) lead to glycogen storage disease VI (Hers disease). Mutations at the muscle enzyme glycogen phosphorylase (GPMM, 842 aa) lead to McArdle's disease. Deficiencies at the glycogen debranching enzyme lead to glycogen storage disease III (Oxford Dictionary of BioMedicine, 2010).