Humboldt State University ® Department of Chemistry

Richard A. Paselk
Chem 431

Biochemistry

Fall 2001
Lecture Notes:: 5 November

© R. Paselk 2001
  
     
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GLYCOGEN METABOLISM, cont.

Phosphorylase can only cleave 1,4-linkages, so now need Debranching enzyme. Debranching enzyme has two activities: a) amylo-a-1,4-transferase moves the terminal three residues of a chain onto another branch; whereas a-1,6-glucosidase hydrolyzes the 1,6-linkage to give free glucose. [Thus muscle can release a small quantity of glucose into the blood without actually doing gluconeogenesis.]

 

Glycogen Control Cascade

Since the Glycogen synthase and phosphorylase reactions are in opposition we need a control system. In muscle it turns out that glycogen synthesis/breakdown is controlled by a very complex system enabling both rapid response to emergencies and exquisite overall control of the opposing activities to respond to a variety of situations. This is accomplished through the Glycogen Cascade Control system. (The diagram shown is actually a simplified representation, especially of the synthase enzyme, which turns out to have 9 phosphorylatable sites which are phosphorylated by a number of different kinases responding to different complex physiological situations and with varying responses by the enzyme.)
 
Response begins with a hormonal signal, such as adrenalin, binding to the receptor on the cell surface. This results in the phosphorylation of GDP to GTP on an intracellular G-protein. The G-protein can now interact with Adenylate cyclase to produce the "second messenger" 3', 5'- cyclic AMP (cAMP). Cyclic AMP then binds to the regulatory subunit of cAMP-dependent protein kinase, releasing the active catalytic subunits (C), which can now phosphorylate inactive o-phosphorylase kinase b to the active m-phosphorylase kinase a (o= original, m= modified, b= inactive, a= active). Phosphorylase kinase a then phosphorylates o-Glycogen phosphorylase b to the active m-Glycogen phosphorylase a, resulting in the breakdown of glycogen with the release of G-1-P.
 
Note the parallel kinase cascade which simultaneously shuts down Glycogen synthase.
 
Finally, one does not always have a warning, that is time to get the endocrine system going to produce adrenalin, thus the release of Calcium in the muscle cells bypasses much of the cascade, activating the normally inactive o-Phosphorylase kinase b , which then acts on both the phosphorylase and the synthase.
 

Glycogen Control in Liver

 
In the liver glycogen metabolism is largely regulated by glucose concentrations, which in turn reflect serum glucose concentrations.
The net result is that glycogen is synthesized when [glucose] is hi, and it is broken down when [glucose] is low.
 

 

 

GLUCONEOGENESIS

In order to provide glucose for vital functions such as the metabolism of RBC's and the CNS during periods of fasting (greater than about 8 hrs after food absorption in humans), the body needs a way to synthesis glucose from precursors such as pyruvate and amino acids. This process is referred to as gluconeogenesis. It occurs in the liver and in kidney. Most of Glycolysis can be used in this process since most glycolytic enzymes are reversible. However three irreversible enzymes must be bypassed in gluconeogenesis vs. glycolysis: Hexokinase, Phosphofructokinase, and Pyruvate kinase. Phosphofructokinase, and/or hexokinase must also be bypassed in converting other hexoses to glucose.
 
Let's begin with pyruvate. How is pyruvate converted to PEP without using the pyruvate kinase reaction? Formally, pyruvate is first converted to oxaloacetate, which is in turn converted to PEP. In the first reaction of this process Pyruvate carboxylase adds carbon dioxide to pyruvate with the expenditure of one ATP equivalent of energy. Biotin, a carboxyl-group transfer cofactor in animals, is required by this enzyme:
 
 
The reaction takes place in two parts on two different sub-sites on the enzyme. In the first part biotin attacks bicarbonate with a simultaneous attack/hydrolysis by bicarbonate on ATP, resulting in the release of ADP and inorganic phosphate (note the coupling by the enzyme of independent processes in this reaction):
 
 
Note that the 14 Angstrom arm of biocytin allows biotin to move between the two sites, in this case carrying the activated carboxyl group. In the second site a pyruvate carbanion then attacks the activated carboxyl group, regenerating the biotin cofactor and releasing oxaloacetate:
 

 

Pathway Diagrams

 

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Last modified 5 November 2001