Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438

Introductory Biochemistry

Spring 2007

Lecture Notes: 4 April

© R. Paselk 2006
 
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Glycogen Metabolism

Start with G-6-P, again note that this molecule is at a metabolic crossroads. First convert to G-1-P using Phosphoglucomutase:

This reaction is very much like PGA Mutase, requiring the bis phosphorylated intermediate to form and to regenerate the phosphorylated enzyme intermediate. Note that this reaction is easily reversible, though it favors G-6-P.

Again a separate "support" enzyme, Phosphoglucokinase, is required to form the intermediate, this time using ATP as the energy source:

UDP-glucose pyrophosphorylase, which catalyzes the next reaction, has a near zero G° ':

It is driven to completion by the hydrolysis of the PPi to 2 Pi by Pyrophosphatase with a G° ' of about -32 kJ (approx. one ATP's worth of energy).

Finally glycogen is synthesized with Glycogen Synthase:

UDPGlucose + (glucose)n UDP + (glucose)n+1

This reaction is favored by a G° ' of about 12 kJ, thus the overall synthesis of glycogen from G-1-P is favored by a standard free energy of about 40 kJ. Note that the glucose is added to the non-reducing end of a glycogen strand, and that there is a net investment of 2 ATP equivalents per glucose (ATP to ADP and UTP to UDP, regenerated with ATP to ADP). Note also that glycogen synthase requires a 'primer.' That is it needs to have a glycogen chain to add on to. What happens then in new cells to make new glycogen granules? Can use a special primer protein (glycogenin). Thus glycogen granules have a protein core.

These reactions will give linear glycogen strands, additional reactions are required to produce branching. Branching enzyme [amylo-alpha-(1,4) to alpha-(1,6)-transglycosylase] transfers a block of residues from the end of one chain to another chain making a 1,6-linkage (cannot be closer than 4 residues to a previous branch). [FYI: For efficient release of glucose residues it has been determined that the optimum branching pattern is a new branch every 13 residues, with two branchs per strand.]

Glycogen is broken down using Phosphorylase to phosphorylize off glucose residues:

(glucose)n + Pi (glucose)n-1 + G-1-P

Note that no ATP is required to recover Glucose phosphate from glycogen. This is a major advantage in anaerobic tissues, get one more ATP/glucose (3 instead of 2!). [FYI: Phosphorylase was originally thought to be the synthetic as well as breakdown enzyme since the reaction is readily reversible in vitro. However it was found that folks lacking this enzyme - McArdle's disease - can still make glycogen, though they can't break it down.]

Phosphorylase can only cleave 1,4-linkages, so now need Debranching enzyme. Debranching enzyme has two activities: a) amylo-alpha-1,4-transferase moves the terminal three residues of a chain onto another branch; whereas alpha-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

Since the Glycogen synthase and phosphorylase reactions are in opposition we need a control system. Glycogen storage/release strategies vary widely with different tissues. In liver, glycogen is used to provide glucose to the serum between meals - it serves a homeostatic function. Glycogen control in liver is thus designed to breakdown and release glycogen when serum [glucose] is low and synthesize glycogen when serum [glucose] is high.

FYI - Glycogen Control in Liver 

In the liver glycogen metabolism is largely regulated by glucose concentrations, which in turn reflect serum glucose concentrations.
  1. In liver glycogen phosphorylase a binds tightly to protein phosphatase-1 and inhibits it. At high concentrations, glucose binds to phosphorylase, causing a release of the protein phosphatase. Protein phosphatase then inactivates m-phosphorylase a by hydrolyzing off Pi to give inactive o-phosphorylase b.
    • Glycogen breakdown and glucose release are inhibited.
  2. Protein phosphatase can now also hydrolyze Pi from inactive Glycogen synthase b to give the active Glycogen synthase a.
    • Glycogen synthesis is activated.
  3. As glucose concentrations drop, glycogen phosphorylase rebinds protein phosphatase and protein kinases rephosphorylate the enzymes, resulting in glucose release.

The net result is that glycogen is synthesized when [glucose] is hi, and it is broken down when [glucose] is low.

Glycogen Control Cascade

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.

Pentose Phosphate Pathway

The Pentose Phosphate Pathway is an alternate pathway for glucose oxidation which is used to provide reducing equivalents in support of biosynthesis. Thus although it involves the catabolism of glucose, it is generally going to be active only when anabolism is taking place.
This pathway is usually treated in two parts: the oxidative portion, and the sugar interconversions portion. In the oxidative part, on the top of the handout, glucose is first oxidized to a lactone, and then oxidatively decarboxylated. Note that in each case NADP+ is the oxidant as opposed to NAD+. Note also that the two DH reactions are both physiologically irreversible, due in part to the very low concentrations of NADPH in cells.
The first enzyme, G-6-P DH, is highly specific for glucose (it is frequently used as the basis of specific glucose assays). In this reaction the #1 (aldehydic) carbon of glucose is oxidized to a lactone (cyclized carboxylic acid). This is the first committed step for this pathway and it is regulated by the availability of NADP+ (substrate availability). Since NADP+ and NADPH are in very low concentrations and the NADP+/NADPH ratio is very low, and since NADP+ is generated only during biosynthetic reactions this results in a close coupling of the oxidative portion of this pathway to reductive biosynthesis.
Next Gluconolactonase opens the ring with the addition of a molecule of water. Then 6-P-gluconate DH oxidizes the #3 carbon to a ketone. This results in the #2 carbon becoming somewhat acidic, thus destabilizing the carboxyl group, which is then lost to give the five carbon ribulose-5-P.

In the non-oxidative portion of the Pentose Phosphate Pathway a series of sugar interconversions takes the RU-5-P to intermediates of other pathways: Ribose-5-P for nucleotide biosynthesis, and F-6-P and Ga-3-P for glycolysis/gluconeogenesis. All of these reactions are near equilibrium, with fluxes driven by supply and use of the three intermediates listed above.


Pathway Diagrams

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Last modified 4 April 2007