Mne Lecture 12 - Metabolic Homeostasis

Front 1

Glycogen:
-structure
-location
-usage (how does its structure fit its function)
-shelf-life

Back 1

-Glycogen is a polymer of glucose, linked through a(1-->4) glycosidic bonds. It is branched by the occurrence of a(1-->6) linkages every 8-12 residues.

-Glycogen is present in the cytoplasm as granules that are 100-400 A in diameter, each of which contains as much as 120,000 glucose units. Glycogen is most predominant in muscle and liver cells where it can make up on average 0.5-2% and 4-10% respectively of the cell weight. Also present in adipose and brain (may have regulatory role or may provide glucose at peak activity).

-glucose can be rapidly mobilized by removing glucose units from the non-reducing ends of glycogen. One of the effects of branching glucose is to increase the number of non-reducing ends from which glucose can be released
**storing as one big molecule v. lots of little glucose molecules lowers the osmotic pressure, since osmotic pressure is proportional to the solute molecules

-Because glycogen binds water, can't store a whole ton in a cell. Glycogen short term storage of glucose, ~ 12 hours (fatty acids for comparison ~1 month).

Front 2

Starch

-structure
-nutritional relevance

Back 2

-Starch structure related to what glycogen looks like
-Starch is mix of amylose (linear glucose unit held by glucose 1-->4 linkage) and amylopectin (looks like glycogen has 1-->4 linkages + branches arising from 1-->6 linkages with branches found every 24-30 residues)

-Nutritional relevance:
-It’s the amylose that is most rapidly metabolized. Relatively small, not as big as amylopectin. Boil potatoes first things that come out of starch granules are amylose, not amylopectin. Because they’re linear 1-->4 linkages that are easily hydrolyzed, can easily mobilize glucose units. Bacteria in potato salad eat amylose and propagate to higher levels.
-Amylopectin is bigger, doesn’t come out starch as easily. 1-->6 linkages harder for enzymes to get at and cleave for both thermodynamic and steric reasons. Also contains less than ½ the energy of 1-->4 linkages too. when we have meal, talk about complex carbs means the food has high fiber, has amylopectin buried in it which makes it harder for enzymes to get to it and mobilize glucose and raise your glucose levels.

Front 3

Advantages and disadvantages of glycogen as storage form

Back 3

Advantages:

a) Active muscles cells mobilize glycogen rapidly by breaking it down to
glucose, which is quickly metabolized. In contrast, they mobilize and
utilize fatty acids very slowly.

b) Glucose can be metabolized both aerobically and anaerobically
whereas fat is metabolized only aerobically.

c) In animals, blood glucose levels must be maintained, a process that
is readily carried out by breaking down glycogen. Fatty acid
metabolism by itself cannot produce glucose to do so.


Disadvantages:

a) Being hygroscopic, glycogen is capable of binding 3-4 times its
weight in water. This property limits the amount of glycogen that can
be stored.

b) That means that it can only serve as short-term storage of metabolic
fuel. Liver glycogen is almost totally depleted 12-18 hours after
fasting. Muscle glycogen can potentially be stored for longer
periods, but it is rapidly depleted during vigorous exercise.

Front 4

Glycogen BREAKDOWN

Back 4

1. Glycogen breakdown is catalyzed by the enzyme phosphorylase which catalyzes the phosphorylation of the glucose unit at the non-reducing end of glycogen.
-->takes inorganic phosphate and glycogen (phosphate involved in nucleophilic attack) and preserves the energy of 1-->4 linkage and attach phosphate at expense of breaking 1-->4 linkage. Make glucose 1-phosphate.

a) The glucose unit is released as glucose-1-phosphate, which is then converted to glucose-6-phosphate (G6P) by the enzyme phosphoglucomutase. G6P can then be further metabolized in the glycolytic pathway, or it can be hydrolyzed by G6P phosphatase to yield glucose (liver)
--> there is slightly more energy in the 6-phosphate linkage than the 1-phosphate linkage, so when you move it from the 1 position to the 6 you release some heat (note that this reaction is still reversible though when we have to MAKE glycogen! The deficit in energy can be made up for by heat being absorbed by the surroundings. When doing glycogenesis need to DRIVE the usage of glucose-1-phosphate to make it thermodynamically advantageous to make it)


b) Debranching enzyme. Glycogen phosphorylase hydrolyzes glucose units until the glucose unit at the non-reducing end is four residues away from the branch point. Then the debranching enzyme catalyzes two successive reactions that remove the branch. Its glycosyl transferase activity transfers a trisaccharide unit with α(1→4) linkages from the "limit branch" to the non-reducing end of another branch. The debranching enzyme also has α(1→6) glucosidase activity that hydrolyzes and releases the remaining glucose unit to yield the debranched glycogen.
-->One of difficulties of glycogen mobilization is as we get close to the 1-->6 branch point the break down of glycogen starts to slow down because as first metabolize glycogen and mobilize glucose and we start hitting the branch points, the glycogen phosphorylase starts hitting branches with more frequency as we get into the innermost parts of the glycogenwhen glycogen gets 4 residues from branch point, glycogen phosphorylase STOPS. To resume with this process, need to engage the DEBRANCHING ENZYME -- takes a trisaccharide, breaks it off, make 1-->4 linkage on chain from which it was derived or another chain. Do this because 1-->6 linkage has lower energy than the 1-->4 linkage. Break off at the 1-->4 to conserve energy (whereas if we were to break at 1-->6 linkage and try to make 1-->4 there would be a deficit in energy that would slow the reaction). Thus we leave behind one glucose unit that has the 1-->6 linkage. DEBRANCHING ENZYME HAS SECOND ACTIVITY: through glucosidase activity releases FREE glucose in UNPHOSPHORYLATED state. Vast majority, however, are mobilized as glucose 1 phosphate --> eventually converted to glucose 6 phosphate


c) The branched structure of glycogen serves to provide a large number of non-reducing ends from which glucose is mobilized. However, the phosphorylase catalyzes its reaction much faster than the debranching enzyme. During vigorous exercise, the outermost residues of glycogen, or approximately 50% of the glucose units, are rapidly utilized. Thereafter, the debranching of glycogen is the rate-limiting step in glucose mobilization. The muscle at this point cannot sustain maximal exertion

Front 5

Where do you get the energy needed to make glycogen?

Back 5

The formation of the glycosidic bonds between glucose units requires energy, and the energy required to form this bond is accomplished by activating glucose-1-phosphate to form UDP-glucose. This reaction is catalyzed by UDP-glucose pyrophosphorylase.

UDP glucose pryophosphorylase catalyzes this reaction where glucose 1 phosphate makes nucloephilic attack on alpha phosphate of UTP and thus we now for UDP glucose releasing a pyrophosphate which is rapidly hydrolyzed to inorganic phosphate by inorganic pyrophosphatase UDP glucose is the substrate for the enzyme glycogen synthase

Front 6

Metabolic homeostasis is really about

Back 6

Glucose levels relatively constant (5 mMol) whether we’re in well fed state or if we’re in the fasting state
brain needs glucose as metabolic fuel so you need to keep levels up so we don’t DIE
under starvation mode – materials are limiting, body HAS to make glucose somehow
it’s about glucose homeostasis, but other processes get tied in

Front 7

von Gierke's disease

Back 7

deficiency in glucose-6-phosphate

symptoms:
hypoglycemia (induced by lack of glycogenolysis induced by epinephrine and glucagon)

Front 8

Andersen's diease

Back 8

-defect in the branching enzyme in glycogen

-leads to cirrhosis of the liver and abnormal glycogen

-if you administer epinephrine, have diminished response (this is how you diagnose)

Front 9

McArdle's disease

Back 9

high muscle glycogen, reduction in blood lactate and pyruvate after exercise (reverse of what we'd see when we exercise, lactate goes up for us),
-no drop in pH (b/c no burst of lactate) following exercise but there is a normal hyperglycemic response to epinephrine (unlike in Andersen's disease)

Front 10

Role of glycogen branching?

Back 10

Glycogen looks like amylopectin and has same features, more difficult to digest at 1-->6 linkages. Glycogen tends to be more heavily branched (every 8-10 residues). Branching increases the number of non-reducing ends –the “business ends” of glycogen. Have glycogen w lots of business ends can mobilize glucose very quickly, at least from the 1-->4 linkages.

Start running and you exert maximal energy for 30 seconds will mobilize up to ½ the glucose units in the glycogen particle. Then that process slows down. As you use up glucose in outermost chains the enzyme that breaks down glycogen starts to encounter the 1-->6 linkages. Getting past the 1-->6 linkage is the rate limiting step of this process. Also helps glycogen present in liver and muscle last a long time. Branching also contributes for making glycogen more compact.

Front 11

Excess protein stored in?

Back 11

excess protein stored in muscle..
Muscle has a function but a lot of it can be wasted away and still maintain muscle fxn

preserving muscle means preserving amino acids and protein

*many amino acids are glucogenic --> carbon skeletons can be used as starting material for gluconeogenesis, can increase number of intermediates in citric acid cycle

*other amino acids are ketogenic --> can be used to make ketone bodies

Front 12

Compare the advantages and disadvantages of the different metabolic fuels.

Back 12

Metabolic fuels:
1. Glucose
2. Fatty Acids and triacylglycerols
3. Ketone bodies
4. Proteins and amino acids

1. glucose and carbs:
a. advantage: can be metabolized aerobic and anaerobic usage.
b. Disadvantage: bulky storage since they bind to water (stores up to 2x its weight in water as glycogen), can easily be depleted in our body.

2. fatty acids:
A. Advantage: hydrophobic, not as bulky as glucose and carbs because they don’t bind water. In form of triacylglycerols can be stored in lipid droplets. Can store lots of energy in small space (6x that of glycogen). Because they're insoluble in water, they must be transported in the circulation as complexes with albumin or components of lipoproteins.
B. Disadvantage: can only be used aerobically. Cannot be used increase number of citric acid intermediates. Fatty acids are not soluble cant cross the blood brain barrier.. They can a little (need to get essential Fas into the brain, like omega 3’s, so there must be a mechanism for those Fas to get to the CNS) BUT for all practical processes because of slow transfer from blood to the brain they cannot be used as an energy source. Fas CAN be oxidized via b-oxidation in brain but has more important role in regulation.

3. Ketone bodies:
A. advantage: essentially soluble form of fatty acids, negatively charged, highly soluble. Like glucose (which is soluble) it is soluble can cross blood brain barrier (up to 33% of brain energy demands in starvation can be met by ketone bodies, but never more. Never a time the brain can do w/o glucose).
B. Disadvantage: **negative charge lowers blood pH which can interfere with energy metabolism. Too many ketone bodies causes blood to undergo an inflammatory response(Also don’t want glucose or fatty acids in extremely high amounts)

Front 13

How do you get glucose released into the blood stream?

Back 13

If we release glucose 6-phosphate in the liver, for example, cannot get to blood stream because GLUCOSE TRANSPORTERS ONLY ALLOW PASSAGE OF FREE GLUCOSE, not glucose 6-phosophate

In muscle, liberate it and it goes to glycolysis.
In liver going to be released. Goes into ER of liver, glucose 6 phosphate must hydrolyze phosphate to form free glucose and then glucose can be released into blood stream through transporter.

GLUCOSE 6 PHOSPHATASE PLAYS IMPORTANT ROLE IN RELEASING GLUCOSE FROM LIVER TO BLOOD STREAM.

Front 14

Glycogen synthesis

Back 14

a) The formation of the glycosidic bonds between glucose units requires energy, and the energy required to form this bond is accomplished by activating glucose-1-phosphate to form UDP-glucose. This reaction is catalyzed by UDP-glucose pyrophosphorylase.
-->start with glucose 1 phosphate + UTP molecule
UDP glucose pryophosphorylase catalyzes this reaction where glucose 1 phosphate makes nucloephilic attack on alpha phosphate of UTP
have UDP glucose releasing a pyrophosphate which is rapidly hydrolyzed to inorganic phosphate by inorganic pyrophosphatase 


b) Glycogen synthase transfers a glucose unit from UDP-glucose to the non-reducing end of glycogen.
-->UDP glucose is the substrate for the enzyme glycogen synthase which does the following reaction.:
Build glucose units starting at the nonreducing end, it’s the OH at the 4-carbon of the glycogen on the nonreducing end that will attack the 1 carbon and that nucleophilic attack released UDP-glucose with attachment of glucose that comes from concomitant UDP-glucose and produce the nonreducing end.

c) However, like DNA polymerase, glycogen synthase requires a primer or pre-existing glycogen chain to form glyosidic bonds. New glycogen molecules are initiated by a protein primer called glycogenin (MW 37,000). Glycogenin then adds glucose units to one of its tyrosine residues, building a chain of glucose units with α(1→4) linkages up to 8 residues in length. Glycogen synthase can then add glucose units to this primer.
-->Glycogen synthase is like DNA polymerase: It cannot start synthesizing glycogen de novo, it can only add onto preexisting glycogen chain. What that means is that glycogen requires primer to start synthesizing glycogen; that protein is provided by protein glycogenin.
Glycogenin has tyosine residue, it is at the tyrosine residue that it builds a glycogen chain staring with udp-glucose and it progressively adds UDP-glucose at that site and makes a glycogen chain that is 8 residues in length (glycogenin). This is used as a primer to now make glycogen.

d) The branching enzyme transfers a seven-residue segment from the non-reducing end of glycogen [α(1→4) linked chains] to the C6-OH group on the same or another chain.
-->Enzyme that does branching makes glycogen + glycogen synthase makes this very large glycogen particle, the center of which has glycogenin and we build glycogen particles through branching and also by making 1-->4 linkages so all the outermost residues are the business ends of the glycogen particle.
-->glycogen branching enzymes. Takes a few residues, about 7 or so, and cleave the 1-->4 linkage and make the 1-->6 linkage. Create molecule with 2 reducing ends and increase the rate at which glycogen synthesis can take place.