Week 7 Lecture 560B on Line Glygoen, Fruct, Gal, Pentose Most Recent

8/2/2019 Week 7 Lecture 560B on Line Glygoen, Fruct, Gal, Pentose Most Recent

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Week 7 Lecture

Biochemistry of Nutrition, 560, B

Dr. Charles Saladino

Introduction

This lecture will cover some of the main metabolic pathways associated with

carbohydrate metabolism. There is a defined and important inter-relationship between

the pathways with homeostatic balance being maintained through various types ofregulation, including covalent, allosteric, hormonal, and second messenger systems.

These pathways will include glycogenesis and glycogenolysis, gluconeogenersis, glucose

isomer metabolism, and the pentosephosphate pathway, otherwise known as the

hexosemonophosphate Shunt, and the Krebs cycle. Let us begin with thepentosephosphate pathway, often given too little time in introductory biochemistry

courses. The structures of the pathway can be found on pages 449 & 650, Devlin) of

your text. You do not have to memorize the pathway, but you do need to

understand its significance and the material I will now cover.

Hexosemonophosphate Shunt or Pentose Phosphate Pathway (PPP)

Purpose of this pathway is two-fold. The first is the production of NADPH (a reducing

agent and the reduced form of NAD+ used in many processes, such as fatty acidsynthesis. This can be considered a cosubstrate for key enzymes. The second

importance of the PPP is that it generates intermediate metabolites essential for the

synthesis of nucleic acids and nucleotides. Because this pathway represents analternative to glycolysis, be sure to reacquaint yourself with the oxidation of glucose to

pyruvate. The PPP occurs in two phases, an oxidative phase and a non-oxidative phase.Again, I refer you to your text as we start with the oxidative phase. Here, glucose-6-P isthe starting metabolite. Remember that this has been formed by the action of the

phosphorylating enzyme, glucokinase. There are three steps during this first phase that

result in the generation of the D isomer of ribulose-5-P. Of importance to note is theformation of NADPH + H+ from NADP+ in steps 1 and 3.

Notice that in the non-oxidative phase, both isomerization and condensation reactions of

several sugar molecules occur. I am not happy with the figure presentation in your text,so I will attempt to clarify it. First, however, lets look for the most significant

intermediates generated by this non-oxidative phase. These products are ribose-5-P (used

for nucleic acids), fructose-5-P, and glyceraldehyde-3-P (G-3-P). (Remember G-3-Pfrom glycolysis?)

Now besides the G-3-P, ribose-5-P, and fructose-6-P, the non-oxidative phase generatesthe interconversion of 3, 4, 6, and 7-carbon sugars. Some of these enter glycolysis. We

also see that the PPP pathway provides a means for cleaving the carbon chain, one carbon

at a time. However, a big however, unlike the Krebs cycle, this pathway does not occur

as a series of consecutive reactions leading from glucose-6-P to six CO2

molecules.


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I now give you a diagram below, where the two, 5-carbon isomerization products

generated from ribulose-5-P (xyulose-5-P and ribose5-P not shown below, but shown inyour text) interact with a transketolase enzyme to form two products. One is the 3-carbon

G-3-P, the other is a 7-carbon sugar (sedoheptulose-7-P), shown below. The G-3-P

formation is very important, because it can re-enter glycolysis.

Now, the G-3-P and the seduloheptose, catalyzed by a transaldolase, join and rearrange to


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form two new sugar products, fructose-6-P (6 carbons) and erythrose-4-P (4 carbons).

(By the way, you should be keeping track of carbons during these reactions.)

The final piece of the puzzle is in the lower right part of the above diagram. Notice that

the 4-carbon erythrose-4-P and the 5 carbon xyulose-6-P join and rearrange (catalyzed by

a transketolase) to form fructose-6-P and G-3-P. Both are glycolytic intermediates. Youshould also note that transketolases will transfer 2-carbon units, whereas transaldolases

transfer 3-carbon units. Each of these carbon units is transiently attached to the enzyme

in the course of the reaction.

Please also note that these reactions of the PPP pathway are all reversible. That means

that the equilibrium can shift in the direction which provides the product when it is in

demand. Just by way of one of many examples, take the situation where more ribose-5-Pis needed than NADPH as in a cell actively engaged in mitosis. Here is what will

happen: Most of the glucose-6-P is converted into fructose-6-P and glyceraldehyde-3-P

(G-3-P) by glycolysis. Then the transketolase and transaldolase convert two molecules of

fructose-6-P and one molecule of G-3-P to yield three ribose-5-P molecules by reversalof the non-oxidative phase of the PPP pathway described above. You could summarize

this by the following reaction (Note the 30-carbon balance on both sides of theequation.):

5 glucose-6-P + ATP -----> 6 ribose-5-P + ADP + H+

So what would be the predominant reaction if NADPH were required in great amounts,

as would be the case in a demand for fatty acid, cholesterol, neurotransmitter, and

nucleotide biosynthesis? The answer is the oxidative phase of the PPP pathway.

Glucose-6-P + 2NADP+ + H2O ribose-6-P + 2NADPH + 2H+ + CO

2

Coordination of Glycolysis and the Pentose Pathway

Obviously by now you realize the glucose-6-P is metabolized by both glycolysis and the

PPP pathway. So how can we partition this metabolite between the two pathways? The

cytoplasmic concentration of NADP+ plays a key role in determining the fate of glucose-6-P. The first reaction in the oxidative phase of the PPP pathway is essentially

irreversible, and under physiologic conditions serves as a control site. The most

important regulatory factor is the level of NADP+, because this is the electron acceptor in

the oxidation of glucose-6-P. Low levels of this key cofactor are made worse by the factthat NADPH competes with NADPH+ in binding to the enzyme. The ratio of NADP+ to

NADPH in the cytosol of the liver of a well-fed rat is 0.014. This low ratio ensures thatNADPH generation is tightly coupled to its utilization in reductive biosynthesis (ex fatty

acid synthesis). In other words, excess NADP+ will allow NADPH to go anywhere. This

all translates into the fact that the non-oxidative phase of the PPP pathway is controlled

by the availability of NADP+.

Some Final Thoughts on the Pentose Pathway


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Please take note that by the time theribulose-5-P has been produced from glucose-6-P,

we have gone from a hexose to a pentose by way of a decarboxylation at step three of theoxidative phase.

Another aside: Now here is an opportunity for you to have done some thinking. Youprobably learned in your pre-requisite (I hope) that when mitochondria are involved,

oxygen is required. The fact that oxidation occurs in the cytosol tells us that this process

is not dependent upon the mitochondria or upon the Krebs cycle. Please remember thatfor next week.

You might have heard of the Calvin Cycle in plants. Here, the cycle begins with CO2

and

goes on to use NADPH in the synthesis of glucose. In the pentose pathway, we begin

with the oxidation of a glucose-derived carbon to CO2

to yield NADPH. Further, the

Calvin cycle converts 6-carbon and 3-carbon molecules to the starting material,ribulose-5-P. The pentose pathway converts 5-carbon sugars (ribose-5-P) into 6-carbon

and 3-carbon intermediates of glycolysis. In photosynthetic organisms, the enzymes arethe same as in mammalian organisms. Some would call this evolutionary economy.What an interesting stroke of creative genius.

Glycogen Metabolism

Glycogen can be considered a readily available storage form of glucose. This glycogen

structure is a very large, branched glucose residue polymerthat can be degradedto indirectly provide glucose when it is required for energy production.Although the majority of glucose residues are linked by -1,4-glycosidic

bonds, branches occur about every ten residues, due to -1,6-glycosidic bond formation. This bond like that formed in joining two amino acids is a dehydration synthesis reaction, where water is removed to form a bond. On yourown, look up the structure of these two bond types, as well asa -glycosidic linkage, and please know them for the next exam. Theyare easily found in the Devlin text or on line. I will note here that -glycosidiclinkages form open helical polymers, but -glycosidic linkages producealmost straight chain polymers to form fibrils, as is seen in cellulose, forexample. Can you suggest for the Discussion Board the advantage(s) of glycogen

having branches, instead of being a long polymer?

Biochemically, glycogen is not as reduced as are fatty acids. Then why would we storeglycogen at all? We could just convert excess energy into fatty acids. Right? Well,

there are several answers to that question. The first is that in between meals, the

controlled breakdown of glycogen and subsequent release of a glucose product makes

that sugar available to those organs where it is needed. So we might think of glycogen asa buffer against non-homeostatic plasma glucose levels. This is especially important for

the brain, whose only source of energy is glucose under-non-starvation conditions.

Further, mobilization of glucose stores (as glycogen) is quick and is a good source of


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energy for sudden bursts of activity. Finally, glucose can supply at least some energy

anaerobically, unlike fatty acids which require the oxygen found in mitochondria.

The liver and the skeletal muscle are the two major storage sites of glycogen. However,there is a greater glycogen concentration in the liver (potentially 7-10%) vs. that of

skeletal muscle (about 2%); but because of its much greater mass, more glycogen overall

is stored in muscle than in the liver. Microscopically, glycogen is found in the cytosol asgranules ranging in size from 10 40 nm. Another point of importance is that the

glycogen stored in muscle is regulated to meet the energy needs of that itssue, where liver

glycogen degradation and synthesis are balanced so as to maintain plasma glucose levelsfor the good of the whole organism.

At this point we need to clarify something. Although I (and many texts) talk of making

glucose available from glycogen, the breakdown of glycogen does not yield a glucosemolecule directly. The degradation process is basically a three-step process:

1) glucose-1-P is released from glycogen; 2) glycogen as a substrate is remodeled to

allow further degradation, as it is during synthesis; 3) glucose-1-P is further converted

into glucose-6-P for further metabolism. This overall breakdown process is calledglycogenolysis. Observe this on page 630-631 of your text. Then look at Figure15.51 to observe glycogenesis. You are responsible for the names of the respective

enzymes for the next exam.

The glucose-6-P product of glycogenolysis basically has three fates: 1) it can continue

into glycolysis; 2) it can enter the now-familiar-to-you pentose pathway to generateNADPH and ribose derivatives; 3) it can be converted to glucose to be released into the

blood stream.

Now in both the degradation and the synthesis of glycogen, we take note of two modified

glucose molecules. These are UDP-glucose and UTP-glucose. The U stands for

uridine, the DP is diphosphate, and the TP is triphosphate. This is the same U as is foundin RNA nucleotides. Please note their respective roles in both process, and if you have

any questions about this, let me know via email or the Discussion Board, as you are, atleast in general, responsible for their respective roles, as it pertains to this

discussion. You would, however, expect a phosphate to be available to help drive the

energy-requiring synthesis of glycogen.

As a point of interest, until not long ago, it was presumed that blood glucose was the soleprecursor for glycogenesis. However, under physiological conditions, it is now known

that most the new glycogen is formed from not from plasma glucose, but from

gluconeogenic (glucose-producing) precursors, especially lactate and alanine. Both ofthese molecules are easily converted into glucose in the liver. Please read page 620-621

of your text to find out the relationship of this gluconeogenic lactate to the Cori

cycle. There is also an alanine cycle which sends alanine from the muscle cells to theliver.

The final topic for glycogen is its regulation. This is quite complex. Various enzymes

that participate in glycogen metabolism allosterically respond to metabolites that signal


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the cells energy needs. In other words, enzyme activity is adjusted to meets the needs of

the cells in which the enzymes are present. Glycogen metabolism is also hormonally

regulated, in order to allow glycogen metabolism to respond to the needs of the wholeorganism. My apologies, but you need to know the main features of Figure 15.54-15.55 in order to appreciate some of this regulation.

Then email me to explain how the insulin/glucagon ratio directly affects the events

shown in the figure. I want some mechanism described here, please.

Feeder Pathways into Glycolysis

The purpose of this section is to account for the fate of two important monosaccharidesafter they have been digested and absorbed. These two sugars are galactose and fructose.

The increase in fructose in the American diet (whatever that is) has been meteoric over

the last couple of decades. This is because of a variety of reasons, not the least of which

is that it is a very inexpensive commodity. In terms of metabolic health and efficiency,this huge increase in the use of fructose is doing none of us any good.

Galactose Metabolism

Please read your text on the Absorption, Transport, and Distribution of galactose

and fructose. Note that they are not absorbed in the same manner, with galactose

utilizing the same Na+-dependant carrier system as glucose does.

Obviously, a major source of dietary galactose is lactose, especially from milk and milk

products. Other sources include the cell turnover, especially cell membranes, upon which

lysosomes work to provide glycoproteins and glycolipids from those membranes.

Like fructose and glucose, galactose must be phosphorylated before it can be mobilized.

This takes place in the liver. Galactose kinase is the enzyme responsible for thisphosphorylation, and it costs one ATP. The product is galactose-1-P. Refer to the

figure below to follow this pathway, and please pay close attention to the color scheme,

so that you will more easily understand what is happening.

First, the newly generated galactose-1-P can not enter glycolysis until it is first converted

to UDP- galactose. This occurs in an exchange reaction. In this reaction, the enzyme

galactose-1-P uridyl transferase transfers the removal of UMP (shown in black) fromUDP-glucose and adds it to the galactose-1-P to form UDP-galactose (shown in red and

black), leaving behind glucose-1-P (shown in blue).

Notice two things that are happening. First, UDP galactose can under go an

isomerization reaction, using the enzyme UDP-galactose 4-epimerase. This reaction

regenerates UDP-glucose, which is recycled back to start a new set of reactions. Note the

efficiency of this, as opposed to using a totally different pathway to generate all the UDP


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glucose needed for this reaction sequence which brings us to a the second point. The

formation of glucose-1-P is now positioned to be converted into glucose -6-P (by another

isomerization reaction using phosphoglucomutase) or even glucose by removing thephosphate group using the enzyme glucose-6-phosphatase. The glucose itself can be

utilized in a variety of ways, including being exported to other tissues from the liver. On

the other hand, the newly-generated glucose-6-P is a metabolite that is part of glycolysis,and hence it can be incorporated into that pathway for oxidation to pyruvate.

There are certainly some side issues here. For example, if energy is abundant, say from ahigh caloric intake, the glucose-1-P might be channeled into glycogenesis instead of

glycolysis. This type of common sense thinking is what I hope you will strive to achieve.

However, the bottom line of what we have shown here is that (and how) the carbons of

galactose can wind up in glucose for glycolysis, because there are no catabolic pathwaysto metabolize galactose.

Fructose

In most organisms, like galactose and other hexoses (such as mannose), fructose can

enter glycolysis after conversion to a phosphorylated derivative. D-fructose is found in

many fruits, but is unfortunately loaded into our diets in a variety of ways, from syrups to


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candies etc. Whereas galactose and glucose are absorbed into the intestinal mucosa by

way of an active, energy-requiring transporter with a countercurrent uptake of sodium,

fructose uptake utilizes a far less efficient, sodium-independent transporter (GLUT-5) forabsorption. All three monosaccharides require another transporter (GLUT-2) to then

enter the portal circulation.

Here we must make a clear distinction between the metabolic events taking place in the

liver vs. those in the muscle. In the liver, the fructose-1-P pathway is utilized. This is

illustrated in this Figure to the right. The first step involves

the phosphorylation of fructose to fructose-1-P, using the enzyme fructokinase and at the

cost of one ATP. This six-carbon product is then split into two, three-carbon sugars,dihydroxyacetone phosphate (DHAP) and glyceraldehyde by the enzyme B aldolase.

Note that the DHAP is a glycolytic intermediate. A triose kinase then converts the

glyceraldehydes to glyceraldehyde-3-P, also a glycolytic intermediate and at the cost of

one ATP. By this means, fructose can enter glycolysis.

In the liver, fructose can theoretically be converted to fructose-6-P by a glucokinase

enzyme. However, the affinity of the glucokinase is 20 times greater for glucose than forfructose. Thus, very little fructose-6-P is formed in the liver, because glucose is so much

more abundant in this organ. Only a heavy intake of fructose would activate this

pathway. Besides, in the liver, glucose is the preferred fuel, as is the case in muscle.

In the muscle, fructose can be converted directly to fructose-6-P at the cost of an ATP.

This is already an intermediate of glycolysis, although again, glucose is the preferredfuel.

There has been a ten-fold increase in fructose consumption in the last 25 years in the

United States, due to the high use of high-fructose corn syrup, which is sweeter and ischeaper to produce than glucose. Although we will not discuss carbohydrate regulation

until the next lecture, I will note here that fructose catabolism in the liver by passes the

phosphofructokinase catalyzed step of glycolysis, wherein fructose-6-P is converted intofructose-1,6-bisphosphate. This is a highly regulated metabolic control point. This could

potentially disrupt fuel metabolism, whereby glycolysis would be directed toward lipid


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synthesis, if ATP production were not needed. Importantly, herein might be a link

between fructose consumption and obesity that is ever-increasing in the United States.

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Week 7 Lecture 560B on Line Glygoen, Fruct, Gal, Pentose Most Recent