CORRELATION BETWEEN METABOLIC PATHWAYSALAKSH CHOUDURY

SHASHWAT MOHAN VAJPEYI

CONTENTS

Importance of regulation. Homeostasis. Importance of cofactor concentrations. Correlation, an explanation using liver as an

example. Then we show various correlations through

citations at regulation of pathways.

AN OVERVIEW

The basis of this seminar is to explain the correlation of various pathways by 3 approaches:-

1. By considering regulation and understanding how regulation interrelates various metabolic pathways.

2. By taking hepatocytes as a basis to understand the correlation of various pathways.

3. And lastly we consider TCA which is the heart of all metabolic reactions by influencing them in some way or the other.

DIAGRAMMATIC REPRESENTATION OF THE CORRELATION

Some basic facts:-

1.A typical eukaryotic cell has a capacity to make about 30000 different proteins.2.These catalyze about thousands of different reactions involving many hundreds of metabolites most shared by more than one pathway.

INTRODUCTION TO REGULATION The basic topic of discussion is the

interconnection between the various metabolic pathways and for understanding this the first and foremost task is to understand the concept of regulation.

Very complex regulatory mechanisms have evolved to ensure that metabolites move through each pathway in the correct direction and at the correct rate to match the organism’s or the cell’s current circumstances.

There can be dramatic change in circumstances can occur for eg; the demand for ATP production can increase 100 folds in the muscle in a few seconds in response to exercise.

HOMEOSTASIS The need for regulation ca be very well

understood by understanding the phenomenon of homeostasis.

A general cellular reaction can be represented as:-

A S P where v1=v2, S =constant. At molecular level, in each metabolic reaction in

a pathway ,the substrate is provided by the preceding reaction at the same rate at which it is converted to product. Thus, although the rate of the metabolite flow, the flux, may be high but the concentration of substrate, S, remains constant.

v1 v2

This above mentioned phenomenon is called as Homeostasis.

When the steady state is disturbed by some change in external circumstances or energy supply, the temporarily altered fluxes through individual metabolic pathways trigger regulatory mechanisms intrinsic to each pathway.

Hence it can be very well stated that homeostasis is the basis of regulation.

Next we move on to explain the phenomenon of homeostasis using the ratio of ATP to AMP as an example.

Basic facts:-In humans about 4000 genes (12% of all genes) encode regulatory proteins that include a variety of receptors, regulators of gene expression and about 500 different protein kinases.

CONNECTING LINK BETWEEN ALL PATHWAYS; [ATP]/[AMP] The ATP concentration in a typical cell is about 5mM. Because ATP is converted to ADP and AMP, the

[ATP]/[ADP] or [ATP]/[AMP] ratio profoundly effects all the reaction rates that employ these cofactors same is the case with [NADH]/[NAD+] and [NADPH]/[NADP+] ratio.

One important mediator of regulation is AMP dependent protein kinase (AMPK).

The action of AMPK increases glucose transport and activates glycolysis and fatty acid oxidation while suppressing energy requiring processes like fatty acid synthesis, cholesterol biosynthesis and protein biosynthesis. Thus we can observe that the cofactor ratios play a key role is the correlation of the regulatory actions of various metabolism.

UNDERSTANDING CORRELATIONUsing liver cells as an example.

cytosol

Intracellular location of major metabolic pathways liver parenchymal cell.

SUGARS

Glucose is phosphorylated to glucose-6-phosphate by enzyme glucokinase•In hepatocytes……it can undergo one of the following metabolic pathways depending upon current metabolic needs.

•It is dephosphorylated to glucose by glucose-6-phosphatase to replenish blood sugar.

•It can be converted to liver glycogen or it can undergo glycolysis to produce pyruvate and then acetyl coA.

• acetyl coA undergoes TCA cycle, with ensuing

electron tranfer and oxidative phosphorylation to yield ATP.

• • Acetyl coA can also serve as a precursor of

fatty acids.

• Finally, glu-6-phosphate can enter PPP pathway,yeilding reducing power NADPH required both for biosynthesis of fatty acid and cholesterol and to yield ribulose-5-phosphate for nucleotide biosynthesis.

FATTY ACIDS

Metabolism of fatty acids in liver

The fatty acid components of fatty acids entering hepatocytes have following fates :

•Some are converted to liver lipids•Fatty acids are primary oxidative fuel in the liver. Free fatty acids may be activated and oxidized to yield acetyl coA and NADH.•Acetyl coA can enter TCA cycle to drive ATP synthesis .•Excess fatty acids may be converted to ketone bodies.• Some of the acetyl coA derived from fatty acids is used for biosynthesis of cholesterol, which is a precursor for steroid hormones and bile salts.•Fatty acids are converted to phospholipids and to TAG’s of plasma lipoprotiens.•Some free Fatty acids become bound to albumin and are carried to heart and skeletal muscles

AMINO ACIDS

Metabolism of amino acids in liver

Amino acids entering liver follow several important metabolic routes:•Amino acids are precursors in biosynthesis of nucleotides, hormones and other nitrogenous compounds.•Amino acids may be transaminated or deaminated and degraded to yield pyruvate and citric acid cycle intermediates with various fates.•Pyruvate can be converted to glucose and glycogen via gluconeogenesis or• it can be converted to acetyl coA which has several possible metabolic fates eg. ATP synthesis and lipid synthesis.•Amino acids donate their amino groups(by transamination) to pyruvate, the product of glycolysis to yield alanine which is transported to the liver and deaminated ,resulting pyruvate is converted to blood glucose by hepatocytes.

UNDERSTANDING KEY REGULATIONS.

As we have seen in the previous slides, all the metabolic pathways are closely interwoven as they share many common substrates as well as enzymes.

Now lets move deeper into their networking by understanding the regulation mechanisms of various pathways; as in understanding how the substrates of one pathway regulate the reactions of the other.

Hence now we can have a close look upon the interconnection between the pathway on a molecular basis. As in comparing the allosteric effects.

UNDERSTANDING ALLOSTERIC REGULATIONHow one cycle substrate can influence the rate of reaction in other.

ALLOSTERIC & HORMONAL MECHANISMS ARE IMPORTANT IN THE METABOLIC CONTROL OFENZYME-CATALYZED REACTIONS A hypothetical metabolic pathway in which reactions A ↔ B and

C ↔ D are equilibrium reactions and B → C is a non equilibrium reaction . The flux through such a pathway can be regulated by the availability of substrate A. This depends on its supply from the blood, which in turn depends on either food intake or key reactions that maintain and release substrates from tissue reserves to the blood, eg, the glycogen phosphorylase in liver and hormone- sensitive lipase in adipose tissue. The flux also depends on the transport of substrate A across the cell membrane. Flux is also determined by the removal of the end product D and the availability of co-substrate or cofactors represented by X and Y. Enzymes catalyzing nonequilibrium reactions are often allostericproteins subject to the rapid actions of “feedback”or “feed-forward” control by allosteric modifiers in immediate response to the needs of the cell . Frequently, the product of a biosynthetic pathway will inhibit the enzyme catalyzing the first reaction in the pathway. Other control mechanisms depend on the action of hormones responding to the needs of the body as a whole; they may act rapidly, by altering the activity of existing enzyme molecules, or slowly, by altering the rate of enzyme synthesis.

IMPORTANT EXAMPLES CITING LINKS THROUGH REGULATION:-

Citrate, the product of the first step of TCA is an allosteric inhibitor of PFK-1 in glycolytic pathway.

AcetylCoA Carboxylase (de-phosphorylation, active) malonyl-CoA inclreases Substrate conc increase in FA Synthesis (plenty of malonyl-CoA)

And this malonyl coA acts as an inhibitor for the carnitine shuttle.

. Major pathways and regulation of gluconeogenesis and glycolysis in the liver. Entrypoints of glucogenic amino acids after transamination are indicated by arrows extended from circles.(See also Figure 16–4.) The key gluconeogenic enzymes are enclosed in double-bordered boxes. TheATP required for gluconeogenesis is supplied by the oxidation of long-chain fatty acids. Propionate isof quantitative importance only in ruminants. Arrows with wavy shafts signify allosteric effects; dashshaftedarrows, covalent modification by reversible phosphorylation. High concentrations of alanineact as a “gluconeogenic signal” by inhibiting glycolysis at the pyruvate kinase step.

CITRIC ACID CYCLEThe heart of all metabolic pathways:

To describe how ultimately TCA being a classic example of an anaplerotic reaction correlates all catabolic and anabolic pathways.

TCA AS THE HEART OF ALL METABOLIC PATHWAYSCatabolism of proteins, fats, and carbohydrates in thethree stages of cellular respiration. Stage 1: oxidation of fatty acids,glucose, and some amino acids yields acetyl-CoA. Stage 2: oxidationof acetyl groups in the citric acid cycle includes four steps in whichelectrons are abstracted. Stage 3: electrons carried by NADH andFADH2 are funneled into a chain of mitochondrial (or, in bacteria,plasma membrane–bound) electron carriers—the respiratory chain—ultimately reducing O2 to H2O. This electron flow drives the productionof ATP.

CONCLUSION.

Hence it can be clearly seen that all the metabolic pathways are closely interwoven.

Indeed it can be rightly said that a cell or an organism is actually a bioreactor with complex mechanisms which have long been developed to maintain the homeostasis in them.

And the credit for this can be given to either evolution or to God the choice for which lies entirely in your belief. But keeping that aside, you should sure take some time out to appreciate the beauty of it.

ANYQUESTIONS

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