1. What are the different classifications of Carbohydrates? Give examples and their clinical significance.

The function of carbohydrates includes energy storage and providing structure. There are three classifications of carbohydrates. The monosaccharide, disaccharides, and polysaccharides. Monosaccharide is the simplest type of carbohydrate which among other properties contains carbon, hydrogen, and oxygen, mostly in a ratio of 1:2:1. Monosaccharides can be grouped into aldoses and

ketoses. Glucose C6H12O6, is a simple sugar (monosaccharide) and an important carbohydrate in biology. Cells use it as a source of energy and a metabolic intermediate. Glucose is one of the main products of photosynthesis and starts cellular respiration.

Glucose (blood sugar, grape sugar dextrose) results from the breakdown of glycogen, a process known as glycogenolysis glucose is needed by the cells to produce energy necessary for the maintenance of life. Moreover, glucose is stored in the liver and becomes glycogen, which can be used by the body whenever it is necessary. The process of converting glucose to glycogen is termed as glycogenesis which will be discussed later on. Glucose is usually found in the blood and from there it will go inside the cells by the means of insulin. The concentration of glucose in the blood is critical to normal body function. Normal blood glucose is around 100-120 mg/dL with highest concentrations after meal. Glucose is regulated by insulin and glucagon which can be found in the islet of Langerhans. Less insulin in the body will result to an influx of serum glucose level which may lead to a condition called hyperglycemia. This condition is more likely caused by a dysfunctional pancreatic secretion of insulin which is known as Diabetes Mellitus. Moreover, if the serum glucose concentration drops too low, glucagon will be released to the system to stimulate the liver to release glucose into the

blood by the process of glycogenolysis.

Another biologically important monosaccharide is Fructose (levulose, fruit sugar), is an isomer of glucose and is the sweetest of all sugars. It is found in large amounts of honey, corn syrup, and sweet fruits. Fructose has a ketone group attached to it that is why it is classified as ketose. Fructose is being utilized by some specialized cells such as the sperm cells for its nourishment. Fructose is absorbed readily in the intestines and goes directly to the hepatic portal vein going to the liver. Inside the liver, the fructose is then converted into glycogen. An increased fructose intake in the diet may contribute to the development of non-alcoholic fatty liver disease.

Moreover, Galactose is also an isomer of glucose, which is an aldose. Galactose is found in the biological systems as a component of the dissacharide

lactose, or milk sugar. This is the principal sugar found in milk. Also, the modified form of galactose

Ketone GroupAldehyde Group

Glucose has an aldehyde group attached to the

first carbon atom

Fructose is classified as ketose(with a

ketone group attached)

which is beta-dextro-n-acetylgalactosamine is an important component of the blood group antigens (ABO). Galactosemia, a genetic disease, is caused by the absence of one or more of the enzymes needed for the conversion. Toxic compounds are formed when galactose accumulates from people who suffer from galactosemia which may lead to severe mental retardation, cataracts, and even early death.

Ribose is a component of many biologically important molecules including RNA and various coenzymes that are required by the many of the enzymes that carry out biochemical reactions in the body. The structure of it is composed of a five-carbon sugar. Also Ribose is a component of DNA, the molecule that carries the genetic information of the cells.

Disaccharides is formed when two simple sugars are joined together by a glycosidic bond. It is an ether bond that is formed by condensing two hydroxyl groups between two monosaccharides. Maltose is composed of two glucose which is connected by a glycosidic bond. Maltose or malt sugar is digested primarily in the mouth by means of salivary amylase which makes some food (which is rich in malt) sweet. Also, it can be further metabolized in the small intestines via the pancreatic enzymes amylase. Another biologically important disaccharide is Lactose which is made up of one molecule of galactose and glucose. Lactose is a principal sugar in the milk of most mammals. To be used by the body as energy, it must be hydrolyzed to produce glucose and galactose. Lactose intolerance is a condition in which the body cannot metabolize lactose due to the absence of enzyme lactase. This causes intestinal cramping, diarrhea and can even lead to dehydration.

Sucrose, also called as table sugar, cane sugar, or beet sugar, is an important carbohydrate in plants. It is water soluble however; it can’t be synthesized by animals. Sucrose is composed of glucose and fructose joined together by a glycosidic bond. A high sucrose diet may precipitate the formation of dental carries, or cavities.

Polysaccharides are made up of polymers (multiple numbers) of sugar units joined by glycosidic linkages. The usually function as structural components or as energy reserves. The most important polysaccharides are starch, glycogen, and cellulose.

Starch is a polymer composed of glucose chains that is joined by glycosidic bond. And is a significant source of carbohydrate in the diet. Starch is composed of two polysaccharide amylose and amylopectin. Both are polymers of glucose. Moreover, starch digestion begins in mouth, where the salivary amylase initiates hydrolysis of the glycosidic linkages which will yield maltose and then further metabolized by an enzyme maltase to become glucose. Glucose is then absorbed into the intestinal cells and transported to liver for storage.

Glycogen is the major glucose storage molecule in animals. The structure of glycogen is similar to that of amylopectin. Glycogen is stored in the liver and skeletal muscle. Glycogen synthesis and degradation in the liver are carefully regulated. Due to glycogen’s compact shape, it takes up little space and highly relevant for mobile animals.

Cellulose is the most abundant polysaccharide, indeed the most abundant organic molecule in the world. Cellulose is a polymer of glucose units linked by glycosidic bonds. Cellulose is a structural

component of the plant cell wall. It cannot be digested by humans. The reason is that is because humans have no enzyme for its digestion which is cellulose. Cellulase is usually found to termites, cows, and goats. In humans, cellulose from fruits and vegetables serves as fiber in the diet.

2. Discuss the ff:

a. Glycolysis

Glycolysis, also known as Embden-Meyerhof Pathway, is a pathway for carbohydrate catabolism that begins with the substrate D-Glucose. The three major products in glycosis are the chemical energy in the form of ATP, chemical energy in the form of NADH, and two three-carbon pyruvate molecules. Glycolysis can be divided into two major segments. The first is the investment of ATP energy. Without this investment, glucose would not have enough energy for glycolysis to continue, and there would be no ATP produced. This segment includes the first five reactions in the pathway. The second major segment involves the remaining reactions of the pathway, those that result in a net energy field.

Reaction 1

Glucose is phosphorylated by the enzyme hexokinase in a coupled phosphorylation reaction. The source of the phosphoryl group is ATP. The phosphoryl group will attach to the 6 th carbon of glucose and will produce glucose-6-phosphate. The cell spends ATP in order to start the reaction. The cell goes into energy debt in these early reactions but this absolutely necessary to get the pathway started.

Reaction 2

The glucose-6-phosphate formed in the first reaction is rearranged to produce the structural isomer fructose-6-phosphate. The enzyme phosphoglucose isomerase catalyzes this isomerization. The result is that the C-1 carbon of the six-carbon sugar is exposed; it is no longer part of the ring structure.

Also, examination of the open chain structures reveals that this isomerization converts aldose into ketose.

Reaction 3

The second energy investment is catalyzed by the enzyme phosphofructokinase. The phosphoanhydride bond in ATP is hydrolyzed, and a phosphoester linkage between the phosphoryl group and the C-1 hydroxyl group of fructose-6-phosphate is formed. The product is fructose-1,6-biphosphate.

Reaction 4

Fructose-1,6-biphosphate is split into two three-carbon intermediates in a reaction catalyzed by the enzyme aldolase. The products are glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate.

Reaction 5

Because G3P is the only substrate that can be used by the next enzyme in the pathway, the dihydroxyacetone phosphate is rearranged to become a second molecule of G3P. The enzyme that mediates this isomerization is triose phosphate isomerase.

Reaction 6

In this reaction, the aldehyde glyceraldehydes-3-phosphate is oxidized to a carboxylic acid in a reaction catalyzed by glyceraldehydes-3-phosphate dehydrogenase. This is the first step in glycolysis that harvests energy, and it involves the reduction of the coenzyme nicotinamide adenine dinucleotide (NAD+). This reaction occurs in two steps. First, NAD+ is reduced to NADH as the aldehyde group of glyceraldehyde-3-phosphate is oxidized to a carboxyl group. Second, an inorganic phosphate group is transferred to the carboxyl group to give 1,3-biphosphoglycerate. The bond between the phophoryl group and carboxyl group has a high-energy bond. These, and all remaining reactions of glycolysis, occur twice for each glucose because each glucose is converted into two molecules of glyceraldehyde-3-phosphate.

Reaction 7

In this reaction, energy is harvested in the form of ATP. The enzyme phosphoglycerate kinase catalyzes the transfer of the phosphoryl group of 1,3-biphosphoglycerate to ADP. This is the first substrate-level phosphorylation of glycolysis, and it produces ATP and 3-phosphoglycerate. It is coupled reaction in which the high-energy bond is hydrolyzed and the energy released is used to drive the synthesis of ATP.

Reaction 8

3-Phosphoglycerate is isomerized to produce 2-phosphoglycerate in a reaction catalyzed by the enzyme phosphoglycerate mutase. The phosphoryl group attached to the third carbon of 3-phosphoglycerate is transferred to the second carbon.

Reaction 9

In this step the enzyme enolase catalyzes the dehydration of 2-phosphoglycerate. The energy-rich products is phosphoenolpyruvate, the highest energy phosphorylated compound in metabolism.

Reaction 10

Here we see the final substrate-level phosphorylation in the pathway, which is catalyzed by pyruvate kinase. Phosphoenolpyruvate serves as a donor of the phosphoryl group that is transferred to ADP to produce ATP. This is another coupled reaction in which hydrolysis of the phosphoester bond in phosphoenolpyruvate provides energy for the formation of the phosphoanhydride bond of ATP. The final product of glycolysis is pyruvate.

It should be noted that reactions 6 to 10 occur twice per glucose molecule, because the starting six-carbon sugar is split into two three-carbon molecules. The ATP produced in glycolysis is four however, 2 is spent so that glycolysis would occur again and thus, the net energy is 2 ATP.

b. glycogenesis

The hormone insulin, produced by the pancreas in response to high blood glucose levels, stimulates the synthesis of glycogen, glycogenesis. When blood glucose rises, as after a meal, the beta cells of the pancreas excrete insulin. It immediately accelerates the uptake of glucose by all the cells of glucose is insulin-dependent. The increased uptake of glucose is especially marked in the liver, heart, skeletal muscle and adipose tissue. In the liver, insulin promotes glycogen synthesis and storage by inhibiting glycogen phosphorylase, thus inhibiting glycogen degradation. It also stimulates glycogen synthase and glucokinase, two enzymes involved in glycogen synthesis. Although glycogenesis and glycogenolysis share some reactions in common, the two pathways are not simply the reverse of one another.

The first reaction of glycogen synthesis in the liver traps glucose within the cell by phosphorylating it. In this reaction, catalyzed by the enzyme glucokinase, ATP serves as a phosphoryl donor, and glucose-6-phosphate is formed.

The second reaction of glycogenesis is the reverse of one of the reactions of glycogenolysis. The glucose-6-phosphate formed in the first step is isomerized to glucose-1-phosphate. The enzyme that catalyzes the rection is phosphoglucomutase.

The glucose-1-phosphate must now be activated before it can be added to the growing glycogen chain. The high-energy compound that accomplishes this is the nucleotide uridine triphosphate (UTP). In

this reaction, mediated by the enzyme pyrophosphorylase, the C-1 phosphoryl group of glucose is linked to the alpha-phosphoryl group of UTP to produce UDP-glucose.

This is accompanied by the release of a pyrophosphate group (PPi). The UDP –glucose can now be used to extend glycogen chains. The enzyme glycogen synthase breaks the phosphoester linkage of UDP-glucose and the growing glycogen chain. UDP is released in the process.

Finally, we must introduce the alpha (1→6) glycosidic linkages to form the branches. The branches are quite important to proper glycogen utilization. The branching enzyme removes a section of a linear alpha (1→4) linked glycogen and reattaches it in an alpha (1→6) glycosidic chain elsewhere in the chain.

c. glycogenolysis

Two hormones control glycogenolysis, the degradation of glycogen. These are glucagon, a peptide hormone synthesized in the pancreas, and epinephrine, a catecholamine released by the adrenal medulla. Glucagon is released from the pancreas in response to low blood glucose while epinephrine is released in response to a threat or a stress. Glycogen phosphorylase is an enzyme involved in glycogen degradation and is activated while glycogen synthase is inactivated.

The enzyme glycogen phosphorylase catalyzes the phosphorolysis of a glucose at one end of glycogen polymer. The involves the displacement of a glucose unit of glycogen by a phosphate group. As a result of phosphorolyis, glucose-1-phosphate is produced without using ATP as the phosphoryl group donor.

Glycogen contains many branches bound to the alpha (1→4) backbond by alpha (1→6) glycosidic bonds. These branches must be removed to allow the complete degradation of glycogen. The extensive action of glycogen phosphorylase produces a smaller polysaccharide with a single glucose bound by alpha (1→6) glycosidic bond to the main chain. The enzyme alpha (1→6) glycosidase, also called the debranching enzyme, hydrolyzes the alpha (1→6) glycosidic bond at a branch point and frees one molecule of glucose. This molecule of glucose can be phosphorylated and utilized in glycolysis, or it may be released into the bloodstream for use elsewhere. Hydrolysis of the branch bond liberates another stretch of alpha (1→4) linked glucose for the action of glycogen phosphorylase.

Glucose-1-phosphate is converted to glucose-6-phosphate by phosphoglucomutase. Glucose originally stored in glycogen enters glycolysis through the action of phosphoglucomutase. Alternatively, in the liver and kidneys it may be dephosphorylated for transport into the bloodstream.

d. gluconeogenesis

Glucose is produced by the process of gluconeogenesis, the production of glucose from noncarbohydrate starting materials. Gluconeogenesis, an anabolic pathway, occurs primarily in the liver. Lactate, all the amino acids except leucin and lysine, and glycerol from fats can all be used to make glucose. However, the aminoacids and glycerol are generally used only under starvation conditions.

At first glance, gluconeogenesis appears to be simply the reverse of glycolysis, because the intermediates of the two pathways are identical. But this is not the case, because steps 1,3, and 10 of glycolysis are irreversible, and therefore the reverse reactions must be carried out by other enzymes. In step 1 of glycolysis, hexokinase catalyzes the phosphorylation of glucose. In gluconeogenesis the dephosphorylation of glucose-6-phosphatase, which is found in the liver but not in muscle. Similarly, reaction 3, the phosphorylation of fructose-6-phosphate catalyzed by phosphofructokinase, is irreversible. That step is bypassed in gluconeogenesis by using the enzyme fructose-1,6-biphosphatase. Finally, the phosphorylation of ADP catalyzed by pyruvate kinase, step 10 of glycolysis, cannot be reversed. The conversion of pyruvate to phosphoenolpyruvate actually involves two enzymes and some unusual reactions. First, the enzyme pyruvate carboxylase adds CO2 to pyruvate. The product is the four-carbon compound oxaloacetate. Then phosphoenol pyruvate carboxylase removes the CO2 and adds a phosphoryl group. The donor of the phosphoryl group in this reaction is guanosine triphosphate (GTP). This nucleotide is like ATP, except that the nitrogenous base is guanine.

A complicated shuttle system transports the oxaloacetate produced in the mitochondria through the two mitochondrial membranes and into the cytoplasm. There, phosphoenolpyruvate caboxylase catalyzes ints conversion to phosphoenolpyruvate. Glycogenesis and glycolysis should be regulated so that the processes will not happen simultaneously or else, the cycle will be repeated and no energy will be obtained.

e. Kreb’s Cycle

Kreb’s Cycle or the citric acid cycle is the final stage of the breakdown of carbohydrates, fats, and amino acids released from dietary proteins. Acetyl CoA and oxaloacetate are the primary substrate that starts this reaction.

The citric acid cycle begins with the transfer of a two-carbon acetyl group from acetyl-CoA to the four-carbon acceptor compound (oxaloacetate) to form a six-carbon compound (citrate).

The citrate then goes through a series of chemical transformations, losing two carboxyl groups as CO2. The carbons lost as CO2 originate from what was oxaloacetate, not directly from acetyl-CoA. The carbons donated by acetyl-CoA become part of the oxaloacetate carbon backbone after the first turn of the citric acid cycle. Loss of the acetyl-CoA-donated carbons as CO2

requires several turns of the citric acid cycle. However, because of the role of the citric acid cycle in anabolism, they may not be lost, since many TCA cycle intermediates are also used as precursors for the biosynthesis of other molecules.

Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the citric acid cycle, three molecules of NADH are produced.

Electrons are also transferred to the electron acceptor Q, forming QH2. At the end of each cycle, the four-carbon oxaloacetate has been regenerated, and the cycle

continues.

f. lipolysis

Lipolysis is the breakdown of lipids. It is the hydrolysis of triglycerides into free fatty acids followed by further degradation, into acetyl units, by beta oxidation. Ketones are produced, and are found in large quantities in ketosis (a state in metabolism occurring when the liver converts fat into fatty acids and ketone bodies, which can be used by the body for energy). Lipolysis testing strips such as Ketostix are used to recognize ketosis.

The following hormones induce lipolysis: epinephrine, norepinephrine, glucagon, growth hormone, testosterone, and cortisol (although cortisol's actions are still unclear). These trigger 7TM receptors (G protein-coupled receptors), which activate adenylate cyclase. This results in increased production of cAMP, which activates protein kinase A, which subsequently activates lipases found in adipose tissue.

Triglycerides are transported through the blood to appropriate tissues (adipose, muscle, etc.) by lipoproteins such as chylomicrons. Triglycerides present on the chylomicrons undergo lipolysis by the cellular lipases of target tissues, which yields glycerol and free fatty acids. Free fatty acids released into the blood are then available for cellular uptake. Free fatty acids not immediately taken up by cells may bind to albumin for transport to surrounding tissues that require energy. Serum albumin is the major carrier of free fatty acids in the blood. The glycerol also enters the bloodstream and is absorbed by the liver or kidney where it is converted to glycerol 3-phosphate by the enzyme glycerol kinase. Hepatic glycerol 3-phosphate is converted mostly into dihydroxyacetonephosphate (DHAP) and then glyceraldehyde 3-phosphate (GA3P) to rejoin the glycolysis and gluconeogenesis pathway.

While lipolysis is triglyceride hydrolysis (the process by which triglycerides are broken down), esterification is the process by which triglycerides are formed. Esterification and lipolysis are, in essence, reversals of one another.

Lipolysis during stress occurs in the fat cells, which, in turn, increases cholestrol during chronic enduring stress.

g. lipogenesis

Much like β-oxidation, elongation occurs via four recurring reactions shown below. In these diagrams, the acetyl and malonyl units are shown as their Acyl carrier protein (ACP) thioesters: This is how fatty acids are synthesized in microorganisms and plants. However, in animals, these same reactions occur on a large dimeric protein, Fatty acid synthase, which has the full complement of enzymatic activities required to synthesize and liberate a free fatty acid.

Step Description Diagram Enzyme

Condensation

The first step is condensation of acetyl ACP and malonyl ACP. This results in the formation of acetoacetyl ACP. Although this reaction is thermodynamically unfavourable, the evolution of CO2 drives

β-Ketoacyl-ACP synthase

the reaction forward.

Reduction of acetoacetyl ACP

In this step, acetoacetyl ACP is reduced by NADPH into D-3-Hydroxybutyryl hydroxylase ACP2. The double bond is reduced to a hydroxyl group. Only the D isomer is formed.

β-Ketoacyl-ACP Reductase

Dehydration of D-β-Hydroxybutyryl ACP

In this step, Water is drawn out creating a double bond between the β and Gamma carbons

β-hydroxyacyl-ACP dehydrase

Reduction of crotonyl ACP

During this final step, crotonyl ACP is reduced by NADPH into butyryl ACP.

Enoyl ACP reductase

Butyryl ACP is then translocated in the CE site, and another malonyl-CoA is brought in the ACP site. In the second step of elongation, butyryl-CE condenses with malonyl ACP to form an acyl ACP compound. This continues until a C16 acyl compound is formed, at which point it is hydrolyzed by a thioesterase into palmitate and ACP.

The end-product of these reactions is always palmitate. No intermediates are released until palmitate is formed. After release from the ACP, palmitate is esterified to Coenzyme A, as this is done with all free fatty acids inside cells to prevent lysis of the cell membranes. If further elongation has to happen, the palmitoyl-CoA, or any other acyl-CoA from a dietary acid, then moves into the endoplasmic reticulum, where it can be elongated up to a length of 20 to 24 carbons by the same chain of reactions as happens in the cytosolic ACP.

iReferences

ihttp://en.wikipedia.org/wiki/Citric_acid_cycle http://en.wikipedia.org/wiki/Glycolysishttp://www.sportsci.org/encyc/adipose/adipose.htmlhttp://www.biochem.arizona.edu/classes/bioc460/summer/460web/lecture/metabolism/lipolysis-lipogenesis.pdfhttp://www.metabolic-database.com/html/citrate_shuttle_animation.htmlhttp://www.wisegeek.com/what-is-lipogenesis.htmhttp://www.answers.com/topic/fatty-acid-synthesis