Carbohydrates and Fiber (Book)
What are disaccharides?
Disaccharides consist of two monosaccharide units joined by covalent bonds. Within this group, sucrose, consisting of one glucose and one fructose residue, is nutritionally the most significiant, furnishing approximately 1/3 of total dietary CHO intake in an average diet. The most common of the disaccharides in the diet are maltose, lactose, and sucrose among others. Maltose=Glucose-Glucose Bond: a(1-4) glycosidic Lactose=Galactose-Galactose: b(1-4) glycosidic Sucrose=Glucose-Fructose: a for glucose, b for fruc
What are oligosaccharides?
Oligosaccharides consist of 2 to 10 monosaccharide units that are also joined by covalent bonds. The number of units is designated by tri, tetra, penta, etc. followed by saccharide. Are formed by condensation reactions to combine/join molecules.
What typically causes a molecule to be optically active?
Optical activity is attributed to the presence of one or more asymmetrical or chiral carbon atoms in the molecule. Chiral carbon atoms have four different atoms or groups covalently bonded to them.
What are some similarities and differences between starch, glycogen, and cellulose.
Starch, glycogen, and cellulose are similar in that they are polymers of repeating glucose molecules. They differ however in their structure, and types of glycosidic bonds as: -STARCH molecules are branched and unbranched with a(1-4) bond, with a(1-6) bonds occuring at branch points in the molecule amylopectin. -GLYCOGEN is even more highly branched than amylopectin, with a(1-4) and a(1-6) types of bonds. -CELLULOSE, the major component of cell walls in plants, is unique in that it has b(1-4) glycosidic bonds which render it resistant to the CHO digestive enzyme a-amylase
In nutrition, why is stereoisomerism important?
Stereoisomerism among monosaccharides, amino acids, and lipids has important metabolic implications because of the stereospecificity of certain metabolic enzymes responsible for digesting and breaking down these molecules. For example, the digestive enzyme alpha amylase can digest and breakdown starch composed of glucose units attached via alpha linkages, however in the presence of molecules of glucose attached with beta-linkages, such as in cellulose, the enzyme cannot break it down during digestion.
Explain how simple sugars link together to form oligo and polysaccharides addressing the type of bond that is formed, the atoms involved and the orientation of the bonds.
Disaccharides contain two monosaccharide units attached to one another through acetal bonds. Acetal bonds are also called glycosidic bonds, are formed between a hydroxyl group of one monosaccharide unit and a hydroxyl group of a second monosaccharide unit, with the elimination of one molecule of water (Condensation reaction). The glycosidic bonds generally involve the hydroxyl group on the anomeric carbon of one member of the pair of monosaccharides and the hydroxyl group on the carbon 4 or 6 of the second member. Furthermore, the glycosidic bond can be alpha or beta in orientation, depending on whether the anomeric hydroxyl group was alpha or beta before the bond was formed, and on the specificity of the enzymatic reaction catalyzing their formation. -Specific glycosidic bonds therefore may be designated alpha 1-4 or a(1-4), b(1-4).
Explain what the D and L designation of monosaccharides meant initially and what they presently mean.
Enantiomers, are a type of stereoisomer or molecules that are said to be mirror images of one another, and are not superimposable. Enantiomers exist in D or L orientation, and if a compound is structurally D, its enantiomer is L. The D or L designation does not predict the direction of rotation of plane polarized light, but rather is simply a structural analogy to the reference compound glyceraldehyde. Within glyceraldehyde, if the -OH group on the chiral carbon is facing right, it is then in the D configuration. If the -OH group on the chiral carbon is facing the left, it is then in the L configuration. In molecules with more than one chiral center, the highest numbered Chiral carbon is referenced with D and L configuration.
What are the six major metabolic pathways for carbohydrate metabolism?
-glycogenesis: The synthesis of glucose (Cytoplasm) -glycogenolysis: the breakdown of glucose -glycolysis: the oxidation of glucose -gluconeogenesis: the production of glucose from noncarbohydrate intermediates (AAs, FAs) -Pentose phosphate pathway: (Hexosemonophosphate shunt): The production of five-carbon (5C) monosaccharides and nicotinamide adenine dinucleotide phsphate (NADPH) Tricarboxylic Acid or TCA Cycle: The oxidation of pyruvate and acetyl-CoA to CO2 and H20 products.
What is the primary function for Glucose Transporter or GLUT1?
GLUT1 is responsible for the basic supply of glucose to erythrocytes, endothelial cells of the brain, and most fetal tissue.
How does the process of glycolysis occur? What steps are involved? Where is it located?
Glycolysis is the major pathway by which glucose is degraded or broken down into two 3-C units, pyruvate. From pyruvate, the metaboliuc course depends on the availability of the reducing units in the cytosol, which further depends on the availability of oxygen in the cell (aerobic metabolism). Glycolysis can function under both anaerobic and aerobic conditions. In anaerobic conditions, pyruvate (3C) is immediately converted to lactate in the muscle cells. This allows still some glucose to be produced in times of oxygen debt, as the lactate will travel to the liver to be converted back into glucose. Many tissues obtain their energy from solely glycolysis including the cells of the brain and GI tract as well as erythrocytes.
How does glucose cross the plasma membrane of the wide variety of tissues?
In certain absorptive cells, such as epithelial cells of the small intestine, and renal tubule, glucose crosses the plasma membrane actively against a concentration gradient, and is pumped by a NA+/K+ ATPase symport system known as SGLT1. However, glucose is admitted to nearly all cells in the body by a carrier mediated transport mechanism that does not require energy, where the specific protein carriers involved is called glucose transporter molecules known as GLUT.
Explain optical activity, in terms of dextrorotatory, levorotatory, chirality.
Many organic substances, including CHO, are optically active in that if a plane-polarized light is passed through a solution of the substances, the plane of light is rotated to the right (R) for dextrorotatory substances or to the left for levorotatory substances. The direction and extent of the rotation are characteristic of a particular compound and are dependent on the substance's concentration, temperature, and the wavelength of light passing through it. R or L direction of light rotation is often expressed as + for right and - for left rotation.
What are monosaccharides?
Monosaccharides are structurally the simplest form of carbohydrate in that they cannot be reduced in size to smaller CHO units via hydrolysis. They are called simple sugars. The most abundant monosaccharide in nature is glucose. Other examples include fructose, and galactose.
Explain the initial process of CHO digestion including where it begins, the primary site where it takes place, the enzymes involved, and the final products.
The digestion of polysaccharides starts in the mouth, with the activation of salivary a-amylase, a glycosidase that specifically hydrolyzes a(1-4) glycosidic linkages. This phase produces few mono-or-disaccharides due do the fast transit time. The salivary amylase continues to act in the stomach until gastric acid penetrates the food bolus and lowers the pH sufficiently to inactivate the enzyme.
Explain the structural difference between amylopectin and amylose.
The most common digestible polysaccharide in plants is starch, which occurs naturally in two forms, amylose and amylopectin, which are both polymers of D-Glucose. -Amylose: Linear, unbranched chain Glucose attached with a(1-4) glycosidic bond (Takes a helical confirmation in water) -Amylopectin: branched chain polymer of glucose, with branch points occuring through a(1-6) bonds, and a(1-4) glycosidic bonds at non-branch points Amylose contributes to 15 to 20% of starch intake while amylopectin contributes to 80 to 85%
What is the difference between liver phosphorylase and muscle phosphorylase?
The muscle and liver forms of phosphorylase are isozymes.
What is glycogenesis? Where does it occur? What are its products?
The term glycogenesis refers to the pathway by which glucose ultimately is converted into its storage form glycogen which is utilized as the body's own reservoir for quick energy response. This process mainly occurs in the liver, and the glycogen produced by the process are stored in both the liver and in the body's skeletal muscle. Muscle stores account for approximately 75% of the body's glycogen stores. Glycogen stores in a muscle are for that muscle only and cannot be released back into circulation.
List and Define the three major classifications of Carbohydrates
Three major classifications include: -Monosaccharides -Oligosaccharides -Polysaccharides
What occurs in aerobic glycolysis?
Under aerobic conditions, pyruvate (3C) can be transported into the mitochondria and participate in the TCA cycle, in which it becomes completely oxidized to CO2 and H20. Complete oxidation results in the release of a large amount of energy much of which is later salvaged as ATP by the mechanism of oxidative phosphorylation in the electron transport chain of the cell.
How are disaccharides digested in the small intestine?
Virtually no digestion of disaccharides or small oligosaccharides occurs in the mouth, stomach or lumen of the small intestine. Digestion takes place almost ENTIRELY within the MICROVILLI or the brush border of the upper SI, after broken down by disaccharidase activity. The resulting monosaccharides immediately enter the enterocytes with the facilitatiton of specific transporters. Some enzymes responsible for digestion of CHO in the enterocyte include lactase, sucrase, maltase, isomaltase, and trehalase.
Where do the monosaccharides glucose, galactose and fructose go after absorption once they have reached the portal vein?
Following transport of glucose, galactose and fructose across the wall of the intestine, they enter the portal circulation, where they are carried directly to the liver for being metabolized. The liver is the major site of metabolism of galactose and fructose which are readily taken up by the liver, and enter the liver cells via facilitated transport and subsequently are metabolized. Both can be converted to glucose derivatives, and once they are, the have the same fate as glucose and can be stored as liver glycogen, returned to the blood stream to maintain circulating glucose levels, or catabolized for energy according to the liver's energy demand.
What is the primary function for GLUT2?
GLUT2 is a low-affinity, high-capacity transporter with preodominant expression in the beta cells of the pancreas, liver, small intestine, and kidney. GLUT2 is involved in the transport of glucose and fructose from enterocytes into the portal blood supply. ITS RATE OF TRANSPORT IS HIGHLY DEPENDENT ON BLOOD GLUCOSE CONCENTRATION. An important fact is that GLUT2 appears to be the sensitive indicator of blood glucose levels and is involved in the release of insulin from the beta cells of the pancreas.
What is the primary function of GLUT3?
GLUT3 is a high-affinity glucose transporter with predominant expression in those tissues that are highly dependent on glucose for functioning, such as the brain and neurons. It is also expressed in cells and tissues that have a high requirement for glucose.
What is the primary function of GLUT4?
GLUT4 is the primary means by which insulin is responsible for the cellular uptake of glucose in muscle and adipose tissues. Other cells and tissues such as the liver, kidneys, erythrocytes and brain do not express GLUT4 and thereofre are not dependent upon insulin for glucose uptake into their cells.
What is the primary function of GLUT5?
GLUT5 is specific for the transport of fructose and will not transport glucose. It is expressed primarily in the small intestine, but to a lesser degree in the kidney, brain, muscle, and adipose tissue.
How are the mono-and-disaccharides absorbed once at the brush border?
Glucose is absorbed into the intestinal mucosa cell by various pathways including active transport, and facilitated transport. One of the main glucose transporter molecules which is involved with the process of active transport and uptake of glucose into the cell is a glucose-galactose receptor sodium-glucose transporter 1 (SGLT1) which simltaneously transports two substances sodium and glucose (or galactose) in the same direction into the cell, and thus is classified as a symporter. This process requires ATP. However, all glucose absorption is not dependent upon SGLT1, and at times of high glucose concentration, such as after a large CHO meal, glucose is transported into the enterocyte by way of facilitated transporter type 2 (GLUT2) which also transports glucose, galactose and fructose out of the enterocyte and is located at the basolateral membrane. After a high-carbohydrate meal, more glucose and fructose is transported into the enterocyte by facilitate transport (GLUT2) than by active transport using SGLT1.
Explain the process occuring within glycogenolysis?
Glycogenolysis refers to the breakdown of glycogen into individual glucose subunits, in the form of glucose-1-phosphate and is catalyzed by the enzyme, PHOSPHORYLASE. Although glycogen phosphorylase cleaves at a(1-4) glycosidic bonds, it cannot and will not hydrolyze a(1-6) bonds. At times of heightened glycogenolytic activity, the formation of increased concentrations of glucose-1-phosphate molecules shifts the glucose phosphate isomerase reaction towards production of glucose-6-phosphate. The glucose-6-P can enter the oxidative pathway for glucose, also known as glycolysis, or become free glucose (in the bloodstream) in the liver and kidneys only. The essential conversion of glucose-6-phosphate to free glucose requires the enzyme glucose-6-phosphatase to act. As the enzyme is only present in the liver or kidneys, and not in muscle cells, free glucose can only be formed from the glycogen in the liver or kidneys and transported through the bloodstreatm from those tissues to other tissues for oxidation reactions (glycolysis).
What are some characteristics of monosaccharides?
Have a functional carbonyl group -Have Optical Activity, dextrotatory (R)+, Levorotatory (L)- -Chiral Carbons -Enantiomers
What is the difference between homopolysaccharide and a heteropolysaccharide?
Homopolysaccharides are high molecular weight polymers, and if the structure is complosed of a single type of monomeric unit, it is called a homopolysaccharide. If it is composed of two or more different types of monosaccharides making up its structure, it is called a heteropolysaccharide. Homopolysaccharides are of far greater importance in nutrition because of their abundance in many forms of natural foods.
What is the meaning of alpha and beta configurations?
In Fisher Projections of the molecules, if the anomeric carbons are arbitrarily positioned to the right, the molecule is in alpha configuration -If the anomeric carbon is positioned to the left, it is said to be in a beta configuration.
Discuss cyclization of monosaccharides and the meaning of the alpha and beta designations.
In a solution, the monosaccharides do not exist in an open chain form, they instead form a cyclic ring structure through a reaction between the carbonyl group and the hydroxyl group. The formation of a cyclic structure forms an additional chiral carbon. The participating groups are the akdegtde ir jetibe if the anomeric carbon and the alcohol group attached to the highest numbered chiral carbon atom.
Explain the metabolic process and reaction occuring in glycogenesis?
In the initial part of the glycogenic pathway, glucose is first phosphorylated upon entering the cell in the cytoplasmic matrix-> producing glucose-6-phosphate via the enzyme hexokinase. This reaction requires 1 ATP to occur. Glucose phosphoylation in the liver occurs via the enzyme glucokinase. Insulin has a postive effect in promoting the activity of phosphoylation of glucose by glucokinase. The next step is the relocation of the phosphate group from the 6-Carbon molecule to the 1-Carbon, making it Glucose-1-Phosphate. Next, energy derived from hydrolysis of UTP to UMP allows the UMP to be coupled to the glucose-1-phosphate molecule to form UDP-glucose. Glucose is incorporated to glycogen in the form of UDP-glucose. The reaction is catalyzed by the enzyme glycogen synthase. The incoming UDP-glucose is then attached and incorperated into a preformed glycogen chain which has been primed by glycogenin. The initial glycogen is formed by binding a glucose residue to a tyrosine residue of a protein called glycogenin. Insulin facilitates the action of glycogen synthase by stimulating the dephosphorylation of glycogen synthase in the liver. This is the primary target of insulin's stimulatory effet on glycogenesis in the liver cell.
List the major hormones involved in CHO regulation, mechanism of action, and the effect of each on blood glucose levels.
Insulin stimulates the uptake of glucose, amino acids, and lipids which leads to their conversion to storage forms in muscle and adipose tissue. The storage form for glucose after insulin response in the body is glycogen which is synthesized through glycogenesis. Insulin increases glyogenesis by promoting the activity of glucokinase in the liver cells, and also by increasing the activity of glycogen synthase which adds UDP-Glucose into glycogen chains. Glucagon, the primary catabolic hormone, has the opposite effects by increasing the breakdown of liver glycogen, a process called glycogenolysis, and lipid stored in adipose tissue and inhibiting the synthesis of proteins. Epinepherine also has an antagonistic effect on insulin as it promotes glycogenolysis through inactivation of phosphorylase a and insulin insensitivity in cells.
How are monosaccharides in D form different from their L form?
Monosaccharides in their D configuration are typically much more important nutritionally than their L configuration, which is due to the fact that those sugars are specifically metabolized exclusively in the D form as the enzymes involved in CHO digestion and metabolism are stereospecific for D sugars typically. These enzymes are thus inactive for L sugars most often.
Explain the role of insulin in the uptake of glucose at the liver, adipose, and muscle cell.
One role of insluin is inhibition of synthesis of glucose by the liver, a process known as gluconeogenesis. Another role is to bind with specific insulin receptors on the cell membrane, which causes the GSV to trans-locate to the cell membrane. The essential part needed to allow insulin to bind to the receptor site on the cell membranes of skeletal muscle, cardiac muscle, or adipose tissue cells are the activation of PI3-kinase and the cascading reactions that follow. The net result of insulin's effects on the cell membrane is to cause translocation of GLUT4 to the cell's membrane, allowing it to help shuttle glucose into the cell. When the cell is insulin insensitive or when blood glucose levels are low, GLUT4 remains in the GSV or glucose storage vesicle within the cell.
What are polysaccharides?
Polysaccharides are long chains of monosaccharide units that may number from several into the hundreds or thousands. The major polysaccharide of interest is glycogen, found in animal tissues, and starch and cellulose of plant tissues. All of these types of polysaccharides only consist of glucose subunits.
How is glycogenolysis regulated through allosteric regulation?
The allosteric activation of phosphorylase b is carried out by AMP to convert it to the active phosphorylase a. When energy levels are low, cellular ATP has been hydrolyzed to AMP, more energy is needed, and the phosphorylase a releases glucose-1-phosphate. The AMP binds to an allosteric site on the phosphorylase b which increases the binding of the glycogen. This allosteric site can also bind ATP, which is an allosteric inhibitor of the enzyme. Glucose-6-phosphate and caffeine are also allosteric inhibitors of the enzyme.
How is glycogenolysis regulated through covalent regulation?
The process of glycogenolysis is highly regulated. Its catalyzing enzyme, phosphorylase is regulated in the body by both covalent and allosteric mechanisms. As phosphorylase is activated for glycogen phosphorylation, glycogen synthetase is appropriately thus inhibited. COVALENT REGULATION: Enhanced by glucagon and the catecholamines, epinepherine and norepinepherine. These hormones react by causing a covalent modification of the enzyme phosphorylase by converting it to an active form through the second messenger cAMP. These hormones bind to a receptor on the cell membrane that causes adenyl cyclase to be activated to produce cAMP. The cAMP causes inactive phosphrylase kinase to become active via phosphorylating it. The active phosphrylate kinase plus ATP converts inactive phosphorylase to active phosphorylase in a cascade reaction.
What is the difference between the reducing end of polysaccharide molecule and the non-reducing end?
The reducing end of a polysaccharide molecule refers to the hemiacetal end (Aldehyde/Ketone), while the non-reducing end refers to the end where no hemiacetal (Aldehyde/Ketone) is present
What happens to glucose once it has been absorbed into the blood stream following intake?
The remainder of glucose which has not been converted to liver glycogen passes into the systemic blood supply to circulate through the rest of the body. It is then distributed among tissues, and glucose enters these cells by facilitated transport, where in skeletal muscle and adipose tissue, the process is INSULIN DEPENDENT, whereas its uptake into the liver, kidney, brain, and other tissues is INSULIN INDEPENDENT and has an uptake of glucose whether insulin is present or absent.
Explain the digestion of carbohydrates starting in the duodenum.
The starches move from the stomach into the duodenum and jejunum, where they are acted upon by PANCREATIC A-AMYLASE and further broken down by hydrolizing the a(1-4) glycosidic bonds in both amylase and amylopectin to produce oligosaccharides maltose, and maltotriose. The partially hydrolyzed amylopectin is not fully digested by a-amylase, as it stops several residues short of the a(1-6) bond, however the oligosaccharides are then acted on by debranching enzymes, and broken down again until it becomes isomaltose a three-unit triose with one a(1-6) glycosidic bond. Isolmaltose is then hydrolyzed by a(1-6) glycosidase or isomaltase which is located in the brush border and is the only glycosidase capable of hydrolyzing a(1-6) glycosidic linkages.
