Carbohydrates are good source of energy and a long chain of sugar. It consist of three classes that is monosaccharides, disaccharides and polysaccharides.
MONOSACCHARIDES
Structure of monosaccharides:
MONOSACCHARIDES
- The simplest unit of carbohydrates and the simplest form of sugar.
- The building blocks of more complex carbohydrates that is disaccharides and polysaccharides.
- Usually colourless, can dissolve in water, and have the appearance of crystal-like substance.
- Example: Glucose, Fructose.
- Formula: (CH20)n , n ≥ 3
Structure of monosaccharides:
Monosaccharides are classified as well based on their functional group. A functional group are categorized by atoms or bonds that are responsible for the chemical activity within the molecule. The functional group include: Ketose and aldose.
Ketose Group:
Ketose Group:
- If a monosaccharides contain a ketone group in the inner atom, then the monosaccharides are classified as ketone group.
- Ketone group is a carbon atom forming double bond with the oxygen atom and single bond with the hydrocarbon groups ( group that contain carbon bonded with hydrogen).
- Below is the general appearance of ketose group:
Aldose Group:
- Monosaccharides that contain aldehyde group at an end carbon.
- Aldehyde group is the carbon atom forming a double bond with the oxygen atom and single bond with hydrogen.
- Below is the general appearance of aldose group.
DISACCHARIDES
- Also called double sugar.
- Composed of two monosaccharides (simple sugar) linked to each other.
- Crystalline water-soluble compound.
- The monosaccharides linked within them by gycosidic bond.
- The monosaccharides joined by the process of dehydration synthesis as shown below:
- Disaccharides also know as complex sugar.
- The sugar are created by through dehydration synthesis and broken down by hydrolysis.
- The common disaccharides are sucrose, lactose and maltose.
Structure of Disaccharides:
POLYSACCHARIDES
- A long chain of monosaccharides that linked by glycosidic bond.
- Typical polysaccharides is between 200 and 2500 monosaccharides long.
- Can be either linear or branch carbon chain.
- Usual structure of polysaccharides consist of 6 carbon monosaccharides linked by oxygen.
- Formula: ( C6H10O5)n, n > 40.
- Example of polysaccharides: Amylose, Glycogen.
- Can be used as energy storage.
- Insoluble in water and not in crystalline form.
- Structure of polysaccharide shown as below:
properties of carbohydrates
General properties of carbohydrates:
Physical properties of carbohydrates:
Chemical properties of carbohydrates:
- Carbohydrates act as energy reserves, also stores fuels, and metabolite intermediate.
- Ribose and deoxyribose form the structural frame of genetic material, RNA and DNA.
- Structural element in the cell wall of bacteria and plant.
Physical properties of carbohydrates:
- Steroisomerism - Compound shaving same structural formula but they differ in spatial configuration. Example: Glucose has two isomers with respect to penultimate carbon atom. They are D-glucose and L-glucose.
- Optical Activity - It is the rotation of plane polarized light forming (+) glucose and (-) glucose.
- Diastereo isomeers - It the configurational changes with regard to C2, C3, or C4 in glucose. Example: Mannose, galactose.
- Annomerism - It is the spatial configuration with respect to the first carbon atom in aldoses and second carbon atom in ketoses.
Chemical properties of carbohydrates:
- Ozazone formation with phenylhydrazine.
- Benedicts test.
- Oxidation
- Reduction to alcohols
CARBOHYDRATE METABOLISM.
Overview:
- Begin with digestion in the small intestine. (monosaccharide absorb into stream).
- Blodd sugar level is controlled by insulin (secreted by pancreas), glucagon, epinephrine.
- If blood sugar level increase, pancrease secrete insulin to stimulate the transfer of glucose into the cells, especially liver and muscle.
In Liver and Muscle:
- Most glucose convert into glycogen (glucogenesis proses).
- Glycogen is stored in liver and muscles until needed some time later.
- If blood sugar level decrease, glucagon and epinephrine release to convert glycogen to glucose. ( glycogenesis process)
In carbohydrates metabolism there is 4 process:
- Glycolysis
- Gluconeogenesis.
- Glycogenesis
- Glycogenolysis.
Glycolysis.
Glycolysis (Figure 8.3), which consists of 10 reactions, occurs in two stages:
The glycolytic pathway can be summed up in the following equation:
D-Glucose 2 ADP 2 Pi 2 NAD → 2 pyruvate 2 ATP
2 NADH 2H 2H2O
- Glucose is phosphorylated twice and cleaved to form two molecules of glyceraldehyde-3-phosphate (G-3-P). The two ATP molecules consumed during this stage are like an investment, because this stage creates the actual substrates for oxidation in a form that is trapped inside the cell.
- Glyceraldehyde-3-phosphate is converted to pyruvate. Four ATP and two NADH molecules are produced. Because two ATP were consumed in stage 1, the net production of ATP per glucose molecule is 2.
The glycolytic pathway can be summed up in the following equation:
D-Glucose 2 ADP 2 Pi 2 NAD → 2 pyruvate 2 ATP
2 NADH 2H 2H2O
![Picture](/uploads/6/1/0/3/61038489/560422_orig.png)
Glycolysis Pathway:
The Reactions of the Glycolytic Pathway:
- Synthesis of glucose-6-phosphate. Immediately after entering a cell, glucose and other sugar molecules are phosphorylated. Phosphorylation prevents transport of glucose out of the cell and increases the reactivity of the oxygen in the resulting phosphate ester. Several enzymes, called the hexokinases, catalyze the phosphorylation of hexoses in all cells in the body. ATP, a cosubstrate in the reaction, is complexed with Mg2. (ATP-Mg2 complexes are common in kinase-catalyzed reactions.) Under intracellular conditions the reaction is irreversible; that is, the enzyme has no ability to retain or accommodate the product of the reaction in its active site, regardless of the concentration of G-6-P.
- Conversion of glucose-6-phosphate to fructose-6-phosphate. During reaction 2 of glycolysis, the open chain form of the aldose glucose-6-phosphate is converted to the open chain form of the ketose fructose-6-phosphate by phosphoglucose isomerase (PGI) in a readily reversible reaction.
- The phosphorylation of fructose-6-phosphate. Phosphofructokinase-1 (PFK-1) irreversibly catalyzes the phosphorylation of fructose-6-phosphate to form fructose-1,6-bisphosphate.
- Cleavage of fructose-1,6-bisphosphate. Stage 1 of glycolysis ends with the cleavage of fructose-1,6 bisphosphate into two three-carbon molecules: glyceraldehyde-3-phosphate (G-3-P) and dihydroxyacetone phosphate (DHAP). This reaction is an aldol cleavage, hence the name of the enzyme: aldolase. Aldol cleavages are the reverse of aldol condensations, described on p. xxx. In aldol cleavages an aldehyde and a ketone are products.
- The interconversion of glyceraldehyde-3-phosphate and dihydroxyacetone
phosphate. Of the two products of the aldolase reaction, only G-3-P serves as a substrate for the next reaction in glycolysis. To prevent the loss of the other three-carbon unit from the glycolytic pathway, triose phosphate isomerase catalyzes the reversible conversion of DHAP to G-3-P. - Oxidation of glyceraldehyde-3-phosphate. During reaction 6 of glycolysis, G-3-P undergoes oxidation and phosphorylation. The product,glycerate-1,3-bisphosphate, contains a high-energy phosphoanhydride
bond, which may be used in the next reaction to generate ATP. - Phosphoryl group transfer. In this reaction ATP is synthesized as phosphoglycerate kinase catalyzes the transfer of the high-energy phosphoryl group of glycerate-1,3-bisphosphate to ADP.
- The interconversion of 3-phosphoglycerate and 2-phosphoglycerate. Glycerate-3-phosphate has a low phosphoryl group transfer potential. As such, it is a poor candidate for further ATP synthesis (G for ATP synthesis is –30.5 kJ/mol). Cells convert glycerate-3-phosphate with its energy-poor phosphate ester to phosphoenolpyruvate (PEP), which has an exceptionally high phosphoryl group transfer potential. (The standard free energies of hydrolysis of glycerate-3- phosphate and PEP are 12.6 and 61.9 kJ/mol, respectively.) In the first step in this conversion (reaction 8), phosphoglycerate mutase catalyzes the conversion of a C-3 phosphorylated compound to a C-2 phosphorylated compound through a two-step addition/elimination cycle.
- Dehydration of 2-phosphoglycerate. Enolase catalyzes the dehydration of glycerate-2-phosphate to form PEP. PEP has a higher phosphoryl group transfer potential than does glycerate-2- phosphate because it contains an enol-phosphate group instead of a simple phosphate ester. The reason for this difference is made apparent in the next reaction.
- Synthesis of pyruvate. In the final reaction of glycolysis, pyruvate kinase catalyzes the transfer of a phosphoryl group from PEP to ADP. Two molecules of ATP are formed for each molecule of glucose.
Gluconeogenesis.
- Gluconeogenesis, the formation of new glucose molecules from noncarbohydrate precursors, occurs primarily in the liver.
- Precursor molecules include lactate, pyruvate, glycerol, and certain -keto acids (molecules derived from amino acids).
- Under certain conditions (i.e., metabolic acidosis or starvation) the kidney can make small amounts of new glucose. Between meals adequate blood glucose levels are maintained by the hydrolysis of liver glycogen.
- When liver glycogen is depleted (e.g., owing to prolonged fasting or vigorous exercise), the gluconeogenesis pathway provides the body with adequate glucose.
- Brain and red blood cells rely exclusively on glucose as their energy source.
Glycolysis and Glucogenesis.
Glycogenesis.
- Glycogen is the storage form of glucose.
- The synthesis and degradation of glycogen are carefully regulated so that sufficient glucose is available for the body’s energy needs.
- Both glycogenesis and glycogenolysis are controlled primarily by three hormones: insulin, glucagon, and epinephrine.
Reaction n glycogenesis:
- Synthesis of glucose-1-phosphate. Glucose-6-phosphate is reversibly converted to glucose-1-phosphate by phosphoglucomutase, an enzyme that contains a phosphoryl group attached to a reactive serine residue.
- Synthesis of UDP-glucose. Glycosidic bond formation is an endergonic process. Derivatizing the sugar with a good leaving group provides the driving force for most sugar transfer reactions. For this reason, sugar-nucleotide synthesis is a common reaction preceding sugar transfer and polymerization processes. Uridine diphosphate glucose (UDP-glucose) is more reactive than glucose and is held more securely in the active site of the enzymes catalyzing transfer reactions.
- Synthesis of glycogen from UDP-glucose. The formation of glycogen from UDP-glucose requires two enzymes: (a) glycogen synthase, which catalyzes the transfer of the glucosyl group of UDP-glucose to the nonreducing ends of glycogen, and (b) amylo-(1,4 →1,6)-glucosyl transferase (branching enzyme), which creates the (1,6) linkages for branches in the molecule.
Glycogenolysis.
Glycogen degradation requires the following two reactions.
- Removal of glucose from the nonreducing ends of glycogen. Glycogen phosphorylase uses inorganic phosphate (Pi) to cleave the (1,4) linkages on the outer branches of glycogen to yield glucose-1-phosphate. Glycogen phosphorylase stops when it comes within four glucose residues of a branch point . (A glycogen molecule that has been degraded to its branch points is called a limit dextrin.)
- Hydrolysis of the a(1,6) glycosidic bonds at branch points of glycogen. Amylo-(1,6)-glucosidase, also called debranching enzyme, begins the removal of (1,6) branch points by transferring the outer three of the four