Carbs 1 and 2

• In plants, glucose is synthesized from carbon dioxide and water by photosynthesis and stored as starch or used to synthesize the cellulose of the plant cell walls.
• Glycogen storage diseases are inborn - errors of metabolism.
• Glycogenesis is stimulated when glucose and energy levels are high, whereas glycogenolysis is increased when glucose and energy levels are low.
• Animals can synthesize carbohydrates from amino acids, but most are derived ultimately from plants.
• Carbohydrates are major constituents of animal food and animal tissues.
• Carbohydrates are characterized by the type and number of monosaccharide residues in their molecules.
• Diseases associated with carbohydrate metabolism include diabetes mellitus, galactosemia, glycogen storage diseases, and lactose intolerance.
• Glucose is the most important carbohydrate; most dietary carbohydrate is absorbed into the bloodstream as glucose formed by hydrolysis of dietary starch and disaccharides, and other sugars are converted to glucose in the liver.
• Glucose is the major metabolic fuel of mammals (except ruminants) and a universal fuel of the fetus.
• Glucose is the precursor for synthesis of all the other carbohydrates in the body, including glycogen for storage; ribose and deoxyribose in nucleic acids; galactose for synthesis of lactose in milk, in glycolipids, and in combination with protein in glycoproteins and proteoglycans.
• Monosaccharides are sugars that cannot be hydrolyzed into simpler carbohydrates.
• Monosaccharides may be classified as trioses, tetroses, pentoses, hexoses, or heptoses, depending upon the number of carbon atoms (3 - 7).
• Monosaccharides may be classified as aldoses or ketoses, depending on whether they have an aldehyde or ketone group.
• The polyhydric alcohols (sugar alcohols or polyols), in which the aldehyde or ketone group has been reduced to an alcohol group, also occur naturally in foods.
• Disaccharides are condensation products of two monosaccharide units, for example, lactose, maltose and sucrose.
• Oligosaccharides are condensation products of three to ten monosaccharides.
• Polysaccharides are condensation products of more than ten monosaccharide units; classified into two classes: Homopolysaccharides, made up of one type of monosaccharide units, and Heteropolysaccharides, made up of two or more types of monosaccharide units.
• The enzyme responsible for making the α(1→4) linkages in glycogen is glycogen synthase.
• The hydrolysis is exergonic, which ensures that the UDP-glucose pyrophosphorylase reaction proceeds in the direction of UDP-glucose production.
• Glycogen synthase catalyzes the α(1→4) linkages in glycogen.
• Elongation of a glycogen chain involves the transfer of glucose from UDP-glucose to the nonreducing end of the growing chain, forming a new glycosidic bond between the anomeric hydroxyl group of carbon 1 of the activated glucose and carbon 4 of the accepting glucosyl residue.
• Pyrophosphate (PPi), the second product of the reaction, is hydrolyzed to two inorganic phosphates (Pi) by pyrophosphatase.
• The UDP released when the new α(1→4) glycosidic bond is made can be phosphorylated to UTP by nucleoside diphosphate kinase [UDP + ATP UTP + ADP].
• Glycogenin then catalyzes the transfer of at least four molecules of glucose from UDP-glucose, producing a short, α(1→4) - linked glucosyl chain.
• This enzyme cannot initiate chain synthesis using free glucose as an acceptor of a molecule of glucose from UDP-glucose.
• The side-chain hydroxyl group of tyrosine-194 in the protein is the site at which the initial glucosyl unit is attached.
• This short chain serves as a primer that is able to be elongated by glycogen synthase, which is recruited by glycogenin, as described in C.
• UDP-glucose is synthesized from glucose 1-phosphate and UTP by UDP-glucose pyrophosphorylase.
• Instead, it can only elongate already existing chains of glucose and, therefore, requires a primer.
• In the absence of a fragment, the homodimeric protein glycogenin can serve as an acceptor of glucose from UDP-glucose.
• Glucose 1-phosphate is generated from glucose 6-phosphate by phosphoglucomutase.
• A fragment of glycogen can serve as a primer.
• Glucose 1,6-bisphosphate is an obligatory intermediate in this reversible reaction.
• Branching in glycogen occurs by the action of the branching enzyme, amylo transglycosylase or branching enzyme.
• α-D-Glucose attached to uridine diphosphate (UDP) is the source of all the glucosyl residues that are added to the growing glycogen molecule.
• The physiologically important monosaccharides include glucose, the "blood sugar," and ribose, an important constituent of nucleotides and nucleic acids.
• This enzyme removes a set of six to eight glucosyl residues from the nonreducing end of the glycogen chain, breaking an α(1→4) bond to another residue on the chain, and attaches it to a nonterminal glucosyl residue by an α(1→6) linkage, thus functioning as a 4:6 transferase.
• In skeletal muscle, glycogenolysis occurs during active exercise, and glycogenesis begins as soon as the muscle is again at rest.
• The remaining glucose residue attached in an α(1→6) linkage is removed hydrolytically by amylo-glucosidase activity, releasing free (nonphosphorylated) glucose.
• Glycogen phosphorylase sequentially cleaves the α(1→4) glycosidic bonds between the glucosyl residues at the nonreducing ends of the glycogen chains by simple phosphorolysis (producing glucose 1 - phosphate) until four glucosyl units remain on each chain at a branch point.