2Several nucleotides can activate sugars Several nucleotides can activate sugars. In the case of glucose for glycogen synthesis it is UTP. See next pageThe first reaction a), is reversible but it is coupled to b) that releases -5 kcal/mol and favors reaction a).
3Sugar nucleotides of glucose or of any other carbohydrate are called activated because sugars bound to nucleotides can be transferred by specific enzymes to proteins and other molecules or can be subjected to enzymatic modifications possible only with activated sugars.UDP-glucuronic is important for detoxification of many drugs and metabolites. Below is shown the formula of conjugated bilirubin:We, primates, lost the ability to make ascorbate. Therefore we are genetically deficient.
5Galactose metabolismFructose is an ketose, therefore itis not a substrate of aldosereductase and causes no cataracts.Glucose and galactose are aldosesand they do cause cataractsDeficiency of galactokinase causes a mild form of galactosemia (galactose deficiency) that causes cataracts.Deficiency of hexose-1-phosphate-uridylyl transferase causes the sever type of galactosemia with liver failure, mental retardation and cataracts.The name galactose-1-phosphate-uridylyl transferase is simply wrong but commonly used including in the NBE. The correct name is hexose-1-phosphate uridylyl transferase. This is so because it transfers equally well glucose as it does galactose. So, the name in the fig is not in agreement with the commission for enzyme nomenclature but it is used in most books and in the NBE.The 4 epimerase uses NAD+ as cofactor and the transition state is 4 keto hexose.Why a deficiency of the 4 epimerase was never reported (as far as I know)?Unrelated to this topic, in glycosaminoglycans a 5 epimerase transforms beta-D- glucuronyl into alpha-L-iduronic. Do you remember that, actually it is “cute” but not too important for medical practice.
6In the next pages we will address some additionalaspects of these issues
7Excess of glucose can deplete NADPH+H+ and cause opacity of the lens (cataracts). Conversion of glucose to sorbitol with ensuing osmotic disturbance. This occurs through the "polyol pathway", a normal reaction sequence in testes but not other tissues. In the first step, Glucose is converted to sorbitol by aldose reductase. The sorbitol formed is then oxidized to fructose.A problem arises when sorbitol production occurs in tissues other than testes. At high glucose levels, aldose reductase activity occurs in other organs which often lack sorbitol dehydrogenase. This leads to an accumulation of sorbitol in these tissues, notably in the lens of the eye, leading to cataract formation due to osmotic damage.The sperm gets its fructose through this pathway.Why the sperm needs fructose and not glucose?Sorbitol (and galactitol) cause cataracts and osmotic disturbances
8We will review sperm energy metabolism and fructose metabolism Sorbitol can act as an alterative energy source for sperm motility and signal transduction in proposed metabolic pathway. ACH blocks glyceraldehyde 3-phosphate dehydrogenase. DAP, dihydroxyacetone phosphate; F-1-P, fructose 1-phosphate; F-6-P, fructose 6-phosphate; GAP, glyceraldehyde 3-phosphate; GLUT, glucose transporter; HK, hexokinase; PFK, phosphofructokinase; PGI, phosphoglucose isomerase; SORD, sorbitol dehydrogenase; TCA, citric acid cycle; TK, triokinase.From: Biol Reprod January; 80(1): 124–133.
9First steps of protein glycosylation by glucose Products of advanced protein glycosylation (advanced glycation end products, or AGEs) accumulate in tissues as a function of time and sugar concentration. AGEs induce permanent abnormalities in extracellular matrix component function, stimulate cytokine and reactive oxygen species production through AGE-specific receptors, and modify intracellular proteins. Pharmacologic inhibition of AGE formation in long-term diabetic animals prevents diabetic retinopathy, nephropathy, neuropathy, and arterial abnormalities in animal models. Clinical trials in humans are currently in progress.
10Glycosylation of hemoglobin and formation of HbA1c As I mentioned above, the degree of glycation of the body's proteins is related to blood glucose levels. Hemoglobin has a half-live of about 100 days. The degree of glycation of hemoglobin gives, therefore, a picture of average blood glucose levels for the previous three month period. Glycated hemoglobin is known as HbA1c. Normally, HbA1c accounts for approximately 5-6 % of the total hemoglobin. Diabetic patients often have HbA1c in excess of 8-10%. It has been usually assumed that glycation of hemoglobin followed reaction with glucose, as shown in the next figure. Current studies have, as noted above, indicated that other reactive species, especially methylglyoxal, may be involved in this process. Note that the degree of glycation of hemoglobin is an indication of the extent of glycation of many of the body's proteins and possible ensuing cellular damage. How is it possible for adiabetic patient to have abnormally low HbA1C?
11Cataracts, polyols, diabetes and galactosemia: Lens metabolismPathway% of glucose utilizationin each pathwayMetabolic roleAnaerobic glycolysis78ATP synthesisPentose shunt14NAPDH synthesisSorbitol pathway5Osmotic regulationCataracts in diabetes and galactosemiaCitric acid cycle3Only in epithelial cellsGlucose and galactose are substrates of aldose reductase that produces sorbitol or galactitol. The function of sorbitol could be to control the osmotic pressure in the lens. The accumulation of sorbitol or galactitol causes cataracts by derangement of the osmotic control of the lens. This leads to changes of permeability, swelling, ionic changes and cataracts. Sorbitol is metabolized by sorbitol dehydrogenase, galactitol is not, therefore it might be more damaging. Fructose is not an aldose, it is not a substrate of aldose reductase, therefore it will not contribute to the formation of cataracts.Inhibition of aldose reductase is being studied as a possible way to inhibit the accumulation of polyols and the formation of cataracts in diabetic patients.
12A type of cataract, called a sugar cataract, is found only in diabetes A type of cataract, called a sugar cataract, is found only in diabetes. This type occurs at any age but is most flagrant when it occurs in young adults in their twenties who are in very poor control of their Type 1 diabetes. Sugar cataracts can grow rapidly with complete loss of vision in the affected eye in as little as 3 days. Poor blood sugar control activates the aldose reductase pathway in which glucose is converted in one step to sorbitol. Because sorbitol breaks down very slowly in the body, it accumulates in the lens and attracts so much water that the precise structure of the lens needed for vision becomes damaged, and a sugar cataract gets formed.
13Lactose synthesis is mediated by a galactosyl transferase that binds galactose with N-acetyl-glucosamineto give N-acetyllactosamine, a constituent of several oligosaccharides.At the onset of lactation a protein specific to the mammary gland increases the affinity of this enzyme for glucose from 1 molar to 1 mM.Lactose is metabolized by LACTASE or β-galactosidase. This enzyme should be lost after weaning.However, many ethnic groups keep β-galactosidase for life. The biology of lactose is very interestingand clinically relevant. A short clinical exercise about lactose intolerance will be done later.ß1,4 Galactosyltransferase is unique among all glycosyltransferases in that its substrate specificity canbe modified by addition of a-lactalbumin. Together, ß1,4 galactosyltransferase and a-lactalbumin formthe lactose synthase complex. Because a-lactalbumin is only expressed in the mammary gland,lactose synthesis only occurs in the mammary gland. In addition, expression of the a-lactalbumingene is closely regulated by hormones, so that lactose synthesis only occurs during the lactatingstate of the tissue.
14This slide gives you an overview of related metabolic steps This slide gives you an overview of related metabolic steps. It is actually an overshoot as far as lactose synthesis goes.
15Synthesis of uridinediphosphoglucose or UDPGlu Synthesis of GlycogenSynthesis of uridinediphosphoglucose or UDPGluDo you remember phosphoglyceromutase? Any similarities with phosphoglucomutase?PPi is hydrolyzed by a pyrophosphorylase in a reaction coupled with the pyrophosphorylaseto dissipate energy as heat thus making the synthesis of UDP-Glu thermodynamically favorable.
16Mechanism of glycogen synthesis Glycogen is NOT synthesized by adding to the existing molecule either free glucose, glucose-1-P or glucose-P-6!The immediate precursor of glycogen is (besides the glycogen primer) UDP-glucose.
17Why do we need to waste 2 ATPs and make glycogen? Deficiency of branching enzyme gives long branches. Causes death at about to years of age. Andersen’s diseaseThe content of glycogen is about 10 % of the wet weight of the liver and 2% of muscle.
18Glycogen synthase only adds glucoses to an existing chain of at least 4 glucose residues. Glycogenin acts by catalyzing the addition of glucose to itself (autocatalysis) by first binding glucose from UDP-glucose to the hydroxyl of Tyr-194 from UDP-glucose, by glycogenin's glucosyltransferase. Once sufficient residues have been added, glycogen synthase takes over extending the chain. Glycogenin remains covalently attached to the reducing end of glycogen.
19Glycogen degradationThe breakdown of glycogen and entry into glycolysis as glucose-6-P is achieved by three enzymes: glycogen phosphorylase, debranching enzyme and phosphoglucomutase.Glycogen phosphorylase produces glucose-1-P plus limit dextrin.The debranching enzyme has a transferase and glycosidase (hydrolase) activities.Hexokinase is bypassed when glucose comes from glycogen!Deficiency of phosphorylase (Mc Adler’s) causes muscle cramps and no lactate formation during exercise.Deficiency of debranching enzyme causes accumulation of limit dextrinIn the next lecture we will begin with regulatory mechanisms involved inglycogen metabolism