Carbohydrate Metabolism Lecture 1 PDF

Carbohydra te Metabolism (Lecture 1) Carbohydrate structure/function & digestion and storage John Barrow [email protected] UNIT II (Intermediary Metabolism) in… Aims of carbohydrate lectures 1 &2 To review: Carbohydrate structures and functions Carbohydrate digestion and absor...

Carbohydra te Metabolism (Lecture 1) Carbohydrate structure/function & digestion and storage John Barrow [email protected] UNIT II (Intermediary Metabolism) in… Aims of carbohydrate lectures 1 &2 To review: Carbohydrate structures and functions Carbohydrate digestion and absorption The initial fate of glucose when it enters the cells of the body Glycolysis and substrate level phosphorylation Gluconeogenesis and anabolic pathways Carbohydrates are... Highly oxidizable Sugar and starch molecules have “high energy” H atom- associated electrons Thus they are a major energy source Carbohydrate catabolism is the major metabolic process for most organisms Function to store potential energy Starch in plants Glycogen in animals Have structural and protective functions In plant cell walls Extra cellular matrices of animal cells Contribute to cell-cell communication ABO blood groups Monosaccharides There are 3 important hexoses (6-C sugars) in human biochemistry Glucose (Glc) Galactose (Gal) Fructose (Fru) Gluco se Fructos Galacto e se Disaccharides Formed from monomers that are linked by glycosidic bonds Covalent bond formed when hydroxyl group of one monosaccharide reacts with anomeric carbon of another monosaccharide What’s an anomeric carbon? Different anomers are mirror images of each other (left- and right-handed forms) It is carbon #1 on the glucose residue It stabilises the structure of glucose Is the only residue that can be oxidised 3 important disaccharides in human biochemistry: Maltose Lactose Sucrose Maltose Don’t have much directly from the diet It is a break-down product of starch It is in beer (from the starch of the barley) Found in many baby foods as a “natural” sweetener Anomeric C-1 is available for oxidation, so maltose can be oxidised (termed a reducing sugar) Lactose Main sugar in milk It is formed from a glycosidic bond between galactose and glucose Anomeric carbon on the glucose is available for oxidation so it is termed a reducing sugar Sucrose Common (table) sugar Only made by plants Approx. 25% of dietary carbohydrate Sweetener in most processed food Does not have a free anomeric C-1 so there is no oxidation site, hence sucrose is termed a non- reducing sugar Polysaccharides Polymers of medium to high molecular weight Distinguished from each other in the, identity of their recurring monosaccharide units length of their chains types of bonds linking monosaccharide units amount of branching they exhibit Homopolysaccharides Single monomeric species Heteropolysaccharides Have two or more monomer species Starch Contains two types of glucose polymer: Amylose (20-25% of starch) D-glucose residues in (α1→4) linkage Can have thousands of glucose residues Amylopectin (75-80% of starch) Similar structure as amylose but branched Glycosidic (α1→4) bonds join glucose in the chains but branches are (α1→6) and occur every 24 – 30 residues Starch Has many non-reducing ends and very few reducing ends Amylose and amylopectin are believed to form alpha helices and form a structure like this, Glycogen Animal cells use a similar strategy as plants to store glucose Polymer of glucose (α1→4) linked sub-units with (α1→6) branches every 8 to 12 residues This makes glycogen more extensively branched than starch 90% is in: Liver (acts to replenish blood glucose when fasting Skeletal muscle (catabolism produces ATP for contraction) Glycogen structure Why store glucose in polymers? Compactness Amylopectin and glycogen have many non- reducing ends This allows them to be readily synthesised and degraded to and from monomers respectively Thus speeds up the formation or degradation The polymers form hydrated gels and are not really “in solution” This means they are osmotically inactive If free glucose were in the cells then [Glc]inside > > [Glc]outside Either Glc would move out of the cell down the concentration gradient Or, the cell would use huge amounts of energy keeping it in the cell Glycoproteins Proteins that have carbohydrates covalently attached Most extracellular eukaryotic proteins have associated carbohydrate molecules Carbohydrate content varies between 1-80% by mass Carbohydrates attached to proteins may, Increases the proteins solubility Influence protein folding and conformation Protect it from degradation Act as communication between cells Glycosaminoglycans (GAGs) In the ‘olden days’ they were called mucopolysaccharides Hints at their function – in mucus and also synovial fluid around the joints Un-branched polymers made from repeating units of hexuronic acid and an amino-sugar, which alternate through the chains Proteoglycans Carbohydrate > > protein Formed from GAGs covalently attaching to proteins They are macromolecules found on the surface of cells or in between cells in the extracellular matrix Therefore form part of many connective tissues in the body Glycoproteins Protein > > carbohydrate Very similar to proteoglycans Usually found on the outer plasma membrane and extra cellular matrix, but also in the blood and within cells in the secretory system (Golgi complex, secretory granules) Some cytoplasmic and The large cartilage nuclear proteins are proteoglycan (aggrecan) also glycoproteins forms an aggregate with hyaluronan and link protein Mucopolysaccharidoses Group of genetic disorders caused by the absence or malfunction of enzymes that are required for the breakdown of glycosaminoglycans Over time the glycosoaminoglycans build up in connective tissue, blood and other cells of the body This build up damages cellular architecture and function Can cause severe dementia, problems with the heart and any other endothelial structure as the glycosaminoglycans build up between the endothelial cells Also, bones tend to be stunted and joints will be inflammed and become severely damaged Hurler, Scheie, Hunter, Sanfilippo syndromes are all examples of mucopolysaccharidoses Hurler syndrome Severe developmental defects: Stop developing at around 4 years Death at around 10 years old Clouding and degradation of the cornea Arterial wall thickening Dementia caused by, amongst other things: Build up of CSF Enlarged ventricular spaces Experimental therapies currently include: Gene therapy Enzyme replacement therapies Carbohydrates in our diet Starch Cereals, potatoes, rice Glycogen Meat (however when the animal dies enzyme activity in the tissue degrades much of the glycogen stores) Cellulose and hemicellulose Plant cell walls – we don’t digest this Oligosaccharides containing (α1→6) linked galactose Peas, beans, lentils – not digested Lactose, sucrose, maltose Milk, table sugar, beer Glucose, fructose Fruit, honey Digestion of carbohydrates Mouth: Salivary amylase hydrolyses (α1→4) bonds of Stomach: starch No carbohydrate digestion Duodenum: Pancreatic amylase works as in mouth Jejunum: Final digestion by mucosal cell- surface enzymes: 1. Isomaltase – hydrolyses (α1→6) bonds 2. Glucoamylase – removes Glc sequentially from non-reducing ends 3. Sucrase – hydrolyses sucrose 4. Lactase Main – hydrolyses products are lactose – Glc, Gal, Fru Absorption of glucose 2 1 3 Dietary glucose Low Na+ High Na+ High dietary Na+ High K+ Low K+ Absorption of monosaccharides Glucose is absorbed through an indirect ATP-powered process ATP-driven Na+ pump maintains low cellular [Na+], so glucose can continually be moved in to the epithelial cells This system continues to work even if glucose has to be moved into the epithelial cells against it’s concentration gradient (i.e. When blood glucose is high) Galactose has a similar mode of absorption as glucose, utilising gradients to facilitate it’s transport Fructose is slightly different, Binds to the channel protein GLUT5 Simply moves down it’s concentration gradient (high in gut lumen, low in blood) Cellulose and hemicellulose These cannot be digested by the gut, but they do have a use Increase faecal bulk and decrease transit time Lack of oligosaccharides in the diet can lead to poor health Many western diets Polymers are broken down by gut bacteria Yielding CH4 and H2 Beans will also have the same effect! Disaccharidase deficiencies Deficiencies may be genetic Or, they can result from, Severe intestinal infection Other inflammation of the gut lining Drugs injuring the gut wall Surgical removal of the intestine Characterised by abdominal distension and cramps Diagnosis would require enzyme tests of intestinal secretions Usually checking for lactase, maltase or sucrase activity Lactose intolerance Most common disaccharidase deficiency Most human populations lose lactase activity after weaning Western whites retain lactase activity into adulthood Theory that this comes from cattle domestication 100,000 years ago If lactase is lacking, then ingestion of milk will give disaccharidase deficiency symptoms This happens for 2 reasons: Undigested lactose is broken down by gut bacteria causing gas build up and irritant acids Lactose is osmotically active, thus drawing water from the gut into the lumen causing diarrhoea Symptoms can be avoided by, Avoiding milk products (many non-western diets do) Using milk products treated with fungal lactase Supplementing diet with lactase Fate of absorbed Glc Glc diffuses through the intestinal epithelium cells into the portal blood and on to the liver Glc is immediately phosphorylated into glucose 6- phosphate by the hepatocytes (or any other cell glucose enters) Glucose 6-phosphate cannot diffuse out of the cell because GLUT transporters won’t recognise it This effectively traps the glucose in the cell Enzyme catalyst, Glucokinase (liver) Hexokinase (other tissues) Glucokinase / Hexokinase Km (for Vmax glucose) High Vmax = efficient enzyme Glucokinase High High Low Km = high affinity for Hexokinase Low Low substrate Blood [Glc] normal – the liver doesn’t “grab” all of the glucose, so other tissues have it Blood [Glc] high (after meal) - liver “grabs” the Glc High glucokinase Vmax means it can phosphorylate all that Glc quickly, thus most absorbed Glc is trapped in the liver Hexokinase low Km means even at low [Glc] tissues can “grab” Glc effectively Hexokinase low Vmax means tissues are “easily satisfied”, so don’t keep “grabbing” Glc Fates of glucose 6-phosphate (G- 6-P) OTHER BLOO na TISSUES D ok i Gl x he se Gl c c pentose pento G- phosphate ses 6-P pathway LIVE glycol NADP R Gl ysis ATP H c (substrate- whe ed level ilis ded G- nee n Pyruv phosphorylat mob 6-P ate ion) citric O acid 2 cycle Lots of ATP glycoge (oxidative n (details as CO2 + phosphorylation) (skeletal opposite) H2O muscle) Glycogen ~90% is in the liver and skeletal muscle In liver: [blood Glc] falls, glucose 6 - phosphatas e glycogen G-6-P Glc into the blood In skeletal muscle: there is no glucose 6-phosphatase, glycogen G-6-P lactate glycol ysis Synthesis of glycogen (Step 1) Glycogen does not form directly from Glc monomers Glycogenin begins the process by covalently binding Glc from uracil- diphosphate (UDP)-glucose to form chains of approx. 8 Glc residues Then glycogen synthase takes over and extends the Glc chains Synthesis of glycogen (Step 2) The chains formed by glycogen synthase are then broken by glycogen-branching enzyme and re- attached via (α1→6) bonds to give branch points Glycogen structure Degradation (Mobilisation) of glycogen Glc monomers are removed one at a time from the non- reducing ends as G-1-P Degradation (Mobilisation) of glycogen Following removal of terminal Glc residues to release G-1-P, by glycogen phosphorylase, Glc near the branch is removed in a 2-step process by de-branching enzyme Transferase activity of de- branching enzyme removes a set of 3 Glc residues and attaches them to the nearest non-reducing end via a (α1→4) bond Glucosidase activity then removes the final Glc by breaking a (α1→6) linkage to release free Glc This leaves an unbranched chain, which can be further degraded or built upon as needed What happens to all this glycogen? G- G- 1-P 6-P 6- ATP se ta (for o ha su lev or ly c ph t u sp muscle bs el yl sis l g ho se os gliyo contracti tr p at ph cno on) e Gl a c lacta te to blood von Gierke’s disease Liver (and kidney, intestine) glucose 6- phosphatase deficiency Symptoms: high [liver glycogen] – maintains it’s normal structure low [blood Glc] – fasting hypoglycaemia This is because glycogen cannot be used as an energy source – all Glc must come from dietary carbohydrate high [blood lactate] – lacticacidaemia Because the lactate produced by skeletal muscle cannot be reconverted to Glc in the liver (this process requires glucose 6- phosphatase – see Cori cycle lectures) Treatment: Regular carbohydrate feeding – little and often Every 3-4 hours throughout the day and night Can be administered through a nasogastric tube and pump, but sudden death has occurred when the pump fails or the tube McArdle’s disease Skeletal muscle glycogen phosphorylase deficiency Symptoms: High [muscle glycogen] – maintains it’s correct structure Weakness and cramps after exercise No increase in [blood glucose] after exercise Most symptoms are not apparent in resting state, when muscles will use other energy sources (Glc and fatty acids from the blood) Usually becomes apparent in 20-30 year olds Children do suffer the disease but may remember pain during adolescence and childhood Treatment: Avoid strenuous activity Make use of your “second wind” Exercise briefly (anaerobically), wait for the pain to subside, continue to exercise (aerobically using oxidative phosphorylation of fatty acids)