Biochemistry

• Carbohydrate metabolism


  Glycolysis, Citric acid cycle (TCA cycle), HMP shunt, Glycogenolysis, gluconeogenesis, glycogenesis
• Carbohydrates (saccharides)


  The most abundant organic molecules in nature
• Carbohydrates


  • Providing a significant fraction of the dietary calories for most organisms
  • Acting as a storage form of energy in the body
  • Serving as cell membrane components that mediate some forms of intercellular communication
• Glycome


  The full set of carbohydrates produced by an organism
• Glycoside hydrolases (glycosidases)


  Enzymes that hydrolyze glycosidic bonds
• Endoglycosidases


  Enzymes that hydrolyze polysaccharides and oligosaccharides
• Disaccharidases


  Enzymes that hydrolyze tri- and disaccharides into their reducing sugar components
• Monosaccharides


  Glucose, galactose, and fructose
• Salivary α-amylase


  Enzyme that acts briefly on dietary starch and glycogen, hydrolyzing random α(1,4) bonds
• Branched amylopectin and glycogen contain α(1,6) bonds, which α-amylase cannot hydrolyze
• Dextrins


  Short, branched and unbranched oligosaccharides resulting from the action of salivary α-amylase
• Carbohydrate digestion halts temporarily in the stomach, because the high acidity inactivates salivary α-amylase
• Pancreatic α-amylase


  Enzyme that continues the process of starch digestion when the acidic stomach contents reach the small intestine
• Disaccharidases


  Enzymes that cleave disaccharides into monosaccharides
• Isomaltase


  Enzyme that cleaves the α(1,6) bond in isomaltose
• Maltase


  Enzyme that cleaves the α(1,4) bond in maltose and maltotriose
• Sucrase


  Enzyme that cleaves the α(1,2) bond in sucrose
• Lactase (β-galactosidase)


  Enzyme that cleaves the β(1,4) bond in lactose
• Trehalase


  Enzyme that cleaves the α(1,1) bond in trehalose
• Transmembrane proteins


  Disaccharidases are located on the luminal surface of the enterocytes
• Sucrase-isomaltase (SI) complex


  A single protein that is cleaved into two functional subunits
• Maltase-glucoamylase (MGA)


  A single membrane protein with two enzymic activities: maltase and glucoamylase
• Glucoamylase


  Enzyme that cleaves α(1,4) glycosidic bonds in dextrins
• Sodium dependent glucose cotransporter 1 (SGLT-1)

  Transporter that absorbs glucose and galactose into enterocytes via secondary active transport
• GLUT-5


  Transporter that absorbs fructose via an energy- and Na+-independent mechanism
• GLUT-2


  Transporter that transports all three monosaccharides (glucose, galactose, fructose) from the enterocytes into the portal circulation
• Ordinarily all digestible dietary carbohydrate is absorbed by the time the ingested material reaches the lower jejunum
• Osmotic diarrhoea


  Caused by the passage of undigested carbohydrate into the large intestine, which draws water from the mucosa
• Bacterial fermentation


  Produces large volumes of carbon dioxide and hydrogen gas, causing abdominal cramps, diarrhoea, and flatulence
• Disaccharide intolerance


  Caused by genetic deficiencies of the individual disaccharidases
• Acquired enzyme deficiency


  Caused by brush border enzymes being rapidly lost in normal individuals with severe diarrhoea
• Lactose intolerance


  Caused by the age-dependent loss of lactase activity starting at approximately age 2 years
• Congenital sucrase-isomaltase (SI) deficiency

  An autosomal-recessive disorder resulting in an intolerance of ingested sucrose
• Oral tolerance tests


  Used to identify specific enzyme deficiencies
• Breath hydrogen test


  Measures H2 in the breath to determine the amount of ingested carbohydrate not absorbed by the body
• Metabolism


  The sum of all the chemical changes occurring in a cell, a tissue, or the body
• Regulatory signals


  Hormones, neurotransmitters, and the availability of nutrients that inform a cell of the metabolic state of the body as a whole
• Glycolysis


  The pathway used by all tissues for the oxidation of glucose to provide energy (as ATP) and intermediates for other metabolic pathways
• Aerobic glycolysis


  The series of ten reactions where pyruvate is the end product and O2 is required to reoxidize the NADH formed
• Anaerobic glycolysis


  The conversion of glucose to lactate, which can occur without the participation of O2