Glycolysis & Lactate Metabolism Flashcards
What is the general structure of carbohydrates? Which isomer is found in nature? What groups may be present? What are some of the features of the structure that determine the function of carbohydrates?
General formula: (CH2O)n
D isomer
May be aldose sugars and contain aldehyde groups (-COH)
or be ketose sugars and contain ketone groups (-C=O)
Hydrophilic (cannot cross membranes without help)
Partially oxidised (need less O2 than fatty acids)
What is a monosaccharide? What are the three main monosaccharides?
Single sugar unit (3 - 9 carbon atoms)
Glucose
Fructose
Galactose
What is the definition of a disaccharide? What are the three main disaccharides, and what do they consist of?
Two sugar units formed by condensation of two monosaccharides.
Lactose = glucose + galactose
Sucrose = glucose + fructose
Maltose = glucose + glucose
What is the definition of an oligosaccharide? What is the definition of a polysaccharide?
Oligosaccharide = 3 - 12 monosaccharides e.g. dextrin
Polysaccharide = 10 - 100 monosaccharides (storage carbohydrates)
What polysaccharides can glucose be stored as? What is the different between the structures of each polysaccharide? What are the consequences of the structural differences?
GLYCOGEN = major store in mammals (liver & skeletal muscle)
a-1,4- bonds (amylase) & a-1,6-bonds (amylopectin)
Highly branched
STARCH = major store in plants
a-1,4-bonds (amylase) & a-1,6-bonds (amylopectin)
Less branched
GI tract enzyme digests starch to glucose & maltose
CELLULOSE = structural polymer of glucose in plants
beta-1,4-linkages
cannot be digested by humans (most of our enzymes only digest alpha glycosidic bonds, we do not produce cellulase)
used as roughage (increases s.a. for absorption & substrate aggregation)
How are carbohydrates digested? By which enzymes and where?
SALIVA = amylase (starch & glycogen ——> dextrins)
PANCREAS = amylase (dextrins ———> monosaccharides)
SMALL INTESTINE =
lactase (lactose ——> glucose & galactose)
sucrase (sucrose —–> glucose & fructose)
pancreatic amylase (a-1,4-bonds)
isomaltase (a-1,6-bonds)
How is glucose transported to respiring tissues?
GLUT1-5 actively transport glucose into intestinal epithelial cells.
Glucose diffuses into the bloodstream (as [glucose] in cell > [glucose] in blood)
Glucose transported into cytoplasm by facilitated diffusion
What cells/tissues have an absolute requirement for glucose? What cells/tissues prefer to utilise glucose?
Absolute requirement:
- RBCs
- WBCs
- kidney medulla
- lens of the eye
Prefers glucose: CNS
Liver & adipose require glucose for specialised functions
What are the three irreversible steps of glycolysis? Which enzymes catalyse these steps?
- Glucose ——–> Glucose-6-phosphate (hexokinase/glucokinase) Requires ATP
- Fructose-6-phopshate ———> Fructose-1,6-bisphosphate (phosphofructokinase) Requires ATP
- Phosphoenolpyruvate ———–> Pyruvate (pyruvate kinase)
Produces ATP
When does lactate production occur? Which enzyme catalyses the breakdown of lactate to pyruvate? In which tissues does lactate metabolism occur? In which organs can lactate be converted back to pyruvate?
Anaerobic conditions in skeletal muscle & GI
Tissues lacking mitochondria (RBCs)
Used to regenerate NAD+ (as oxidative phosphorylation cannot occur)
Liver & heart: lactate ——–> pyruvate (lactate dehydrogenase)
What is the normal [lactate] in the blood? What is the definition of hyperlactaemia? What is the definition of lactic acidosis? What can increased [lactate] cause? What can impair lactate metabolism?
Normal [lactate] = 1mM
Hyperlactaemia = 2-5mM (above renal threshold; therefore blood pH decreases)
Lactic acidosis = 5mM circulatory collapse (shock) & congestive heart disease
Lactate utilisation impaired in: liver disease, thiamine deficiency, enzyme deficiency
How is fructose metabolised? Where is fructose metabolised? What diseases can result from enzyme deficiencies?
Liver:
(fructokinase) (aldolase)
Fructose —————> F-1-P ———–> 2 x G-3-P (GLYCOLYSIS)
Fructokinase deficiency = essential fructosuria (no clinical signs except fructose in urine)
Aldolase deficiency = fructose intolerance (F-1-P builds up in liver and causes damage) Management = fructose-free diet
How is galactose metabolised? Where is galactose metabolised? What diseases result from which enzyme deficiencies?
Liver:
(galactokinase) (transferase)
Galactose ———> Galactose-1-P ———-> G-1-P (GLYCOLYSIS)
UDP-glucose —-> UDP-galactose
(epimerase)
Galactokinase deficiency = galactose accumulation ——-> galactitol (depletes NADPH ——> e.g. cataracts) (rare)
Transferase deficiency = galactose & galactose-1-P accumulation ——–> see above + damages kidneys, brain (mental retardation), and liver (jaundice)
Management: lactose-free diet
