Lecture 4 - carbs Flashcards
what are carbohydrates
- source of energy
o diet
o endogenous
▪ stored form of energy - structural components
o cell membrane
o RNA and DNA
o cell walls of bacteria and plants - Glucose
o Major clinically significant carbohydrate
4 structural properties may be used to classify carbohydrates:
- The size of the base carbon chain (number of carbons in the molecule)
- Location of the CO group
- Number of sugar units
- The stereochemistry \
C L A S S I F I C A T I O N ( C A R B O N N U M B E R )
basic formula where n is the number of carbon
3-triose- glyceraldehyde
dihydroxyacetone
4- tetrose - erythrose
5-pentose - rib or deoxyribose
6-hexose- glucose galactose
7-heptose-sedoheptulose
A L D O - HEX - O S E F A M I L Y
- ALDO
– aldehyde group where the carbonyl group is found on the end C so C=O on terminal C
-carbs from aldehydes are called aldoses - HEX
– 6 carbons - OSE
– carbohydrate - 3 naturally occurring forms
– D-glucose
– D-galactose
– D-mannose - Chemical formula: C6H12O6
THE K E T O - HEX - O S E F A M I L Y
- KETO
– ketone group where the carbonyl group C=O is found on a C that is in the middle
-carbs from ketones is called ketoses - HEX
– 6 carbons - OSE
– carbohydrate - 1 naturally occurring form
– D-fructose - Chemical formula: C6H12O6
what are isomers
- Isomers
o compounds with the same molecular formula but different structures
▪ identical number of atoms and atomic elements - difference in configuration around one specific C atom
o glucose, fructose, galactose, mannose C6H12O6 (epimer)
what is a Stereoisomers
o compounds identical in composition and differ only in spatial configuration
* D- and L- configuration
D- -OH on the right
L- -OH on the left
o most sugars in humans are of the D form with OH on the right
enantiomers
Non-superimposable mirror images are called enantiomers
(optical isomers)
– they have a “chiral” carbon
what is a M O N O S A C C H A R I D E S
simple sugar
* a single polyhydroxy aldehyde or ketone unit that cannot be
hydrolyzed to a simpler form
–c=o and -oh groups form a ring structure (with the chiral carbon) through intramolecular hemiacetal or hemiketal linkage
-when an aldehyde and alcohol come together a ring structure is formed through INTRAMOLECULAR HEMIACETAL OR HEMIKETAL LINKAGES
in a HEMIACETAL FORM (alde+alco) - THE HYDROXYL forms a bridge between c1 and c5 and the carbonyl forms alcohol
makes a carboxylic acid OH-C-OR
the anomeric carbon is the c attached to C=O
carbonyl
c=O
carboxyl
COOH
ketone and alcohol reaction - hemiketal
It is the =O (carbonyl group) that reacts with the –OH (alcohol or
“hydroxyl” group)
The H+ from the alcohol group moves positions to form a new OH and the remaining O (from the alcohol) forms the “bridge
fructose
❑ Carbonyl group (at C2 - anomeric carbon) and chiral carbon (at C5) form the hemiketal linkage to form the ring structure
❑ Hemiketal linkage: between the carbonyl group (-C=O) and the chiral carbon
❑ Chiral carbon is the highest numbered carbon with 4 different “groups
A and B isomers
Two chiral ring forms are possible
o MUTAROTATION alters the equilibrium between the three forms
* spontaneous opening and closing of the hemiacetal or hemiketal structure
* carbon about which this rotation occurs is the anomeric carbon
o (-OH group down) and (-OH group up) anomers
chemical properties of monosacc
- some carbohydrates are reducing substances
– free aldehyde or ketone group required - the anomeric carbon must remain unaltered
- the ring must be able to open up!
– Cu2+ or Fe3+ ions can be reduced by reducing sugars
* the resulting oxide is red
* lab application:
o Urinalysis: clinitest tablets with Benedicts reagent /clinitest (Copper 2 Sulphate reduced to copper 1 oxide)
if testing water or egg white the result will be blue or negative but if testing glucose or other sugars the test will be red or post
what are disacc
-two monosaccharides joined covalently by a glycosidic bond,
with a formation of h2o
o aldehyde or ketone group of one monosaccharide joins to alcohol or aldehyde or ketone group of another
o Cx(H2O)y
- physiologically important disaccharides
maltose = glucose + glucose
lactose = glucose + galactose
sucrose = glucose + fructose
how does disaccharide act as a reducing sugar
-can be a reducing sugar only if there is a free aldehyde or ketone group
- When the anomeric carbon has a hydroxyl group available to participate, it is considered “reducing”
- If the anomeric carbon is involved in the linkage, then it cannot be a reducing sugar
so if the anomeric carbon is axial or equatorial alone thats okay but if its joined like \o/ then no
what happens when a sugar is reducing
- it reduces another compound and is oxidized i.e the carbonyl group is oxidized to a carboxyl group
- All monosacccharides are reducing sugars because they are capable of opening up the ring form
- The reducing end of the disaccharide is the monosaccharide with a free anomeric carbon that is not involved in a glycosidic bond and is capable of converting to the open chain form.
how are disaccharides hydrolyzed
intestinally by disaccharidases like maltASE, lactase, sucrase
o monosaccharides are absorbed and transported to liver
o the only CHO to be directly used for energy or stored as glycogen is glucose
- fructose and galactose cannot be used until they are converted to glucose
what is starch
polysaccharides
o Storage form of carbohydrate in plants
o Composed of two polymers:
* Amylose
▪ linear chain of alpha-1,4-linked glucose units more than a 1000 glucose will make an amylose
- Amylopectin
▪ highly branched chain: with alpha 1-4 and alpha1-6 links
▪ up to 1 million glucose units
what is Glycogen
polysaccharides
o animal storage form of carbohydrate
* liver (hepatocytes)
* muscle
o similar to amylopectin, more highly branched (every 8-12 units)
* enhanced solubility
what is Cellulose
polysaccharides
structural polysaccharide in plants
o high tensile strength from beta-1,4 linkages look like ////
o humans do not have cellulases
-purest form of cellulose is in
cotton
what are two major hormones that help maintain glucose levels between 4.1-5.6 mmol/l
- Insulin:
▪ Increases glycogenesis and glycolysis
▪ Increases lipogenesis
▪ Decreases glycogenolysis - Glucagon:
▪ Increases glycogenolysis
▪ Increases gluconeogenesis
o Regulatory hormones:
* Insulin, Glucagon, Epinephrine, Cortisol, Growth hormone, ACTH
o Disorders of carbohydrate metabolism:
* Hyperglycemia (high blood glucose), Hypoglycemia (low blood glucose)
Glycogenesis
pathway in glucose metabolism
- converts glucose to glycogen for storage
- occurs when blood glucose is high resulting in a lowered blood glucose
- mainly in liver hepatocytes, influenced by insulin
GlycogenoLYSIS
pathway in glucose metabolism
breakdown of glycogen to glucose
* occurs when blood glucose is LOW, results in a raised blood glucose
- mainly in liver, some in muscles, influenced by glucagon and other hormones
GlucoNEOgenesis
pathway in glucose metabolism
* formation of glucose from non-CHO sources (glycerol, amino acids, lactate)
* occurs when blood glucose is low, results in a raised blood glucose
-it is significant in hypoglycemic states
o Glycolysis and Lipogenesis
pathways in glucose metabolism
o Glycolysis
* Metabolism of glucose into pyruvate or lactate (energy production)
o Lipogenesis
* Conversion of carbohydrates to fatty acids
what monosaccharides are of clinical significance
Glucose and galactose
o Hyperglycemia (increased blood glucose) determines a diagnosis of diabetes mellitus (DM)
- Diabetes mellitus
▪ Glucose is underutilized
▪ Insulin secretion is impaired or there is tissue insensitivity to its action (or both)
▪ Type 1
▪ Type 2
▪ Gestational diabetes mellitus
what is type 1 diabetes
-diagnosed in childhood
* ABOSULTE insulin deficiency - autoimmune destruction of the insulin-secreting beta cells of the pancreas
what is type 2 diabetes
appears in adulthood (most common)
* insulin deficiency (insulin is present, but the target tissues are not sensitive enough)
- glucotoxicity - beta cells of the pancreas increasingly unresponsive to glucose
- environmental factors
what is gestational diabetes
insulin resistance during pregnancy
* maternal hyperglycemia associated with increased neonatal morbidity and mortality
Consequences of untreated prolonged hyperglycemia or DIABETES
Retinopathy
o Renal dysfunction
oPeripheral circulatory problems
o Myocardial infarction
oStroke
Lab findings in hyperglycemia
o↑ glucose in (serum and urine)
o↑ osmolality in (serum and urine)
o ketonemia and ketonuria in (serum and urine)
o↓ pH (blood and urine)- acidosis
o↑ urine-specific gravity
o electrolyte imbalance
o↓ insulin
what is HYPOglycemia and why is it clinically significant
from a monosaccharide - Glucose (decreased blood glucose)
* not as common
* defective glucose counter-regulation by glucagon, epinephrine
- unawareness in half of long-standing
Type 1 diabetics
o no neurogenic warning symptoms
o decreased epinephrine response
consequences of untreated hypoglycemia
-severe CNS dysfunction : headache, confusion, dizziness, seizures, loss of
consciousness, death
- Lab findings in hypoglycemia
o ↓↓ blood glucose levels
o ↑ insulin
glucose in infants is lower than in adults
hypogly in infants is transient - prematurity, maternal diabetes, GMD, maternal toxemia
less transient if there are inborn metabolism errors
hypoglycemia in infants is called :
galactosemia
o congenital enzyme deficiency
o cannot convert galactose to glucose
o “failure to thrive”
o screen at birth
o easy to treat
o Lab findings
* hypoglycemia
* hyperbilirubinemia
* ↑ galactose
causes brain damage, cataracts, jaundice, enlarged liver, kidney damage,
if a galactosemic baby is given milk the unmetabolized milk sugars will build up and damage the liver, eyes, kidneys and brain
hypoglycemia in adults causes
Decreased glucose production or increased glucose utilization
o drugs - most prevalent cause
o ethanol - inhibits gluconeogenesis
o hepatic disease
o pancreatic tumours
o septicemia
o defective glucose counter regulation
what are some disaccharide deficiencies of clinical significance
-can lead to forms of glycogen storage disease ie Lactase deficiency
-deficiencies of individual disaccharidases
what is Lactase deficiency:
- inability to digest lactose (milk sugar)
- occurs after weaning
- unabsorbed lactose is fermented by intestinal bacteria leading to cramps, bloating, gas formation, diarrhea
- premature infants are more prone to lactose intolerance: the enzyme levels do not increase significantly until the third trimester of pregnancy
o Diagnosis would be lactose intolerance
* lactose tolerance test
* breath hydrogen
* stool acidity
* reducing substances
o Treatment
* avoid lactose-containing foods
* lactase tablets
Polysaccharides deficiency clinical significance
Glycogen storage disease -deficiency of an enzyme involved in glycogen metabolism - glycogenolysis
* inherited defects will present in childhood
* affects liver and skeletal muscle
o LIVER forms are marked by hepatomegaly and hypoglycemia
o MUSCLE forms have mild symptoms that manifest after strenuous exercise
* no specific treatment or cure
o liver transplant
