cho after midterm Flashcards
what is glutathione
Glutathione is a tripeptide composed of glutamate, cystein, glycine. Reduced glutathione (GSH) maintains the normal reduced state of the cell.
Glutothione functions
It serves as a reductant. • Conjugates to drugs making them water soluble. • Involved in amino acid transport across cell membranes. • Cofactor in some enzymatic reactions. – rearrangement of protein disulfide bonds. The sulfhydryl of GSH is used to reduce peroxides (ROS) formed during oxygen transport. – Reactive oxygen species (ROS) damage macromolecules (DNA, RNA, and protein) and ultimately lead to cell death. • The resulting oxidized form of GSH is two molecules linked by a disulfide bridge (GSSG).
what does the enzyme glutathione reductase use
uses NADPH as a cofactor to reduce GSSG back to two moles of GSH. Thus, the pentose pathway is linked to the supply of adequate amounts of GSH.
Regulation of Blood Glucose
- obligate glu users: cannot use FAs, AAs for nrg
eg, brain, nervous tissue, RBCs, WBCs, renal
medulla
- need to maintain blood glu levels (therefore, control on lower
limit [fasting level])
eg, human, pig, horse: 4-5.5 mmol/L (70-100 mg/dL)
cat, dog (carnivores): 3 mmol/L (~60 mg/dL)
cow, sheep (ruminants): 1.5-2 mmol/L (~35-40 mg/dL) - other NB requirements for glucose:
a) lactose production in the mammary gland
b) main energy source for the fetus
UTILIZATION OF STORED CARBOHYDRATE
IN THE POST-ABSORPTIVE STATE
blood glucose ↓ to below fasting level (regulation)
[↑ glucagon from α cells, Islets of Langerhan, pancreas]
↓
↓ glucose uptake into cells
↓
↑ glycogenolysis and gluconeogenesis
(↑ glucose output from liver)
[muscle glycogen breakdown, no response to ↑ glucagon]
- Glycogenolysis
pathway to glucose and enzyme
glycogen → glu 1-P → glu 6-P → glu
- enzyme phosphorylase → break
glycogen bonds
Remember Liver contains glucose 6-phosphatase.
• Muscle does not have this enzyme.
why?
The liver releases glucose to the blood to be taken
up by brain and active muscle. The liver
regulates blood glucose levels.
The muscle retains glucose 6-phosphate to be use
for energy. Phosphorylated glucose is not
transported out of muscle cells.
Allosterism
A change in the activity and conformation of an enzyme resulting from the binding of a compound at a site on the enzyme other than the active binding site
Epinephrine and Glucagon Stimulate
Glycogen breakdown
Muscle is responsive to epinephrine. • Liver is responsive to glucagon and somewhat responsive to epinephrine. • Both signal a cascade of molecular events leading to glycogen breakdown.
Fed vs. Fast
glucagon vs insuling
Glucagon = starved state; stimulates glycogen breakdown, inhibits glycogen synthesis. • High blood glucose levels = fed state; insulin stimulates glycogen synthesis and inhibits glycogen breakdown.
Gluconeogenesis (GNG)
where does it happen
whate are the substrates (4)
glycolysis in reverse (sort of ?)
- mainly in liver (kidney in starvation)
- Converts pyruvate and related three- and four-carbon
compounds to glucose
Substrates: glycerol
lactate (Cori cycle)
pyruvate
part (if not all) of C-skeleton of most AAs
(ie, loss of NH2 by trans- or de-amination)
Essentially reversal of glycolysis except
that three reactions in glycolytic sequence
are not reversible: reactions catalyzed by
glucokinase and hexokinase,
phosphfructokinase, and pyruvate kinase
- GNG requires these reactions to be
bypassed by other enzymes
Gluconeogenic enzymes (muscle/adipose lack
these):
glycolysis vs. GNG
Glycolysis Enzyme
Glucokinase
Phosphofructokinase-1
Pyruvate kinase
GNG Enzyme Glucose 6-phosphatase Fructose 1,6-bisphosphatase Phosphoenolpyruvate carboxykinase Pyruvate carboxylase (2 step reaction)
Pyruvate Carboxylase
Pyruvate + CO2 + ATP + H2O oxaloacetate + ADP + Pi + 2 H+ • Pyruvate Carboxylase fixes CO2 . Enzymes which fix CO2 . require the cofactor BIOTIN. Biotin is a vitamin and is always involved in CO2 fixation. • This reaction takes place in the mitochondrial matrix.
Phosphoenolpyruvate
Carboxykinase
Oxaloacetate + GTP phosphoenolpyruvate + GDP + CO2 • This reaction takes place in the cytosol • PEP is now synthesized and the sum of the two reaction is: • Pyruvate + ATP + GTP + H2O PEP + ADP + GDP + Pi + H+
where is pyruvate carboxylated
Pyruvate is carboxylated in the mitochondria. by
Pyruvate Carboxylase
Oxaloacetate can’t pass out of
the mitochondria. and must become malate by malate DH
what happens to oxaloacetate in the cytosol
Oxaloacetate decarboxylated and
phosphorylated in the cytosol. by
Phosphoenolpyruvate Carboxykinase
Cori cycle
Cori cycle (removal of muscle lactate)
muscle lactate → blood → liver lactate → glu
Lactate from active muscle is converted to glucose in liver.
Acetyl CoA
Acetyl CoA (or Cs from metabolite converted to acetyl CoA, eg, FAs) cannot form net glucose -2 Cs enter TCA at acetyl CoA -2 CO2 lost with each turn of TCA
what are the obligate glucose users
brain, nervous tissue, RBCs, WBCs, renal
medulla
biotin where is it used
Biotin is a vitamin and is always
involved in CO2
fixation. is a cofactor in pyruvate carboxylase
The importance of gluconeogenesis
Liver glycogen, in absence of CHO intake, lasts for 16h (in the human)
After this period, animal solely dependent on liver GNG to maintain
blood glucose
eg, prolonged fasting, nearly “0” CHO diet
Anaerobic Glycolysis during
Intensive Exercise
glu / glycogen → pyruvate → lactate
• anaerobic, no O2
(in cytosol)
Intensive muscle exercise:
1. Up to 100x ATP requirement of resting or low-activity
- ATP generation is major limiting factor for
maintaining intensive activity
2. Lack of sufficient O2 to sustain e- transport chain
3. ATP derived from substrate-generated glycolysis
(glycogen, glucose)
4. Muscle anaerobic glycolysis “kept going” by:
a) lactate production
b) the Cori cycle
5. Stimulus to glycogenolysis is epinephrine (adrenal
medulla)
6. In muscle: glycogen → 2 lactate (yields 3 ATP / 6Cs)
glucose → 2 lactate (yields 2 ATP / 6Cs)
In liver: 2 lactate → glucose costs 6 ATP
Anaerobic Glycolysis during
Intensive Exercise (Con’t)
in muscle, in liver
- In muscle: glycogen → 2 lactate (yields 3 ATP / 6Cs)
glucose → 2 lactate (yields 2 ATP / 6Cs)
In liver: 2 lactate → glucose costs 6 ATP
Acetate is not
glucogenic Remember: Acetate is never converted to glucose in animals. Acetate in not glucogenic. The free energy (ΔG°’=-33.4kJ/mol) derived from the oxidative decarboxylation of pyruvate is large. The pyruvate DH reaction essentially irreversible
TOPICS RELATED TO
UTILIZATION OF CHO
- Lactose Intolerance
- lack / relative lack of digestive enzyme lactase
- genetic versus acquired
Lactose remains in small intestine:
a. osmotic effect
b. attacked by bacteria → lactic acid (irritant)
→ methane & H2 (bloating)
c. symptoms: diarrhea, nausea, vomiting
CHO - Zero Carbohydrate Diet for Weight
Reduction (like Atkins, Zone, South Beach)
- very restrictive food choices → nutrient
deprivation
- produces mild ketosis (reduces appetite)
Risks: - ↑ risk of gout, osteoporosis, heart disease ?
- produces dehydration - Glycogen Loading
- used by athletes involved in
endurance sports
- can ↑ time of maximum
performance 2 fold
- reason: as 75-90% max O2
capacity reached, muscle
metabolism → anaerobic
Strategies to maximize glycogen stores
- Consume a high carbohydrate diet on a daily basis
- Take in sources of glucose during exercise
- Beverages or foods - Carbohydrate loading
– Days 7-4 before competition: moderate carbohydrate
intake with training 1-2 hours per day
– Days 3-1: high carbohydrate intake with limited
training
– Consuming a high carbohydrate meal within two
hours after exercise has also been shown to increase
muscle glycogen by 300%
Strategies to maximize glycogen stores
Risks:
a. ↑ body wt / feeling of heaviness,
stiff muscles
b. accentuated hypoglycemia follows
depletion of large glycogen load
CHO TOPICS
4. High Fibre Diets for Weight Control
- normal, desirable intake: 20-30 g/d
If some is good, is more better?
a. Hypothesis: ↑ fibre in food, ↑ satiety, ↓ energy density
b. Properties of fibre in GI tract:
i. attracts / holds H2O (pectins gums) → ↓ transit time
ii. provides bulk (cellulose)→ stimulates peristalsis
iii. binds cholesterol & bile acids (lignin, gums, pectins)
iv. phytic acid found with fibre in seeds / grains binds
divalent cations (Ca, Mg, Zn, Fe)
v. bacteria in gut can attack fibre (energy source)
- Diabetes Mellitus
anomaly in carbohydrate metabolism
- Oral Health (Dental Caries)
free sugars
- DRI position on free sugars
- Galactose and Fructose Intolerance
- inborn error of metabolism
- galactosaemia: mental retardation / cataracts
in infants (avoid milk)
- Hepatic Glycogen-Storage Diseases (Glycogenoses)
- genetic disorders in carbohydrate metabolism LIVER: Type I - von Gierke's disease Type III - Cori's disease Type IV - Andersen's disease Type VI - Hers' disease MUSCLE: Type II - Pompe's disease Type V - McArdle's disease Type VII Type VIII
Von Gierke Disease
Edgar Otto Conrad von Gierke,
German pathologist,1877-1945
• Hereditary metabolic disorder with autosomal
recessive inheritance
• Due to an inborn lack of glucose-6-phosphatase
• Consequently, glucose 6-phosphate can not be
converted to glucose
• This results in low blood sugar - hypoglycemia
Without glucose-6-phosphatase: • Levels of glucose 6-phosphate increase – Glycolysis increases • Levels of lactate and pyruvate in blood increase – Glycogen levels increase • Glycogen is deposited in liver and kidney cells. A child who has Von Gierke disease, would have an enlarged liver -- hepatomegaly. Treatment • Prevent hypoglycemia with frequent feedings of foods of high starch foods (broken down to glucose) •Avoid CHO that must be converted to G6P to be utilized (fructose and galactose)
Pompe Disease
J.C. Pompe, 20th century Dutch pathologist
Autosomal recessive inheritance • Due to an inborn lack of alpha-1,4 glucosidase (acid maltase), an enzyme which cleaves 1,4 and 1,6 alpha-glycosidic linkages. • Absence of this enzyme leads to an accumulation of glycogen in lysosomes.
• Organs affected by accumulation of glycogen
– skeletal muscles (mostly), CNS, heart, liver,
• leading to:
– bulky muscles including macroglossia (enlarged
tongue) and cardiomegaly (enlarged heart)
– hypotonia (loss of muscle tone) and muscle
weakness, including congestive heart failure
(from heart muscle weakness)
McArdle Disease
Cori’s type V glycogenosis
Results from a defect in muscle glycogen phosphorylaseB
.
• More than 30 known mutations can produce the same
clinical picture (phenotype).
• In about half of patients, there is a single base change
(R49X) in which the codon for Arg becomes a stop codon,
resulting in a partially completed non-functional protein.
• The remaining mutations are a mixture of nonsense (stop
codons) and missense (changed amino acid) mutations
that result in incorrect folding and/or loss of catalytic
activity.
Glycogen phosphorylase activity is lost.
Notice the difference between normal pattern dark
staining for phosphorylase (figure A) compared to the
lack of dark staining (except in a few blood vessels) in a
patient with McArdle disease (figure B).
Occasional vacuoles are filled with glycogen (asterisks
in figure C) since glycogen can not be broken down to
glucose.
• During normal exercise, O2 can not be transported
to muscles cells fast enough. Thus, a stored
reserve of glucose (glycogen) is used as fuel.
• Since glycogen can not be broken down to glucose
in patients with McArdle disease, lactate does not
build up.
• After several minutes of exercise, patients
experience severe muscle pain, probably from
increased ADP.
McArdle Disease
Symptoms:
- muscular pain
- fatigability
- muscle cramping following exercise, which
disappears with rest - later in life - severe cramps and myoglobinuria
after exercise. - kidney failure can be associated with
rhabdomyolysis (muscle breakdown)
