Biochemistry II Flashcards
What are some inborn errors of fructose and galactose metabolism?
- Classical galactosemia
- Essential fructosuria
- Hereditary fructose intolerance (type 2)
What is classical galactosemia?
Defect: deficiency of Gal-1-phosphate uridylyl transferase leads to galactose accumulation in the body
- *Symptoms and signs:**
- Aversion to milk
- failure to thrive
- hepatomegaly
- cataracts
- mental retardation
- hypoglycemia
What is the mechanism of classical galactosemia?
- accumulation of Galactose-1-phosphate in liver and galactose in blood and all tissues
- high Gal-1-P in liver leads to liver damage
- high galactose –> accumulation of galactitol in lens of eye and brain, which lead to cataracts and mental retardation
galactose ==> Galactitol
(Enzyme: aldose reductase)
(NADPH –> NADP)
Why is neonatal screening done for classical galactosemia?
Detecting the disorder early allows for elimination of all dietary galactose and prevents the development of more serious complications
- it is thought that the majority of mental retardation occurs in the womb
Where does fructose enter glycolysis?
- After* control point, PFK-1, as Dihydroxy Acetone Phosphate (DHAP) and Glyceraldehyde-3-Phosphate (G3P)
- this is so it can rapidly produce lactate and/or fatty acids
Where is fructose metabolized?
Mostly in the Liver
What are the reactions that allow fructose to enter glycolysis?
What happens to fructose in tissues other than the liver?
It is phosphorylated to Fructose-6-kinase by hexokinase, then enters glycolysis
How is glucose converted into fructose?
Glucose ==> Sorbitol
(Enzyme: Aldose Reductase)
(NADPH –> NADP)
Sorbitol ==> Fructose
(Enzyme: Sorbitol Dehydrogenase)
(NAD –> NADH)
How does Glycerol enter glycolysis?
Glycerol ==> Glycerol-3-P
(Enzyme: Glycerol Kinase)
(ATP –> ADP)
Glycerol-3-P ==> Dihydroxy Acetone Phosphate
(Enzyme: Glycerol-3-P dehydrogenase)
(NAD –> NADH)
(Also part of the glycerol phosphate shuttle)
How is galactose metabolized?
How do people with lactose intolerance acquire galactose?
UDP-glucose is made from Glucose-1-P by UDP-glucose pyrophosphorylase
UDP-Glucose is then made into UDP-Galactose by epimerase
Why is galactose necessary for life?
UDP-galactose is needed for glycoprotein and proteoglycan synthesis
Why does a build up of galactitol lead to?
In Classical galactosemia, galactose is not metabolized and is then converted to galactitol
- galactitol accumulates in cells, increasing their osmotic pressure and promoting cell swelling
- cell swelling leads to damage of nerves, lenses of the eye, and liver cells
What is Essential Fructosuria?
-Defect: hepatic fructokinase (fructose cannot be made into fructose-1-P)
-
Symptoms: Fructosemia
Fructosuria after fructose injection
Benign condition
What is Hereditary Fructose Intolerance (Type 2)?
-Defect: Hepatic Aldolase B with greater affinity for Fructose-1-P cleavage than F-1,6-BP, but will not cleave F-1-P and will not release it from active site
-Symptoms: get sick when ingest fructose
fasting hypoglycemia
hepatomegaly
failure to thrive
Note: Doesn’t seem to be an issue with breast milk
Why does Hereditary fructose interolerance (Type 2) cause hypoglycemia while fasting?
- Fructose-1-P is a competitive inhibitor of phosphorylase and aldolase
- cells continue to make an accumulation of F1P during fasting which causes depletion of Pi and ATP
- Pi is a substrate for glycogen phosphorylase, therefore, glycogen stores cannot be used to get free glucose
Fill in the blank
- 2 NADP+
- 2 NADPH
- Ribulose-5-Phosphate
- Xylulose-5-Phosphate
- Ribose-5-Phosphate
- Nucleotide synthesis
- CO2
8, 9, 10. Fatty Acid Synthesis
Glutathione Reduction
Other rxns such as detox
PPP Oxidative rxns
Fill in the blank
- Glucose-6-P
- NADP –> NADPH
- 6-Phosphogluconolactone
- H20 –> H+
- 6-Phosphogluconate
- NADP –> NADPH
- Ribulose-5-phosphate
- CO2
- Glucose-6-Phosphate dehydrogenase
- Lactonase
- 6-Phosphogluconate dehydrogenase
What is the overall reaction of PPP oxidative reactions
Glucose-6-phosphate + 2 NADP + H20 ==>
Ribose-5-phosphate+ 2 NADPH + 1 H+ +CO2
What is the flow of the PPP when more Ribose-5-Phosphate than NADPH is needed?
- Oxidative phase is off
- only glycolysis and non-oxidative phase rxns are necessary
What is the flow of the PPP when both NADPH and Ribose-5-Phosphate are required?
- Only oxidative phase is used
- Isomerase converts Ribulose-5-P to Ribose-5-P
What is the flow of the PPP when more NADPH than Ribose-5-Phosphate is needed?
- Both oxidative and non-oxidative rxns are required
- Ribose-5-P is converted back to G3P by non-oxidative and glycolysis rxns
What is the flow of the PPP when both NADPH and pyruvate are required?
- Both oxidative and non-oxidative rxns are required
- G3P is further converted to Pyruvate through glycoslysis
What is the importance of the PPP?
- Produces NADPH for reductions
- Produces pentoses for nucleotide synthesis
What are the pathways requiring NADPH?
Synthesis:
Fatty acid biosynthesis
Fatty acid chain elongation
Cholesterol biosynthesis
Neurotransmitter biosynthesis
Nucleotide biosynthesis
Superoxide synthesis
Detoxification:
Reduction of oxidized glutathione
Cytochrome P450 monooxygenases
Why is NADPH more effective for reductions?
Intracellular ratio of
NADPH/NADP+ >> NADH/NAD+
What is a blood indicator of free radical damage? Why?
Malondialdehyde in blood
- Free radicals extract hydrogen atoms from lipid to form lipid radicals
- reaction with O2 propagates radical chain rxn and forms lipid peroxy radical and lipid peroxide
- electron rearrangements result in lipid degredation
What dietary chemicals can terminate free radical chain reactions?
Antioxidants
i.e. vitamin E
What is the main biological defense against ROSs?
Glutathione
- Glutathione with free sulfhydryl group
- Reduces hydrogen peroxide and lipid peroxides by formation of disulfide
GSH + GSH ==> GSSG
(Enzyme: Glutathione peroxidase)
(H2O2 –> 2H2O)
What is the mechanism that makes Glucose-6-Phosphate Dehydrogenase Deficiency harmful?
Glucose-6-Phosphate Dehydrogenase allows for the oxidative reactions of the PPP to produce NADPH
-NADPH is necessary for Glutathione defenses against ROS damage
- Without G6P Dehydrogenase, cells are vulnerable to
ROSs
What is Glucose-6-Phosphate Dehydrogenase Deficiency?
- X-linked enzymopathy affects males
Affected males usually have 10% normal enzyme activity, sufficient to handle normal oxidative stress - Crisis occurswhen cells are assaulted by high levels of oxidants (i.e. antimalarials)
- RBCs are most susceptible because they cannot repair oxidative damage by replacement of lipids, proteins, etc.
What are Heinz bodies?
Occur in erythrocytes of G6P deficient people after drug exposure
- they are particles of denatured hemoglobin that has become crosslinked, adhering to the RBC membrane
visible when stained with basic dyes
How can a drug deplete NADPH?
In a normal individual, NADPH depletion is fast, but so is the G6P dehydrogenase rxn
In a G6P dehydrogenase deficient individual, NADPH depletion is just as fast, but they are unable to produce NADPH to keep up with the depletion
G6P Dehydrogenase rxn is too slow
What are the three energy systems of Muscles?
- Immediate: “Phosphagens” (ATP and creatine-P)
- Short Term: Anaerobic glycolysis
-
Long Term: Aerobic
- Aerobic glycolysis
- fatty acid oxidation
- The 3 systems are used in overlapping sequence
Why are RBCs more suscleptible to Glucose-6-Phosphate dehydrogenase Deficiency?
Because they do not have the ability to replace and repair affected structures (lipids, proteins, etc) and, thus, cannot otherwise repair oxidative damage
How can thiamine deficiency affect the PPP?
Transketolase is dependent on thiamine pyrophosphate as a cofactor
Reactions in PPP:
Glyceraldehyde-3-P + Sedoheptulose-7-P ==> Fructose-6-P + Erythose-4-P
Erythose-4-P + Xylulose-5-P ==> Fructose-6-P + Glyceraldehyde-3-P
What allows aerobic metabolism to take over long term exercise?
Increased blood flow
What are the approximate time scales of each muscle energy system?
Phosphagens: ~10s
Anaerobic: ~2min
Aerobic: ~2hrs
Which energy system is able to produce the most ATPs per sec? Why?
Phosphagen system:
Creatine Phosphate contains a high energy bond that can easily be given to ADP or AMP:
ADP + phosphocreatine ==> ATP + creatine
Allows for quick production of ATP until phosphocreatine is exhausted
Why was it previously believed that Phosphocreatine and glycogen was not used in moderate exercise?
When measured, PCr and Glycogen stores remain consistent through exercise
- no net loss was observed
When in actuality, PCr and Glycogen stores are restored between muscle twitches
- net loss is only observed during intense exercise because there is not enough time between twitches to restore levels
What are key regulatory steps of metabolism OUTSIDE of the mitochondrial matrix?
GLUT-4 Transporters
Hexokinase
PFK-1
Glycogen phosphorylase
Carnitine Palmityl Transferase I (CPTI)
How is GLUT-4 Regulated?
- Activated by insulin and/or exercise
- Activated by AMP (via AMP-PK)
More GLUT-4 transporters are transported to the cell surface to allow more glucose into the cell
How is hexokinase regulated?
Through Feedback Inhibition:
Limits drain on blood glucose when using glycogen
Hexokinase is inhibited by it’s product, Glucose-6-P
Lack of inhibition can result in depleted phosphate stores, while lack of activation by substrate allows unphosphorylated glucose to leave the cell
How is Glycogen degredation regulated?
- Activated by AMP (Allosterically), epinephrine, Ca2+
Epinephrine activates Adenylate cyclase
Ca2+ and Ca2+-calmodulin help push the reaction of Phosphorylase kinase A (inactive) to Phosphorylated Phosphorylase kinase (active)
AMP helps push the reaction of Phosphorylase B (inactive) to Phosphorylated Phosphorylase B (active) (which breaks down glycogen)
How is PFK-1 Regulated?
- Activated by insulin, F-2,6-BP, and AMP (allosterically)
- Inhibited by citrate, and ATP
How is Carnitine Polmityl Transferase I (CPT-1) regulated?
It is blocked by malonyl-CoA
-Acetyl CoA Carboxylase-2 (ACC-2) synthesizes MCoA from Acetyl-CoA and CO2
(inhibited by AMP-PK)
- Malonyl CoA Decarboxylase decarboxylates MCoA to AcCoA
(Activated by AMP-PK)
What regulation is directly or indirectly activated by AMP?
Glucose transport into cell
Glycolysis
Glycogenolysis
FA transport into Mitochondria
What points of ATP production are controled by NAD+?
Pyruvate -> AcCoA +CO2
TCA Cycle
Beta-oxidation of FA
What is respiratory control?
The process in which ADP controls the rate of oxphos, and the rate in which O2 is used in the cell and ATP produced
What are three major proteins found in connective tissue?
Collagen
Elastin
Laminin
What is elastin?
A major protein found in elastic fibers in the ECM of connective tissue of smooth muscle cells, endothelial and microvascular cells, chondrocytes, and fibroblasts
Allow tissues to expand and contract
particularly important for blood vessels
What is laminin?
After Type IV collagen, most abundant protein in basal laminae
provides additional structural support for tissues
Able to bind Type IV collagen to other molecules present in ECM
What are the major proteins present in connective tissue? What is their purpose?
Collagen: tensile strength of tissue (resist tearing)
Proteoglycan: Resilliency of tissues
Elastic Fibers: Elasticity of tissues
Laminin: Structural support of basal lamina
What is the general structure of collagen?
- All types contain 3 long polypeptide chains with at least one strech wound together to form a triple-helix
General sequence:
GLY-X-Y
(X usually Pro, Y usually hydroxyproline or hydroxylysine)
- Right handed collagen super helix is formed by intertwining three, left-handed helical strands
- collagens are glycoproteins, but the amount of carbohydrate is variable
What is the formula for chain designation of collagens?
Col(collagen type)alpha(chain type)
i.e. three-chain a2b structure that makes up Type I collagen = [Col(I)alpha1]2Col(I)alpha2
What is Type I collagen?
Accounts for 90% of total body collagen and occurs in skin, bone, tendons, cornea, soft tissues, and scars.
Has the least carbohydrate
a2b = [Col(I)alpha1]2Col(I)alpha2
What can defects in Type I collagen cause?
Osteogenesis imperfecta
What is Type II collagen?
Occurs in cartilage (made by chondrocytes) and vitreous humor
10% carbohydrate
c3 = [Col(II)alpha1]3<br></br> (homotrimer)
What is Type III collagen?
Occurs mainly in blood vessel walls, other hollow organs, and fetal skin
(also scars and adult soft tissue)
Contains disulfide bridges between chains
d3 = [Col(III)alpha1]3
(homotrimer)
What can defects in Type III collagen cause?
Ehlers-Danlos type 4
- aortic rupture, GI tract, pregnancy problems
- skin fragility, poor wound healing –> surgical issues
What is Type IV collagen?
Collagen of basement membranes
has disulfide bridges and has highest carbohydrate content
non-fibrillar type, interruptions of the triple helix due to highest content of carbohydrate
Mesh forming collagen
All homotrimers w/different subunits (subunits 1-5):
[Col(IV)alpha(1-5)]3
How are the chemical bonds oriented in a collagen triple helix?
H-bonds occur between each Gly-N-H and the C=O of the succeeding X residue on neighboring chain
Hydroxylation of Y residues give a polar outside surface to stabilize overal structure with H-bonds to H2O
Every 3rd residue must be Gly since there is no room near the helix axis for the side chain of ANY other AA
What is the structure of Procollagen?
Procollagen is formed from preprocollagen after removal of signal peptides in the ER
3 Domains of procollagen:
- Globular (N and C terminal)
- Triple Helical
- Non-triple helical
Aminoprocollagen peptidase cleaves at the non-triple-helical domain near the N terminal
Carboxylprocollagen peptidase cleaves the non-triple-helical domain near the C terminal
Describe the steps of collagen synthesis
- Polypeptide chains of preprocollagen are synthesized on the rER and the signal (pre) sequence is cleaved
- Proline and lysine residues are hydroxylated by a rxn that requires O2 (activated by Fe2+) and Vitamin C
- Galactose and glucose are added to hydroxylysine residues (addition of carbohydrates)
- Triple helix forms, and procollagen is secreted from the cell and cleaved to form collagen
- Cross-links are produced.
What happens biochemically during scurvy?
With a lack of Vitamin C, Hydroxylation of collagen is not possible, leading to:
Loss of Strength
easy bruising and petechiae
bad breath
gum disease
loss of teeth
poor wound healing
What occurs biochemically during Copper deficiency?
Lysyl Oxidase is unable to work and patient presents with tissue dysfunction
Lysyl oxidase converts lysine residues to aldehydes, allowing for intermolecular cross-link reactions to occur in collagen
Fibers are unable to fit together in ECM
What is the general presentation of Ehlers-Danlos sydrome?
A heterogenous group of connective tissue disorders
major manifestations are:
skin fragility
skin hyperextensibility
joint hypermobility
What is the collagen defect and clinical manifestation of Ehlers-Danlos IV?
Defect:
Decrease in Type III collagen
Manifestation:
Arterial, intestinal or uterine rupture
thin, translucent, easily bruised skin
This is because Type III collagen is often found in blood vessels and the GI tract
What is the collagen defect and clinical manifestation of Ehlers-Danlos VI?
**Defect:** Decreased hydroxylysine (deficiency of lysyl hydroxylase)
Manifestation:
Hyperextensible skin and joints
poor wound healing
This is because hydroxylation of lysine allows for stabilization of collagen with H2O Hbonds
What is the collagen defect and clinical manifestation of Ehlers-Danlos VII?
Defect:
N-terminal propetide not cleaved
Manifestations:
Joint hypermobility
easily bruised skin
hip dislocations
Lack of procollagen cleavage prevents collagen from maturing and leaves globular regions of collagen intact
What are the collagen defect and clinical manifestations of Osteogenesis Imperfecta Type I?
Defect:
Autosomal dominant defect causing
Decreased synthesis of Type I collagen
Manifestations:
Blue sclera
long bone multiple fractures prior to puberty
Type I collagen is prevalent in bone formation and is 90% of total body collagen
What are the collagen defect and clinical manifestations of Osteogenesis Imperfecta Type II?
Defect:
Point mutations and exon rearrangements;
defects in Type I collagen
(Autosomal dominant)
Manifestations:
Perinatal lethality
malformed and soft, fragile bones
Dark sclera
absent calvarial (skull) mineralization
What are proteoglycans?
Highly osmotically active extracellular polymers with gel-like properties taht protect cells and provide tissues with mechanical resiliency
Associated with collagen in ECM
Specific properties depend on type and amount of Glycosaminoglycans (GAGs) in structure
Proteo = protein cores (many per single giant molecule)
glycan = carbohydrate chains (glycosaminoglycans)
What are some common Glycosaminoglycans?
Hyaluronate
Chondroitin-6-sulfate
Dermatan Sulfate
Keratan Sulfate
Heparin
What is the cause of Hurler Syndrome?
- Autosomal Recessive disorder caused by deficiency of alpha-L-Iduronidase ==> inability to degrate proteoglycans
- Results in abnormal intra-lysosomal accumulation of dermatan and heparan sulfate ==> cell death, organ dysfunction
- Osmotically active polymers accumulate in tissues:
neuronal involvement leads to delays and retardation
hydrocephalus resulting from meningeal involvement
etc.
Diagnosis at 6-24 Months. Need to demonstrate enzyme deficiency in cultured fibroblasts from patient
- Palliative care
What are symptoms of Hurler syndrome?
- Characterized by:
Developmental delays
mental retardation
coarse facial features with macroglossia
skeletal abnormalties
organomegaly
CV disease; valvular dysfunction
joint stiffness
hydrocephalus
corneal clouding
umbilical and inguinal hernias
