Carbohydrates Flashcards
(163 cards)
1
Q
Carbohydrates are chains of ……… with attached ………. & ……….. groups. The chemical formula is ……….. .
A
carbon, hydrogen, hydroxyl, Cn(H2O)n
*n is minimum of 3 carbons
2
Q
- if the hydroxyl group on a monosaccharides is replaced by hydrogen, it forms……..
- if it is replaced by amino group, it becomes ………..
A
- deoxyribose in DNA
2. glucosamine in proteoglycans
3
Q
Monosaccharides containing aldehyde are known as ………., while those containing keto groups are called ………
A
aldoses, ketoses
4
Q
What is the most important monosaccharide?
A
D-Glucose
5
Q
Aldehyde formula is ……….
while keton formula is ……….
A
R-C(=O)H
R-C(=O)R
6
Q
Monosaccharides numbering starts with the carbon atom nearest to ………..
A
carbonyl group
7
Q
Define the Anomeric carbon
A
The carbonyl group carbon in monosaccharides
8
Q
Define Penultimate carbon
A
The carbon next to the last carbon in the chain
9
Q
Define D & L isomers
A
The configuration of hydroxyl group around the penultimate carbon atom. If it is on the right, it is a D-sugar, on the left is L-sugar.
*most important monosaccharided are D-sugars
10
Q
Define Epimers
A
Term used to describe two monosaccharides that share the same formula but differ in configuration around a single carbon atom (like glucose and galactose).
Enzymes that catalyze the interconversion between epimers are called epimerases
11
Q
Glucose differs from fructose in the position of carbonyl atom. T/F
A
True. Glucose is an aldose, while fructose is considered a ketose (carbonyl is at C-2)
12
Q
The ring structure in monosaccharide is formed between what components?
A
It is formed between the hydroxyl group of the penultimate carbon with the anomeric carbon.
13
Q
The anomeric carbon can exist in two configuration in the ring form. T/F
A
True. The alpha (hydroxyl down), and the beta (hydroxyl up)
14
Q
Glucose can be modified into:
- Gluconic acid by ………
- Glucouronic acid by……..
- Glucosamine by………..
A
- Oxidation of the aldehyde at C-1. (the phosphorylated form is important in HMS)
- Oxidation of alcohol at C-6 producing uronic acid (important in proteoglycans)
- By substitution of an amino group for the hydroxyl at C-2 (important in proteoglycans and glycoproteins)
15
Q
Disaccharides are formed by……..
A
two monosaccharides with a glycosidic linkage connecting the anomeric carbon and a hydroxyl group on the second monosaccharide.
The bond is designated as alpha or beta depending on the anomeric carbon configuration in the linkage
16
Q
Galactose and glucose share the same formula. Galactose is an ……..of glucose
A
Epimer. The difference the hydroxyl group configuration on C-4
17
Q
Lactose is hydrolyzed to ………. and …… by lactase which is found along the brush border membrane of the small intestine
A
Glucose, Galactose
18
Q
The monosaccharides in lactose are joined by what linkage? while in maltose they are joined by ….
A
beta-1,4 glycosidic linkage
alpha-1,6 glycosidic linkage
19
Q
Sucrose is made of …… and …… which are linked through their anomeric carbons by a ……. linkage.
A
glucose, fructose
alpha-1,2 glycosidic
* sucrose is hydrolyzed by sucrase found along the brush border in small intestine
20
Q
Define oligosaccharides
A
have between 2 and 10 monosacchrides linked by glycosidic bonds. Found in mucoproteins and glycolipids
21
Q
Define polysaccharides and list their functions
A
Have over 10 monosaccharides.
- Structural component
- storage
- dietary fiber
* The most common are starch, glycogen, cellulose and proteoglycans
22
Q
Define starch and list its types
A
Starch is the plant polysaccharides
It is two types.
Amylose is a long unbranched chain of monosaccharides connected by a alpha-1,4 bonds.
Amylopectin is same as amylose but with branches connected by alpha-1,6 bonds
23
Q
Define Glycogen
A
Is the animal polysaccharides. Structurally similar to amylopectin but is more branched
24
Q
Define cellulose
A
Linear plant polysaccharide. Composed of glucose units linked together by beta-1,4 bonds.
*There is no enzyme in humans that hydrolyzes the bonds between glucose units linked by beta-1,4
25
Define proteoglycans
polymers of monosaccharides linked covalently to a protein core (rich in serine & threonine)
The carbohydrate portion of proteoglycans is called glycosaminoglycan
Proteoglycans are the major structural component of extracellular matrix
26
Define glycosaminoglycan GAG
The polysaccharide portion of proteoglycan. A chain of repeated disaccharides, usually hexosamines and uronic acids.
The polymers are usually sulfated, except hyalouronic acid (which is non sulfated and not connected to a protein core, it doesn't form a proteoglycan). Hexosamines are acylated
27
Why proteoglycans serve as a shock absorber?
Because they are highly asymmetrical and have a high density of negative charges allowing them to absorb large amount of water.
28
Why Staphylococci can invade connective tissue in humans?
because it has hyalouronidase activity (degrades hyalouronic acid)
29
What is the difference between proteoglycans and glycoproteins?
Glycoprotein also surround a protein core, but there's no repeating disaccharides like proteoglycan. Also it has fewer number of monosaccharide (oligosaccharides)
The amino acid side chains of carbohydrates involved in the linkage are:
1. Asparagine: in plasma and cell surface
2. Serine: in mucous and connective tissue
3. 5-hydroxylysine: collagen
* TSH, laminin, alkaline phosphatase are also glycoproteins
30
Glycoproteins are found in ........
1. Plasma
2. Connective tissue (collagen)
3. On cell surface as antigens
4. Mucus
31
Where is the glycoproteins synthesized?
in the endoplasmic reticulum and the packaging is completed in Golgi.
32
What are GLUT?
Proteins in the plasma membrane that transport glucose into cells
33
Why phosphorylation of glucose occurs inside the cells?
To keep the intracellular concentration of glucose low favoring more glucose uptake by the cells.
* Phosphorylation is coupled with GLUT function
* The membrane is impermeable to phosphorylated compounds which are negatively charged
34
GLUT proteins differ in?
1. Rate of transport
2. Affinity for glucose
3. Tissue specificity
35
What is the location of GLUT 4? And what are they sensitive to ?
in skeletal muscles and adipose tissue. These are sensitive to insulin (which increases their function)
36
What is the location of GLUT 2? Why are they different that the other GLUTs?
The liver. These have low affinity for glucose (high Km) and are not saturated by increased glucose conc. in the portal circulation. In fasting state, this high Km ensures more glucose is delivered to other tissues than taken up by the liver
37
What are S-GLUT?
These are GLUT proteins located in the kidney and intestinal membrane. They require sodium for glucose transport (cotransport -- symported)
38
What enzymes are required for glucose phosphorylation inside the cells?
1. Hexokinase in most cells, inhibited by increased G-6-P inside the cell (this enzyme has low Km and LOW Vmax)
2. Glucokinase for liver cells. Not inhibited by G-6-P inside the hepatocytes (has high Km and HIGH Vmax). Most effective when glucose level is very high in the portal vein
39
Where does glycolysis take place?
in the cytosol
40
Glycolysis converts .........
glucose to pyruvate
41
In glycolysis, ..... ATP consumed and ...... are produced per mole of glucose
2, 4
* net production is 2
* This is called the embden-meyerhoff pathway
42
Under aerobic conditions, one mole of glucose completely oxidized to CO2 and H2O result in ..... ATP
36-38
43
Under anaerobic conditions, what happens to pyruvate and why?
Pyruvate is converted to lactate in order to regenerate NAD+ from NADH for glycolysis to continue.
44
How many ATPs produced during anaerobic respiration?
2 ATP only
45
The most common form of glucose in the body is ?
beta-D-glucose
46
Glycolysis is divided into two stages. What are they?
First stage that consumes ATP
| Second stage that produces ATP
47
What is the difference between PFK-1 and PFK-2?
PFK-2 produces Fructose-2,6-bisphosphate
48
Key enzymes in stage 1 glycolysis are?
```
Hexokinase (or glucokinase)
Phosphofructokinase 1 (PFK-1)
```
49
What is the rate limiting step in glycolysis?
conversion of fructose-6-phosphate to fructose-1,6-bisphosphate by PFK-1
This is the primary site of regulation
50
What is the function of PFK-2?
Formation of fructose-2,6-bisphosphate which is an allosteric activator for glycolysis and inhibitor for gluconeogenesis
see p.477
51
Function of PFK-1?
add a phosphate group to C-1 of Fructose-6-phosphate, with the formation of fructose-1,6-bisphosphate
52
The cleavage of fructose-1,6-bisphosphate by aldolase yields ..... & ......, which are interconvertible by......
dihydroxyacetone phosphate, glyceraldehyde-3-phosphate
* these two are interconvertible by triosephosphate isomerase, that's why from this step each mole of glucose can be considered to produce 2 mols
53
What intermediates in stage 2 glycolysis have enough energy to drive the production of ATP?
1,3 bisphosphoglycerate, phosphoenolpyruvate
54
What are the important enzymes in stage 2 glycolysis?
1. Glyceraldehyde-3-phosphate dehydrogenase (produce NADH)
2. 3 phosphoglycerate kinase (produces ATP)
3. pyruvate kinase (produces ATP)
4. Lactate dehydrogenase is also important in anaerobic conditions
55
...... is the only oxidation reaction in glycolysis
oxidation of glyceraldehyde-3-phosphate
see p.478
56
what is the function of phosphoglycerate mutase?
moves the phosphate group from carbon 3 to carbon 2, forming 2-phosphoglycerate in glycolysis
57
What is the function of enolase?
dehydrates 2-phosphoglycerate to form phosphoenolpyruvate, which is a high energy compound.
58
What is the function of lactate dehydrogenase?
Reduces pyruvate to lactate, using NADH to generate NAD+ which is important to continue glycolysis under anaerobic conditions
59
What is the fate for lactate?
transported to the liver where it can be used to resynthesize glucose
60
PFK-1 is inhibited by molecules that indicates high energy status like ....... and .....
ATP, citrate
61
Pyruvate kinase is inhibited by ........ and activated by......
ATP, Acetyl-CoA
| it is activated by accumulation fructose-1,6-bisphosphate
62
Most important activation of glycolysis in the liver is ..........
fructose-2,6-bisphosphate
| This compound is increased by insulin and decreased by glucagon
63
In the liver, pyruvate kinase is inhibited by .....
phosphorylation (covalent regulation)
64
What are the possible fates of pyruvate?
1. In the cytosol, it is reversibly converted to lactate (anaerobic respiration) or to alanine (a vehicle for transporting aminogroups from the muscles to liver to incorporate into urea.
2. In the mitochondria, it is irreversibly carboxylated into oxaloacetate, which enters TCA or gluconeogenesis. Or it is converted to Acetyl-CoA (used for fatty acid synthesis, or oxidized in TCA for ATP production)
see p.480
65
What is Pyruvate dehydrogenase enzyme (PDH)?
converts pyruvate to Acetyl-CoA in the mitochondria by oxidative decarboxylation. This reaction is irreversible
Pyruvate (C3)+CoA+NAD ------> Acetyl-CoA (C2)+NADH+CO2
* The enzyme is composed of 3 parts (decarboxylase, dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase)
* The reaction requires 5 coenzymes (FAD, NAD+, thiamin pyrophosphate, lipoic acid, CoA)
66
Pyruvate dehydrogenase is inhibited by?
ATP, CoA, NADH
67
When PDH is phosphorylated, it is active. T/F
False. It is inactive.
* PDH phosphorylation is catalyzed by kinase driven by increased ATP & NADH
* PDH dephosphorlyation is catalyzed by phosphatase signaled by insulin
see p.482
68
Why PDH regulation is very important?
Because it conserves carbohydrates and proteins when fatty acids are being oxidized.
* all 3 fuels produce Acetyl-CoA and NADH
69
PDH enzyme complex is similar to what enzymes?
1. alpha ketoglutarate dehydrogenase in TCA cycle
2. Keto acid dehydrogenase in aminoacid catabolism
* all three enzymes share the same requirement for the 5 coenzymes (NAD, FAD, CoA, Thiamine pyrophosphate, lipoic acid)
* All three enzymes catalyze the oxidative decarboxylation of alpha-ketoacids
70
What are the net products of anaerobic respiration?
2ATP, 2 lactate
| * no NADH. Because NADH from oxidation of glyceraldehyde-3-p will be used to reduce pyruvate into lactate
71
What are the net products of aerobic respiration?
2ATP, 2NADH, 2pyruvate
72
Why the cytosolic NADH oxidation in the mitochondria yields 2 or 3 ATPs?
it depends on the type of transportation (shuttle). Transport of electrons from the cytosol via malate produces NADH and 3 ATP. While alpha-glycerol phosphate shuttle produces FADH2 and 2 ATP
see p.483
73
What is the effect of Fluoride on enolase?
fluoride inhibits enolase by combining with 2-phosphoglycerate and making the substrate unavailable
74
what are the sites for gluconeogenesis?
Liver, kidney and epithelium.
| *The pathway requires both mitochondial and cytosolic enzymes (pyruvate carboxylase is mitochondrial)
75
What is the difference between gluconeogenesis and glycogenolysis?
Glycogenolysis gives rise to preformed glucose. While gluconeogenesis is the synthesis of glucose from C3 and C4 precursors
76
Function of gluconeogenesis?
provide glucose for RBC and CNS when glucose level is low and the liver glycogen is depleted in fasting state.
77
What are the primary precursors for gluconeogenesis?
1. Lactate in RBC and muscles
2. Aminoacids from protein degradation in skeletal muscles
3. glycerol from triacylglycerol degradation in adipose tissue
78
In gluconeogenesis, in order to pass the irreversible pyruvate kinase reaction, what reactions must occur?
Two irreversible reactions occur
1. Pyruvate carboxylase: converts pyruvate into oxaloacetate. Operates in the mitochondria (the only enzyme from gluconeogenesis in the power house, rest of them in cytosol). This is an anaplerotic reaction
2. PEPCK: converts oxaloacetate into phosphoenolpyruvate (PEP)
79
Function of biotin in pyruvate carboxylase reaction?
A carrier for the activated carboxyl group derived from the bicarbonate. One ATP is needed for the activation
See P. 485 for the reaction
80
What is the function of PEPCK reaction?
Phosphorylation and then decarboxylation of oxaloacetate to produce phosphoenolpyruvate. One GTP is consumed in this reaction. This reaction occurs in the cytosol.
81
A total of ...... ATP is consumed to convert pyruvate to phosphoenolpyruvate
Four
| * This is for two moles of pyruvate. For one mole conversion, only 2 is needed
82
Function of Fructose-1,6-bisphosphatase enzyme in gluconeogenesis?
Pass the irreversible reaction of PFK-1 to hydrolyze Fructose-1,6-bisphosphate to fructose-6-phosphate and inorganic phosphate.
* This is the rate limiting step in gluconeogenesis
83
How is glycolysis inhibited when gluconeogenesis is activated and vice versa?
Because the the activators for gluconeogenesis (ATP, Citrate) are the inhibitors for glycolysis, and the inhibitors (AMP, fructose-2,6-bisphosphate) are the activators for glycolysis.
The regulated enzymes are fructose-1,6-bisphosphatase (gluconeogenesis) and PFK-1 (glycolysis)
84
What is the last step in gluconeogenesis?
The removal of phosphate from glucose-6-phosphate by glucose-6-phosphatase and the release of glucose
* This enzyme is associated with the endoplasmic reticulum (cytosolic enzyme)
85
Why muscle glycogen can not serve as a source for blood glucose??
Because the enzyme glucose-6-phosphatase is found only in the liver, kidney and the intestine.
86
How many ATPs are needed to reverse the 3-phosphoglycerate kinase reaction in glycolysis during gluconeogenesis?
two
87
What is the function of Cori cycle?
To conserve glucose by recycling lactate from RBCs and skeletal muscles. Lactate is taken up by the liver and reconverted to glucose via gluconeogenesis
88
Function of Alanine cycle?
To reconvert alanine released by skeletal muscles into glucose. Alanine is taken up by the liver and is converted to pyruvate, which is used for glucose synthesis
See. P 347
89
What aminoacids are non glucogenic and why?
Leucine and lysin
These are strictly ketogenic and can not be converted to glucose (which is an aldose)
* The nitrogen arising from the conversion reaction is disposed of as urea
90
The hydrolysis of Triacylglycerol in adipose tissue produces ...... The liver can convert it to ......... which is a gluconeogenic intermediate
1. glycerol
| 2. glycerol-3-phosphate, which can be oxidized to dihydroxyacetone phosphate
91
The oxidation of odd numbered fatty acids produces ........, which is converted to succinyl-CoA and can be used for gluconeogenesis
Propionyl-CoA
| * Even numbered fatty acids produces Acety-CoA, which is not a glucose precursor.
92
Fructose-1-phosphate can be cleaved into ....... and ....... which are glucogenic intermediates.
glyceraldehyde, dihydroxyacetone phosphate
| * glyceraldehyde can be reduced to glycerol or phosphorylated to glyceraldehyde-3-phosphate
93
What are the methods for regulating gluconeogenesis??
1. Substrate availability: during fasting, aminoacids derived from protein degradation is the source for gluconeogenesis. Fatty acids oxidation provide the energy required.
2. Enzymatic control
3. Hormonal control
94
Acetyl-CoA inhibits ........ of glycolysis and activates ....... gluconeogenesis
Pyruvate kinase, pyruvate carboxylase
95
Fructose-1,6-bisphosphatase of gluconeogenesis is activated by .......... and inhibited by ...........
activated by ATP, Citrate. Inhibited by AMP and fructose-2,6-bisphosphate
* Those compounds have the opposite effect on PFK-1 of glycolysis
96
Glucose-6-phosphatase and glucokinase both have ...... Km value
High
* they are both not saturated under normal condition
see p.488
97
Glucagon ....... glucose synthesis and release
promotes
| * Opposing the effect of insulin
98
Increase glucagon level leads to ....... cAMP
increase
| * cAMP decreases in response to insulin
99
What is the second site for glycolysis regulation and why is it important??
The conversion of phosphoenolpyruvate into pyruvate (catalyzed by pyruvate kinase).
This reaction is irreversible
* One ATP is formed from this reaction
100
What is the effect of glucagon on fructose-2,6-bisphosphate?
Glucagon leads to its degradation, hence removing its stimulation of PFK-1 (glycolysis) and inhibition on fructose-1,6-bisphosphatase (gluconeogenesis)
101
Glucacgon promotes the synthesis of gluconeogenesis enzymes. T/F?
True. It promotes PEPCK, fructose-1,6-bisphosphatase, glucose-6-phosphatase
* Synthesis of these enzymes also require catecholamines and glucocorticoids
102
Glucagon ....... fatty acid and protein degradation
increases
* proteins to supply aminoacids for gluconeogenesis
* Fatty acids oxidized in the liver for energy
103
What is the relation between alcohol and hypoglycemia?
Oxidation of ethanol in the liver leads to accumulation of NADH and depletion of NAD+
* NAD+ is need for oxidation of lactate to pyruvate and alpha-glycerol-phosphate to DHAP. This leads to gluconeogenesis failure.
* Ethanol is oxidized to acetaldehyde, which in turn is oxidized to acetate. Leads to steatosis
104
What is the effect of glucagon on pyruvate kinase??
Glucagon leads to its phosphorylation, and hence inactivation
105
What is the effect of branching in glycogen?
Branching increases solubility and break down
106
What is glycogenin? and what is its function?
Glycogenin is the core protein in glycogen particle, and the primer for the synthesis of the first 8 glucose molecules in glycogen
107
All the synthesis and degradation of glycogen occur at the ............
end of branching points
108
What is the difference between white and red muscles in regard to glycogen metabolism?
```
White muslces (fast) degrade glycogen to uses glycolysis to generate ATP from the glucose. The end product is lactate.
While in red muscles (slow), Glycogen is degraded and pyruvate is completely oxidized in TCA cycle.
```
109
The most important enzyme in glycogenolysis is ..........., with the function of ..........
glycogen phosphorylase, cleaving the alpha-1,4-glycosidic linkage by the addition of phosphate
110
Define glycogen phosphorolysis?
The addition of inorganic phosphate to cleave the alpha-1,4-glycosidic bonds by the enzyme glycogen phosphorylase. The molecule released is glucose-1-phosphate
* This is the rate limiting step in glycogenolysis
111
Glycogen phosphorylase becomes ineffective in degrading glycogen when......
it gets 4-5 units away from the branching point on the partially degraded glycogen
112
What is the function of alpha-1,4-glucan transferase?
cleaving 4 glucose units from the branch point and transfer them to the end of another chain (another alpha-1,4 bond is formed)
113
What is the function of alpha-1,6-glucosidase?
Removes the remaining glucose unit from the branch point on the partially degraded glycogen
114
Complete glycogenolysis requires the following enzymes?
1. glycogen phosphorylase
2. alpha-1,4-glucan transferase
3. alpha-1,6-glucosidase
115
Before glucose-1-phosphate is added to the glycogen primer, it must be activated by .........
formation of UDP-glucose
| * most monosaccarides react with UTP to form UDP-sugar
116
In glycogenesis, glucose-6-phosphate is converted to glucose-1-phosphate by the enzyme.......
phosphoglucomutase
117
In glycogenesis, the formation of UDP-glucose is catalyzed by ......
UDP-glucose pyrophosphorylase
118
One of the phosphate groups in UDP-glucose is derived from ....., the other is from ......
1. glucose-1-phosphate
| 2. UTP (uridine triphosphate)
119
Glycogen phosphorylase in muscles is activated by ...... and inhibited by ......
1. epinephrine, Ca (glucagon in liver)
| 2. Insulin, Glucose-6-phosphate, ATP
120
Glycogen phosphorylase in liver is activated by .... and inhibited by ....
1. Glucagon, epinephrine
| 2. Insulin, glucose
121
Glycogen synthase is activated by ..... and inhibited by ....
1. glucose, insulin
| 2. glucagon, epinephrine
122
Glycogenesis is coupled to ..... intake in cells.
K+
| * Insulin and glucose are given to treat hyperkalemia (insulin shifts K back into the cells)
123
The linkage between glucose and UDP is high energy bond. T/F
True
124
What is the function of glycogen synthase enzyme?
The transfer of glucose from UDP-glucose to the end of an existing glycogen chain
125
If glycogen is totally degraded, what initiates glycogenesis?
The protein core (glycogenin) starts the initial synthesis.
126
The rate limiting step in glycogenesis is ........
The reaction catalyzed by glycogen synthase, which leads to the formation of alpha-1,4-bonds
127
Branching points in glycogen is created by ......
alpha-1,4 alpha-1,6 glucan transferase, which creates an alpha-1,6-linkage
* there is always at lease 4 glucose molecules between branch points
128
The primary mode of regulation for glycogen synthase/phosphorylase is .....
hormonal
| * the hormone action is mediated by phorphorylation/dephosphorylation
129
Activation of glycogen phosphorylase is initiated by .......
glucagon in liver, or epinephrine in muscle
130
Explain why the binding of few hormone molecules initiates the release of millions of glucose-1-phosphate in glycogenolysis
The binding of a single glucagon to the liver receptor increases the synthesis of cAMP, which leads to activation of cAMP kinase (protein kinase A). Each molecule of cAMP kinase leads to phosphorylation of many phosphorylase kinase molecules, and each molecule of the latter can phosphorylate many molecules of glycogen phosphorylase.
131
What is the function of phosphorylase kinase enzyme?
The activation of glycogen phosphorylase in glycogenolysis
132
In skeletal muscle, phosphorylase kinase enzyme can be activated by the binding of .....
Calcium. So the same molecule that activates contraction in muscles, activates glycogenolysis at the same time (without activation by protein kinase A)
133
The activity of glycogen phosphorylase can be fine tuned by the accumulation of allosteric effectors although the primary mode of regulation is covalent modification. T/F
True
see p.493
134
In glycogenolysis, the activation protein kinase A leads to phosphorylation of ......, which in turn phosphorylates ......
1. Phosphorylase kinase
| 2. glycogen phosphorylase
135
Phosphorylation of glycogen phosphorylase leads to its ......., while phosphorylation glycogen synthase leads to its .......
activation, inactivation
136
The phosphorylated form of glycogen synthase can be activated by .......
glucose-6-phosphate
137
For glycogen synthase, the D form is the ...... while the L form is the .....
1. inactive (phospho enzyme)
| 2. active (dephospho)
138
HMP shunt branches off at .......... in glycolysis then re-enters at ........
glucose-6-phosphate
fructose-6-phosphate
* The HMP shunt pathway occurs in all cells
139
The products of the HMP shunt are ........., ....., .......
CO2, NADPH, pentose phosphate
140
HMP shunt pathway produces ATP. T/F?
False
141
The function of HMP shunt is ......... and ........
1. supply pentoses: the synthesis of nucleotides and some coenzymes requires ribose-5-phosphate
2. NADPH: required in the reductive reactions for synthesis of fatty acids, cholesterol and steroids
142
Why RBCs have high level of NADPH?
to maintain the reduced form of glutathione, which prevents hemolysis by neutralizing the effect of strong oxidizing agents (superoxide and hydrogen peroxide)
143
Why the absence of either NADPH or NADPH oxidase results in chronic infection?
because NADPH oxidase reduces molecular O2 to superoxide in phagocytic cells, which is used in these cells cidal process
144
The reactions in the HMP shunt pathway can be divided into ...... and ...... phases
oxidative, nonoxidative
145
The oxidative phase of HMP shunt produces ....... NADPH per mol of glucose
two
* the oxidative phase consist of 3 irreversible reactions
* glucose-6-phosphate is oxidized to lactone, forming one NADPH. Lactone is hydrolyzed to gluconate by lactonase. Then gluconate is oxidized to form another NADPH, CO2 and ribulose-5-phosphate
146
The major function of the nonoxidative phase of HMP shunt pathway is to ......
recycle the excess pentoses.
* The nonoxidative phase consist of reversible reactions
* Ribulose-5-phosphate is isomerized to ribose-5-phosphate, which is used for nucleotide synthesis. The excess is recycled.
* Transketolase: transfers C-2 units using thiamine pyrophosphate as a C-2 carrier
* Transaldolase: transfers C-3 units
see p.497
147
For every ..... mols glucose-6-phosphate that enters the HMP shunt, ...... mols of CO2 are produced, and .... mols of fructose-6-phosphate are returned to glycolysis
6,6,5
148
The rate limiting reaction in the HMP shunt pathway is ..............
initial reaction catalyzed by glucose-6-phosphate dehydrogenase
149
glucose-6-phosphate dehydrogenase amount is increased in ....... and ........ after a carbohydrate rich meal.
liver and adipose tissue.
| * High carb meals lead to fatty acid synthesis, which requires a lot of NADPH
150
Glucose-6-phosphate dehydrogenase is allosterically activated by ........... and inhibited by ...........
NAD+
| NADH and palmitoyl-CoA
151
The primary site for fructose metabolism is the ........ . It is derived from the hydrolysis of sucrose at the ........
liver
| brush border of intestine
152
The three enzymes that metabolizes fructose in the liver are...........
aldolase B, fructokinase, glyceraldehyde kinase
* they convert fructose into trioses intermediates in glycolysis
* the rapid phosphorylation of fructose may lead to low levels of intracellular phosphate and diminished ability to synthesis ATP
153
Why fructose is metabolized much faster than glucose?
because it bypasses the rate limiting step in glycolysis.
154
Fructose is converted into fructose-1-phosphate by ..........
fructokinase enzyme
155
Fructose-1-phosphate is split into two C3 segements by ......., producing ........ and ......
aldolase B
| glyceraldehyde and DHAP
156
Glyceraldehyde can be reduced to ........, or phosphorylated to ........
glycerol
| glyceraldehyde-3-phosphate (catalyzed by triokinase)
157
Deficiency of fructokinase results in ..........
Fructosuria (fructose in blood and urine)
158
Galactose metabolism occur almost entirely in the ........
liver
159
What are the enzymes required for galactose metabolism??
1. galactokinase: phosphorylation of galactose into galactose-1-phosphate
2. Uridyl transferase: exchange of galactose-1-phosphate for glucose-1-phosphate moiety of UDP-glucose
3. UDP-galactose-4-epimerase: recycle UDP-galactose to UDP-glucose
160
Reducing sugars contain .......
Aldehyde groups that are oxidized to carboxylic acids
Example is glucose, galactose, lactose, fructose (after isomerization)
* non reducing sugars like sucrose
161
Inulin is a .....
polymer of fructose joined by beta-2,1 glycosidic linkage. Highly soluble, used to determine the GFR
162
Dextran is a polymer of ....., while Levan is a polymer of ....
glucose, formed from the hydrolysis of sucrose via glucosyl transferase
* Formed and used by the S. Mutans to attach to tooth surface
* Levan is a polymer of fructose. Also used by S mutans for the same purpose
163
Entner-doudoroff pathway is .....
an alternative to glycolysis, occurs in aerobic bacteria
* Converts glucose to pyruvate + glyceraldehyde-3-phosphate
* Yields 1 ATP, 1 NADH, and 1 NADPH
