Biochemistry - Metabolism (Carbohydrate) Flashcards
1
Q
Which metabolic processes occur in only mitochondria?
A
Fatty acid production (B-oxidation), acetyl-CoA production, TCA cycle, oxidative phosphorylation, ketogenesis.
2
Q
Which metabolic processes occur in only cytoplasm?
A
Glycolysis, fatty acid synthesis, HMP shunt, protein synthesis (RER), steroid synthesis (SER), cholesterol synthesis.
3
Q
Which metabolic processes occur in both the mitos and the cytoplasm?
A
Heme synthesis, Urea cycle, Gluconeogenesis. (HUG!).
4
Q
What does a kinase do?
A
Uses ATP to add high-energy phosphate group onto substrate (Eg, phosphofructosekinase).
5
Q
What does a phosphorylase do?
A
Adds inorganic phosphate onto substrate without using ATP (eg glycogen phosphorylase).
6
Q
What does a phosphatase do?
A
Removes phosphate group from substrate (eg fructose-1,6-bisphosphatase)
7
Q
What does a dehydrogenase do?
A
Catalyzes oxidation-reduction reactions (eg pyruvate dehydrogenase)
8
Q
What does a Hydroxylase do?
A
Adds hydroxyl group -OH onto substrate (eg tyrosine hydroxylase)
9
Q
What does a carboxylase do?
A
Transfers CO2 groups with the help of biotin (eg pyruvate carboxylase)
10
Q
What does a mutase do?
A
Relocates a functional group within a molecule (eg Vitamin12 dependent methylmalonyl-CoA mutase).
11
Q
How are carbohydrates in the diet broken down to glucose?
A
Salivary amylase in the mouth, and intestinal amylase in the small intestine break down to tri and disacchs. Intestinal saccharidases such as sucrase and lactase convert from tri/di to monosaccharides.
12
Q
How do mono/disaccharides cross the intestinal wall?
A
SGLT is an ATP-requiring Na+/Glu cotransporter on enterocytes that brings glucose into the cell. GLUT-2 brings glucose into the blood from the enterocyte.
13
Q
How does glucose get into cells in the body?
A
GLUT1, GLUT2, and GLUT4.
14
Q
Where is GLUT1 found?
A
High affinity transporters found on brain capillaries and RBCs.
15
Q
Where is GLUT2 found?
A
Liver and pancreas. Not saturated at physiologic conditions.
16
Q
Where is GLUT4 found?
A
Skeletal muscle, heart, adipose cells. These transporters are insulin sensitive.
17
Q
What does the first step of glycolysis accomplish?
A
Phosphorylation of glucose to trap it in the cell. Perfomed by hexokinase (most tissues exc liver and panc betas) or glucokinase (liver, panc betas). Requires ATP.
18
Q
What are the important features of hexokinase?
A
High affinity for glucose (low Km); can phosphorylate glucose even when plasma concentration is low. Inhibited by reaction product glucose-6-phosphate. Has a lower capacity (low Vmax).
19
Q
What is the main purpose of glucokinase?
A
Glucokinase functions as a glucose sensor in the liver and pancreas, and in states of hyperglycemia, enhances liver metabolism of glucose and pancreatic insulin secretion; GLUT-2 transporters on these cells facilitate large loads.
20
Q
How do the kinetics and feedback mechanisms of glucokinase differ from hexokinase?
A
Glucokinase as a higher Km (lower affinity), so requires high glucose concentration to work. Has a higher Vmax, can handle higher capacity. Glucokinase is not inhibited by G6P, but by fructose,6P instead. Glucokinase is induced by insulin.
21
Q
What does a gene mutation of glucokinase cause?
A
MODY - maturity onset diabetes of the young.
22
Q
What two steps of glycolysis require ATP?
A
Step 1 and Step 3 - phosphorylation of glucose as it enters the cell, and conversion of fructose6P to fructose 1,6P by phosphofructokinase.
23
Q
What is the rate limiting step of glycolysis? What enzyme does this?
A
Conversation of fructose6P to fructose16P by phosphfructokinase. Requires ATP, irreversible.
24
Q
What inhibits phosphofructokinase?
A
ATP, citrate.
25
What activates phosphofructokinase?
AMP, and fructose26biphosphonate?
26
When is fructose26biphosphonate created, and why?
Created from fructose6P by PFK2, which is active in the fed state. Activates PFK1, enhancing glycolysis.
27
What happens to PFK2 in the fasting state?
In the fasting state, glucagon is released, which leads to phosphorylation of PFK2 by protein kinase A, and it becomes FBPase-2, which decreases F,26,BP, and increases F6BP. This enhances gluconeogenesis.
28
What two steps of glycolysis produce ATP?
Conversion of 1,3BPG to 3PG by phosphoglycerate kinase and conversion of PEP to pyruvate by pyruvate kinase.
29
What activates/inhibits pyruvate kinase?
fructuose1,6BP activates. ATP and alanine inhibit.
30
What is the net production of ATP per 1 molecule of glucose by glycolysis?
2 ATP, and 2NADH (if lactate not produced).
31
What is the effect of arsenic on glycolysis?
Reduces to net zero ATP. Bypasses the production of 1,3BPG, thus skipping an ATP generating step. (x2 for the other half of the molecule).
32
What are the 4 possible pathways for pyruvate?
1. Lactate (lactate dehydrogenase)
2. Alanine (ALT)
3. Acetyl CoA + NADH (pyruvate dehydrogenase)
4. Oxaloacetate (pyruvate carboxylase)
33
Under what conditions is pyruvate converted to lactate?
Cells without mitochondria, or in low oxygen states (anaerobic glycolysis), or when there is a high NADH/NAD+ ratio that exceeds the capacity of the respiratory chain to take NADH.. Major pathway in RBCs, WBCs, kidney medulla, lens, cornea, testes.
34
What does lactate dehydrogenase require?
b3 (niacin).
35
What is the purpose of pyruvate dehydrogenase and when is it active?
Mitochondrial enzyme complex linking glyolysis and the TCA cycle. Active in fed states.
36
What are the 5 cofactors necessary for pyruvate dehydrogenase?
```
B1 (thiamine; pyrophosphate/TPP)
B2 (FAD, riboFlavin)
B3 (NAD, niacin)
B5 (CoA, pantothenic acid)
Lipoic acid
```
37
What inhibits lipoic acid and what does it lead to?
Arsenic. Vomiting, rice water stools, garlic breath.
38
In what three ways does exercise activate pyruvate dehydrogenase?
Inc NAD+/NADH ratio. Inc ADP. Inc Ca2+.
39
What other complex is pyruvate dehydrogenase complex similar to?
Alpha ketoglutarate complex in TCA cycle. (converts alpha ketoglutarate to succinyl-CoA)
40
What happens with pyruvate dehydrogenase complex deficiency?
X-linked disorder, causes building up pyruvate --> lactate/alanine. Findings: Lactic acidosis, neurologic deficits, elevated serum alanine in infancy.
41
What is the treatment for pyruvate dehydrogenase deficiency?
Increased intake of ketogenic nutrients: high fat content or increased Lysine and Leucine. (onLy pureLy ketogenic amino acids).
42
What is the purpose of pyruvate carboxylase and what does it require?
Creates oxaloacetate, which can be put into the TCA cycle, or used for gluconeogenesis. Requires biotin.
43
What does the TCA cycle produce per acetylCoA?
3 NADH, 1 FADH2, 2 CO2, 1 GTP, which translates to about 10ATP.
44
What is the order of molecules in TCA?
Citrate Is Krebs Starting Substrate For Making Oxaloacetate. (Citrate, isocitrate, alphaKetoglutarate, SuccinylCoA, Succinate, Fumarate, Malate, Oxaloacetate).
45
Which enzyme in the TCA is similar to pyruvate dehydrogenase complex? What step?
alphaketoglutarate dehydrogenase also requires B1,2,3,5 and lipoic acid.
46
What are the "ins" of the ox phos chain?
NADH (complex I), FADH2(complex II), oxygen(complex IV).
47
How do the NADHs from glycolysis get inside the mitochondria?
Malate-aspartate or glycerol-3-phosphate (triose) shuttles.
48
How does the oxidative phosphorylation chain work?
Flow of electrons creates a proton gradient that, coupled to ox phos, drives the production of ATP by Complex V, ATP synthase.
49
How much ATP is produced per NADH?
2.5
50
How much ATP is produced per FADH2?
1.5
51
How much ATP is produced from one molecule of glucose undergoing aerobic respiration?
32 net
52
Why does utilization of the glycerol-3-phosphate shuttle in muscle yield less ATP?
30 vs 32; this shuttle brings in FADH2 instead of NADH, which yields 1 less ATP in ox phos (2 per molecule of glucose).
53
How does rotenone function as a poison?
Directly inhibits electron transport (complex I), causing a decr proton gradient and block of ATP synthesis.
54
How does cyanide function as a poison?
Directly inhibits electron transport (complex IV), causing a decr proton gradient and block of ATP synthesis.
55
How does antimycin A function as a poison?
Directly inhibits electron transport (complex III), causing a decr proton gradient and block of ATP synthesis.
56
How does CO function as a poison?
Directly inhibits electron transport (complex IV), causing a decr proton gradient and block of ATP synthesis.
57
How does oligomycin function as a poison?
Directly inhibits mitochondrial ATP synthase, causing an incr proton gradient. No ATP is produced because electron transport stops.
58
How do uncoupling agents work?
Increase permeability of membrane, causing a decreased proton gradient and increased oxygen consumptions. ATP synthesis stops but electron transport continues. Produces heat.
59
Examples of uncoupling agents?
2,4-dinitrophenol used illicitly for weight loss. Aspirin overdose. Thermogenin in brown fat.
60
What is the purpose of the HMP/pentose shunt?
1) Provides a source of NADPH, which is required for reductive reactions to prevent damage from reactive oxygen species. 2) Yields ribose for nucleotide synthesis and glycolytic intermediates.
61
Where are the sites of pentose shunt activity?
Liver, adrenal cortex (fatty acid/steroid synthesis), RBCs, lactating mammary glands.
62
What is involved in the oxidative phase of the pentose shunt?
G6PDehydrogenase converts G6P to 2NADPH, ribulose-5-P, and CO2. Irreversible. Rate limiting step.
63
What is involved in the nonoxidative phase of the pentose shunt?
Ribulose-5-P is converted to Ribose-5-P, glucose-3-phosphate, and fructose6P. Requires B1.
64
How does G6PD production of NADPH contribution to detoxification of free radicals?
1. NADPH is necessary for Glutathione Reductase to make GSSG (oxidized) --> 2GSH (reduced).
2. GSH is necessary for glutathione peroxidase, which changes hydrogen peroxide to water.
65
What are common oxidizing agents?
fava beans, sulfa drugs, primaquine, antituberculosis drugs, and infection (free radicals via inflammatory response).
66
What are the symptoms of G6PD deficiency?
Decreased NADPH in cells leads to hemolytic anemia, with heinz bodies resulting from denatured hemoglobin precipitates within RBCs, and Bite cells resulting from phagocytic removal of heinz bodies by splenic macrophages. X-linked recessive.
67
What reaction oxygen species are produced from oxygen?
Oxygen --> superoxide --> hydrogen peroxide --> water.
Addition of an electron each time. Hydroxyl radicals are the dangerous biproduct of these intermediates reacting with each other or with metals such as iron.
68
What is essential fructosuria and its inheritance pattern?
Defect in fructokinase, autosomal recessive. Benign and asx, since fructose not trapped in cells. Fructose appears in blood and urine.
69
What does aldolase B do in fructose handling? Why are its kinetics faster than glycolysis pathway?
Converts from fructose-1-P to DHAP or GAP. So, the precursor (fructose 6 phosphate --> fructose --> fructose 1 phosphate) bypasses the rate limiting step of conversion to 1,6fructosebiphos by PFK1.
70
What is fructose intolerance? Why does this deficiency matter?
Hereditary (autosomal recessive) deficiency of aldolase B. Fructose 1P accumulates, which decreases available phosphate and therefore ATP.
71
What are the symptoms of fructose intolerance and why?
As ATP falls, AMP rises. In the absence of Pi, AMP is degraded, causing hyperuricemia (and jaundice). The decreased availability of hepatic ATP affects gluconeogenesis (causing hypoglycemia with vomiting), and protein synthesis (causing a decrease in blood clotting factors and other essential proteins). Also inhibits glycogenolysis.
72
How to test for fructose intolerance, and then treat?
Urine dipstick with be negative for sugar (tests for glucose only), but reducing sugar can be detected in urine. Treat by decreasing intake of fructose and sucrose (fructose + glucose).
73
What is galactokinase deficiency? What are the sx?
Hereditary auto recess defect of enzyme that converts galactose to galactose-1-phosphate. Galactitol accumulates if galactose in diet, relatively mild. Sx: galactose in blood and urine, infantile cataracts. May present as failure to track objects or develop social smile.
74
What is classic galactosemia?
Abscence of galactose-1-phosphate uridyltransferase, which converts galactose1P to glucose1P.. Autosomal recessive. Damage occurs from accumulation of toxic substances (galactitol) and decreased available phosphate.
75
What are the sx and treatment of classic galactosemia?
Failure to thrive, jaundice, hepatomegaly, infantile cataracts, intellectual disability. Can lead to E.Coli sepsis in neonates. Treatment: exclude galactose and lactose (galactose + glucose) from diet.
76
What is sorbitol, and how is glucose converted to it?
Sorbitol is the alcohol counterpart of glucose. Aldose reductase converts glucose to sorbitol, another way of trapping glucose in the cell.
77
How is sorbitol further processed and in which cells?
Sorbitol can be converted to fructose in cells containing sorbitol dehydrogenase. These cells include liver, ovaries, and seminal vesicles.
78
Which cells have only/primarily aldose reductase without sorbital dehydrog, and are thereby at risk for sorbitol accumulation?
Schwann cells, retina, kidney = aldose reductase only. Lens has primarily aldose reductase.
79
What are sx of sorbitol accumulation in the setting of chronic hyperglycemia (eg diabetes)?
Cataracts, retinopathy, peripheral neuropathy due to osmotic damage.
80
What are three types of lactase deficiency?
1. Primary (age-dependent decline due to absence of lactase-persistent allele; seen in asian/african/native americans).
2. Secondary: loss of brush border due to gastroenteritis, autoimmune dz, etc.
3. Congenital lactase deficiency: rare.
81
What is the function of lactase and what does deficiency cause?
Lactase functions on brush border to convert lactose into glucose and galactose. Stool demonstrates lower pH and breath shows greater hydrogen content with lactose tolerance test. Sx include bloating, cramps, flatulence and osmotic diarrhea.
82
What are the three "irreversible" steps of glycolysis, ie, the steps requiring circumvention in gluconeogenesis, the conversion of pyruvate etc to glucose?
Hexokinase, PFK1, pyruvate kinase (PEP->pyruvate).
83
Where does gluconeogenesis occur and why?
Mainly in the liver, to maintain euglycemia during fasting state. Also in kidney and intestinal epithelium.
84
What are the four irreversible enzymes contributing to gluconeogenesis?
Pathway Produces Fresh Glucose: Pyruvate Carboxylase (mito), Phosphoenolpyruvate carboxykinase (cytosol), Fructose-1,6-bisphosphatase (cytosol), Glucose-6-phosphatase (in ER).
85
What is the role of pyruvate carboxylase in gluconeogensis?
Pyruvate carboxylase converts pyruvate to oxaloacetate. (Requires biotin, ATP. Activated by acetyl-CoA).
86
How is oxaloacetate transported to cytosol?
Hops on board the malate-aspartate shuttle.
87
What is the role of phosphoenolpyruvate carboxykinase in gluconeogensis?
PEP carboxylase in the cytosol converts oxaloacetate to PEP. Requires GTP. This step overcomes the irreversible conversion of PEP to pyruvate.
88
What is the role of fructose-1,6-biphosphatase in gluconeogenesis?
In cytosol, converts F16biphos to F-6-biphos. Reverses the irreversible step of PFK1. Is rate limiting step. Activated by citrate, inhibited by fructose-2,6-biphosphate.
89
What is the role of glucose-6-phosphatase in gluconeogenesis?
Glucose 6 phosphate to glucose in the ER. Overcomes 1st step of glycolysis, allows glucose to leave cell.
90
Big picture: what is the function of glycogen?
To maintain blood glucose concentrations between meals. It is synthesized when blood glucose levels are high (eg after a meal) and degraded when blood glucose levels are low.
91
What is the structure of glycogen? What are the three major steps in synthesizing?
About 30,000-40,000 glucose molecules, with many branches. Synthesis involves 1) activating glucose molecules, 2) synthesizing long polymers of glucose molecules 3) remodeling the polymers by creating branches
92
How are long polymers of glucose created?
1. Glucose1P is converted to UDP-glucose by UDP-glucose-pyrophosphorylase.
2. Glycogen synthase links the glucose from UDP-glucose to existing glycogen chain via alpha 1-4 bond, and releases UDP.
93
How are branches on glycogen created?
"Branching enzyme" aka a transglycosylase breaks off pieces of linear polysaccharide and reattaches via alpha 1,6 linkage to glycogen strand.
94
How does the degradation of glycogen occur (glycogenolysis)?
1. Glyocogen phosphorylase (needs VitaminB6!) releases G1Ps off branched residues until 4 glucose units remain. This remainder is called the "limit dextrin".
2. Debranching enzyme (4-alpha-glucotransferase) moves three units over to the linear part and makes a 1,4-alpha linkage.
3. Debranching enzyme (alpha-1,6-glucosidase) liberates the remaining unit in the branch.
95
How does the use of liberated glycogen differ in muscle and in hepatocytes?
In muscle, glycogen is broken down to provide glucose for metabolism within the cell. In liver, glycogen is broken down to create glucose that will be sent to the blood to maintain adequate blood sugar to supply other tissues (eg, brain).
96
What is the effect of insulin on glycogen storage in the liver?
Tyrosine kinase receptor activates glycogen synthase; glycogen is stored. Glycogen phosphorylase is deactivated.
97
What is the effect of insulin on glycogen storage in muscle?
Upregulates expression of GLUT4 receptors; increases uptake of glucose into the cells.
98
What is the effect of glucagon on glycogen storage in the liver?
Activates glycogen phosphorylase - glucagon tells the liver we need more blood sugar. Glycogen will be broken down and glucose made available.
99
What does the muscle degrade glycogen in response to?
Exercise! Muscle does not have glucagon receptors. Muscle responds to increased epinephrine, and increased cytoplasmic Ca2+.
100
What's up with lysosomal degradation of glycogen?
Lysosomes degrade a small amnt of glycogen via alpha1,4-glucosidase, acid maltase, as part of normal housekeeping.
101
What is von Gierke disease? What are the Sx and Tx?
Type I glycogen storage disease. Deficient glucose-6-phophatase (Gets G6P back to glucose). Sx: severe fasting hypoglycemia, elevated liver glycogen, elevated serum lactate, elevated triglyc, elevated uric acid, and hepatomegaly. Tx: Frequent oral glucose/cornstarch, avoid fructose and galactose.
102
What is pompe disease? Sx?
Type II glycogen storage disease. Deficient lysosomal alpha1,4,glucosidase (acid maltase). Sx: Pompe trashes the Pump. Cardiomegaly, HCM, exercise intolerance, and systemic findings leading to early death.
103
What is Cori disease?
Type III glycogen storage disease. Defective debranching (1,6glucosidase) enzyme. Milder form of type I with normal glucose levels.
104
What is McArdle disease? Tx:
Type V glycogen storage disease. Deficient skeletal muscle glycogen phosphorylase (myophosphorylase). Sx: McArdle hurts Muscle. Elevated glycogen in muscle, cannot break down --> painful muscle cramps, myoglobinuria with strenuous exercise, and arrythmia from electrolyte abnormalities. Treat with B6 (helps glycogen phosphorylase).
