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MCAT Biochemistry > Carbohydrates Metabolism I > Flashcards

Carbohydrates Metabolism I Flashcards

Ch 9

1
Q

GLUT 2

A

low affinity
captures excess glucose for storage
found in the liver (for glucose storage) and pancreatic beta-islet cells (as part of the glucose sensor)
-high Km

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2
Q

GLUT 4

A

found in adipose tissue and muscle and is stimulated by insulin

  • low Km (~ 5mM)
  • mediates insulin-stimulated glucose uptake
  • ONLY INSULIN RESPONSIVE GLUCOSE TRANSPORTER
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3
Q

glycolysis

A

occurs in the cytoplasm of all cells and does not require oxygen
-yields 2 ATP per molecule of glucose

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4
Q

glucokinase

A

converts glucose to glucose-6-phosphate
present in the pancreatic beta-islet cells as part of the glucose sensor
- responsive too insulin in the liver

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5
Q

hexokinase

A

converts glucose to glucose 6-phosphate in peripheral tissues

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6
Q

phosphofructokinase-1 (PFK-1)

A

phosphorylates fructose 6-phosphate to fructose 1,6-bisphosphate in the rate limiting step of glycolysis

  • activated by AMP and fructose 2,6-bisphosphate (F2,6BP)
  • inhibited by ATP and citrate
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7
Q

phosphfructokinase-2 (PFK-2)

A

produces the F2,6-BP that activates PFK-1

-activated by insulin and inhibited by glucagon

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8
Q

glyceraldehyde 3-phosphate dehydrogenase

A

produces NADH, which can feed into the ETC

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9
Q

3-phospholgycerate kinase and pyruvate kinase

A

each perform substrate level phosphorylation (placing an inorganic phosphate onto ADP to form ATP

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10
Q

substrate level phsophorlation

A

placing an inorganic phosphate onto ADP to from ATP

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11
Q

what are the enzymes that catalyze irreversible reactions

A

glucokinase/hexokinase
PFK-1
pyruvate kinase

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12
Q

what happens to the NADH that is produced in glycolysis

A

it is oxidized by the mitochondrial ETC when oxygen is present

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13
Q

what happens to the NADH if the oxygen or mitochondria are absent?

A

the NADH is oxidized by cytoplasmic lactate dehydrogenase

ex: red blood cells, skeletal muscles (during short, intense bursts of exercise) and any cell deprived of oxygen

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14
Q

galactose

A

comes from lactose in milk
-is trapped in the cell by galactokinase and coverted to glucose 1-phosphate via galactose-1-phsophate uridyltransferase and an epimerase

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15
Q

fructose

A

comes from honey, fruit, and sucrose (table sugar)

-trapped in cell by fructokinase and then cleaved by aldolase Bto from glyceraldehyde and DHAP

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16
Q

pyruvate dehydrogenase

A

refers to a complex of enzymes that convert pyruvate to acetyl-CoA

  • stimulated by insulin
  • inhibited by acetyl-CoA
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17
Q

glycogenesis

A

glycogen synthesis

-production of glycogen using two main enzymes: glycogen synthase & branching enzyme

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18
Q

glycogen synthase

A

creates alpha-1,4-glycosidic links between glucose molecules
-activated by insulin in the liver and muscle

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19
Q

branching enzyme

A

moves a block of oligoglucose from one chain and adds it to the growing glycogen as a new branch using an alpha-1,6-glycosidic link

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20
Q

glyconeolysis

A

breakdown of glycogen using two main enzymes:

  • glycogen phosphorylase
  • debranching enzyme
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21
Q

glycogen phosphorylase

A
  • removes single glucose 1-phosphate molecules by breaking alpha-1,4 glycosidic links
  • in liver: activated by glucagon to prevent low blood sugar
  • in exercising skeletal muscles: activated by epinephrine and AMP to provide glucose for the muscle itself
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22
Q

debranching enzyme

A

moves a block of oligoglucose from one branch and connects it to the chain using an alpha-1,4-glycosidic link
-removes the branchpoint, releasing a free glucose molecule

23
Q

gluconeogenesis

A

occurs in both the cytoplasm and mitochondria

-predominantly in the liver

24
Q

the three irreversible steps of glycosis that are bypassed with different enzymes in gluconeogenesis

A
  • pyruvate carboxylase
  • frusctose-1,6-bisphosphate
  • glucose-6-phosphatase
25
pyruvate carboxylase
converts pyruvate into oxaloacetate, which is then converted to PEPCK - pyruvate carboxylase is activated by acetyl-CoA from beta-oxidation - PEPCK is activated by glucagon and cortisol
26
what are the two enzymes that bypass pyruvate kinase
pyruvate carboxylase | PEPCK
27
fructose-1,6-bisphosphatase
converts fructose-1,6-bisphosphate to fructose-6-phosphate - RATE LIMITING STEP OF GLUCONEOGENESIS - activated by ATP directly and glucagon directly - inhibited by AMP directly and insulin indirectly
28
glucose-6-phosphatase
converts glucose-6-phosphate to free glucose | -found in the endoplasmic reticulum of the liver
29
pentose phosphate pathway (PPP)
also known as the hexose monophosphate shunt (HMP) - occurs in the cytoplasm of most cells - generating NADPH and sugars for biosynthesis
30
what is the rate limiting step of the PPP
glucose-6-phosphate dehydrogenase - activated by NADP+ and insulin - inhibited by NADPH
31
what is diabetes mellitus caused by
disruption of the insulin/GLUT 4 mechanism
32
type 1 diabetes
insulin is absent and cannot stimulate the insulin receptor
33
type 2 diabetes
receptor becomes insensitive to insulin and fails to bring GLUT 4 transporters to the cell surface
34
how can cells with GLUT 4 transporters increase their intake of glucose?
by increasing the number of GLUT 4 transporters on their surface
35
rate limiting step of glycolysis
phosphofructokinase-1
36
rate limiting step of fermentation
lactate dehydrogenase
37
rate limiting step of glycogenesis
glycogen synthase
38
rate limiting step of glyconeogenesis
fructose-1,6-bisphosphatase
39
rate limiting step of the pentose phosphate pathway
glucose-6-phosphate dehydrogenase
40
rate limiting step of glycogenolysis
glycogen phosphorylase
41
what are the three intermediates that pyruvate is converted into
1. acetyl-CoA for the citci acid cycle via pyruvate dehydrogenase 2. lactate during fermentation via lactatedehydrogenase 3. oroxaloacetate for gluconeogenesis via pyruvate carboxylase
42
how is citrate produced?
by the citrate synthase from acetyl-CoA and oxaloacetate | -takes place in the mitochondria
43
what happens when the citrate acid cycle slows down
citrate accumulates - in the cytosol, citrate acts as a negative allosteric regulator of phosphofructokinase-1, the enzyme that is the rate limiting step of glycolysis
44
what are the primary functions of NADPH
- involvement in biosynthesis of lipids and cholesterol - production of bactericidal bleah in the lysosomes of certain white blood cells - maintenance of a supply of reduced glutathion for protection against free radical damage
45
rate limiting step of making glycogen
glycogen synthase | alpha-1,4 glycosidic bonds to make branching enzymes
46
branching enzyme
makes branches as granule grow, hydrolyzes(breaks) an alpha-1,4 glycosidic link and forms an alpha-1,6 glycosidic link to create branch
47
what does the number of branches created in a molecule of glucagon tell us
the more branches, the greater the efficiency with which energy can be stored
48
rate limiting step of glycogenolysis
glycogen phosphorylase
49
what does glycogen phosphorylase do
it is the rate limiting step of glycogenolysis | -phosphorylases alpha-1,4 glycosidic links at periphery but stops near branch points
50
what is the difference between glycogenolysis and gluconeogensis
glycogenolysis: is the break down of glycogen to to release glucose molecules gluconeogensis: formation of glucose from non-carbohydrate sources
51
what are the debranching enzymes? | where do you see them and what do they do?
debranching enzymes are seen in glycogenolysis two enzyme complex: - glucosyltransferase breaks alpha-1,4 bond at branching point and forms a new alpha-1,4 bond - glucosidase hydrolyzes alpha-1,6 releasing free glucose
52
Generation of glucose directly from the glycogen molecule is the domain of which enzymes?
branching enzymes seen in glycogenolysis
53
what is the function of a phosphatase enzyme?
dephosphorylation
54
what is generated in the pentose phosphate pathway?
NADPH, which is used to reduce glutathione

MCAT Biochemistry (8 decks)

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