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MCAT Biochemistry > Carbohydrates Metabolism I > Flashcards

Carbohydrates Metabolism I Flashcards

Ch 9

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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
Q

pyruvate carboxylase

A

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
Q

what are the two enzymes that bypass pyruvate kinase

A

pyruvate carboxylase

PEPCK

27
Q

fructose-1,6-bisphosphatase

A

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
Q

glucose-6-phosphatase

A

converts glucose-6-phosphate to free glucose

-found in the endoplasmic reticulum of the liver

29
Q

pentose phosphate pathway (PPP)

A

also known as the hexose monophosphate shunt (HMP)

  • occurs in the cytoplasm of most cells
  • generating NADPH and sugars for biosynthesis
30
Q

what is the rate limiting step of the PPP

A

glucose-6-phosphate dehydrogenase

  • activated by NADP+ and insulin
  • inhibited by NADPH
31
Q

what is diabetes mellitus caused by

A

disruption of the insulin/GLUT 4 mechanism

32
Q

type 1 diabetes

A

insulin is absent and cannot stimulate the insulin receptor

33
Q

type 2 diabetes

A

receptor becomes insensitive to insulin and fails to bring GLUT 4 transporters to the cell surface

34
Q

how can cells with GLUT 4 transporters increase their intake of glucose?

A

by increasing the number of GLUT 4 transporters on their surface

35
Q

rate limiting step of glycolysis

A

phosphofructokinase-1

36
Q

rate limiting step of fermentation

A

lactate dehydrogenase

37
Q

rate limiting step of glycogenesis

A

glycogen synthase

38
Q

rate limiting step of glyconeogenesis

A

fructose-1,6-bisphosphatase

39
Q

rate limiting step of the pentose phosphate pathway

A

glucose-6-phosphate dehydrogenase

40
Q

rate limiting step of glycogenolysis

A

glycogen phosphorylase

41
Q

what are the three intermediates that pyruvate is converted into

A
  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
Q

how is citrate produced?

A

by the citrate synthase from acetyl-CoA and oxaloacetate

-takes place in the mitochondria

43
Q

what happens when the citrate acid cycle slows down

A

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
Q

what are the primary functions of NADPH

A
  • 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
Q

rate limiting step of making glycogen

A

glycogen synthase

alpha-1,4 glycosidic bonds to make branching enzymes

46
Q

branching enzyme

A

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
Q

what does the number of branches created in a molecule of glucagon tell us

A

the more branches, the greater the efficiency with which energy can be stored

48
Q

rate limiting step of glycogenolysis

A

glycogen phosphorylase

49
Q

what does glycogen phosphorylase do

A

it is the rate limiting step of glycogenolysis

-phosphorylases alpha-1,4 glycosidic links at periphery but stops near branch points

50
Q

what is the difference between glycogenolysis and gluconeogensis

A

glycogenolysis: is the break down of glycogen to to release glucose molecules
gluconeogensis: formation of glucose from non-carbohydrate sources

51
Q

what are the debranching enzymes?

where do you see them and what do they do?

A

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
Q

Generation of glucose directly from the glycogen molecule is the domain of which enzymes?

A

branching enzymes

seen in glycogenolysis

53
Q

what is the function of a phosphatase enzyme?

A

dephosphorylation

54
Q

what is generated in the pentose phosphate pathway?

A

NADPH, which is used to reduce glutathione

MCAT Biochemistry (8 decks)

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