ch 9- Carbohydrate Metabolism I

Question 1

GLUT 2

Answer 1

a low affinity transporter in hepatocytes and pancreatic cells; after a meal, blood travelling through the hepatic portal vein from the intestine is rich in glucose which GLUT 2 the excess of for storage. K sub m of GLUT 2 is high - about 15mM

Question 2

GLUT 4

Answer 2

in adipose tissue and muscles and responds to glucose conc in peripheral blood; K sub m is similar to normal glucose level in blood - about 5 mM

Question 3

Glycolysis

Answer 3

a cytoplasmic pathway that converts glucose into two pyruvate molecules, releasing a modest amount of energy captured in 2 substrate-level phophorylations and one oxidation reaction; energy carriers produced are NADH and can feed aerobic respiration pathway to generate energy for cell; not carrying out is incompatible with life

Question 4

Hexokinase

Answer 4

widely distributed enzyme in tissues used during glycolysis; phosphorylates glucose to form glucose 6-phosphate, trapping glucose in the cell, and is inhibited by its product. low K sub m - reaches max velocity at low glucose conc; irreversible

Question 5

Glucokinase

Answer 5

Glycolysis - enzyme found only in liver cells and pancreatic beta-islet cells where it traps and phosphorylates glucose; in the liver it is induced by insulin. Acts as the glucose sensor along with GLUT 2; high K sub m; irreversible

Question 6

Phosphofructokinase-1 (PFK-1)

Answer 6

rate-limiting enzyme for glycolysis and main control point in glycolysis. Fructose 6-phosphate is phophorylated to fructose 1,6-bisphosphate using ATP; inhibited by ATP and citrate and activated by AMP, fructose 2,6 bisphosphate and insulin. Insulin stimulates and glucagon inhibits in hepatocytes; irreversible

Question 7

Phosphofructokinase-2 (PFK-2)

Answer 7

Glycolysis - activated by insulin, inhibited by glucagon. converts a tiny amount of fructose 6-phosphate to fructose 2,6 bisphosphate (F2,6-BP) which activates PFK-1. found mostly in the liver.

Question 8

lactate dehydrogenase

Answer 8

rate limiting enzyme for fermentation; key fermentation enzyme in mammalian cells; oxidizes NADH to NAD+, replenishing the oxidized coenzyme for glyceraldehyde-3-phsphate dehydrogenase; prevents glycolysis from stopping when all available NAD+ is reduced to NADH.

Question 9

glycogen synthase

Answer 9

rate limiting enzyme for glycogenesis

Question 10

gylcogen phophorylase

Answer 10

rate limiting enzyme for glycogenolysis

Question 11

fructose 1,6-bisphosphatase

Answer 11

rate limiting enzyme for gluconeogenesis

Question 12

Glucose-6-phosphate dehydrogenase

Answer 12

rate limiting enzyme for pentose phosphate pathway

Question 13

reduction and oxidation in biomolecules

Answer 13

oxidation is increasing bonds to oxygen or other heteroatoms (atoms besides C and H); and reduction is increasing bonds to hydrogen

Question 14

Glyceraldehyde-3-Phosphate Dehydrogenase

Answer 14

enzyme during glycolysis that catalyzes an oxidation and addition of inorganic phosphate (P sub i) to its substrate, glyceraldehyde 3-phosphate resulting in production of high energy intermediate 1,3-bisphosphoglycerate and the reduction of NAD+ to NADH; reversible

Question 15

3-Phosphoglycerate kinase

Answer 15

performs substrate level phosphorylations; enzyme during glycolysis that transfers high-energy phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate; reversible

Question 16

substrate-level phosphorylations

Answer 16

reaction that directly phosphorylates ADP to ATP using high-energy intermediate; an example of an enzyme that causes this is 3-Phosphoglycerate kinase; not dependent on oxygen and are the only means of ATP generation in an anaerobic tissue

Question 17

Pyruvate Kinase

Answer 17

last enzyme in aerobic glycolysis; catalyzes a substrate-level phosphorylation of ADP using the high energy substrate phosphoenolpyruvate (PEP); activated by fructose 1,6-bisphosphate from the PFK-1 reaction. Irreversible

Question 18

feed-forward activation of pyruvate kinase

Answer 18

the product of an earlier reaction of glycolysis (fructose 1,6-bisphosphate) stimulates, or prepares a later reaction in glycolysis (by activating pyruvate kinase)

Question 19

Dihydroxyacetone phosphate (DHAP)

Answer 19

intermediate of glycolysis - used in haptic and adipose tissue for triacylglycerol synthesis. formed from fructose 1,6-bisphosphate, can be isomerized to glycerol 3-phosphate which can then be converted to glycerol, the backbone of tracylglycerols

Question 20

1,3-Bisphosphoglycerate (1,3 BPG) and phosphoenolpyruvate

Answer 20

other intermediates of glycolysis - high energy, used to generate ATP by substrate level phosphorylation. This is the only ATP gained in anaerobic respiration

Question 21

Irreversible enzymes in glycolysis pathway

Answer 21

keeps pathway moving in one direction; Glucokinase or hexokinase, PFK-1, and pyruvate kinase catalyze reactions that are irreversible

Question 22

glycolysis in erythrocytes

Answer 22

anaerobic glycolysis is only pathway for ATP production yielding a net 2 ATP per glucose

Question 23

bisphosphoglycerate mutase

Answer 23

found in RBCs (erythrocytes) - produces 2,3-bisphosphoglycerate (2,3-BPG) from 1,3 BPG in glycolysis; mutases are enzymes that move a functional group from one place in a molecule to another. In this case phosphate is moved from 1 to 2 position

Question 24

2,3 bisphosphoglycerate (2,3 BPG)

Answer 24

produced from 1,3 BPG; binds allosterically to the beta chains of hemoglobin A (HbA) and decreases its affinity for oxygen; abnormal increase in this may shift the curve far enough so that HbA is not fully saturated in the lungs; does not bind well to fetal Hemoglobin (HbF). therefore this has higher affinity for oxygen

Question 25

physiological changes that promote a right shift of oxygen dissociation curve (Bohr effect)

Answer 25

high 2,3 BPG, low pH, High [H+], high pCO2. All occur during exercise (exercise is the RIGHT thing to do)

Question 26

galactose

Answer 26

important monosaccharide; important source is disaccharide lactose which is hydrolyzed to galactose and glucose by lactase, a brush border enzyme of the duodenum; reaches liver through the hepatic portal vein

Question 27

galactokinase

Answer 27

phosphorylates galactose in the liver, trapping it in the cell

Question 28

galactose-1-phosphate uridyltransferase

Answer 28

works with an epimerase to convert galactose 1-phosphate from phosphorylated galactose into glucose 1-phosphate

Question 29

epimerases

Answer 29

enzymes that catalyze the conversion of one sugar epimer to another (epimers are diastereomers that differ at exactly one chiral carbon)

Question 30

Fructose and fructose metabolism pathway

Answer 30

important monosaccharide (not as important as glucose); found in honey and fruit and part of the disaccharide sucrose (table sugar). Sucrose is hydrolyzed by the duodenal brush border enzyme sucrase to glucose and fructose which are absorbed in hepatic portal vein. liver phosphorylates fructose using fructokinase and traps it. frutose 1-phosphate is then cleaved into glyceraldehyde and DHAP by aldolase B

Question 31

pyruvate deydrogenase complex (PDH) reaction

Answer 31

pyruvate from aerobic glycolysis is converted to acetyl CoA in the mitochondria for entry into the citric acid cycle if ATP is needed or for fatty acid synthesis if ATP is not needed; irreversible; activated by insulin in the liver but is unresponsive to hormones in nervous system. pyruvate dehydrogenase is inhibited by its product, acetyl CoA and when there is enough acetyl CoA it is instead converted into oxaloacetate to enter gluconeogenesis

Question 32

Glycogen

Answer 32

branched polymer of glucose; storage form of glucose. core protein of glycogenin with linear chains of glucose emanating out. Synthesis and degradation occur primarily in liver and skeletal muscle. Stored in cytoplasm as granules composed of polyglucose chains. if chains are branched, the glucose density is highest at the periphery allowing more rapid release of glucose on demand. in liver prevents low blood sugar between meals, in muscles energy reserve for muscle contraction

Question 33

glycogenesis

Answer 33

synthesis of glycogen granules; begins with core protein called glycogenin. Glucose addition to granule through glucose 6-phosphate, then converted to glucose 1-phosphate, which is then activated by coupling to a molecule of uridine diphosphate (UDP), which them permits its integration into the glycogen chain by glycogen synthase. Activation occurs when glucose 1-phosphate interacts with uridine triphosphate (UTP), forming UDP-glucose and pyrophosphate (PP sub i)

Question 34

glycogen synthase

Answer 34

rate limiting enzyme of glycogen synthesis; also forms the alpha-1,4 glycosidic bond found in linear glucose chains of the granule. Stimulated by glucose 6-phosphate and insulin, inhibited by epi and glucagon through a protein kinase cascade that phophorylates and inactivates the enzyme

Question 35

Branching enzyme

Answer 35

glycogenesis - responsible for introducing alpha-1,6-linked branches into the granule as it grows; also hydrolyzes one of the alpha-1,4 bonds to release a block of oligoglucose which is then moved and added in a lightly different location; also forms an alpha-1,6 bond to create a branch.
alpha-1,4 keeps the same branch moving 4ward. alpha 1,6 puts a branch in the mix

Question 36

Glycogenolysis

Answer 36

process of breaking down glycogen

Question 37

glycogen phosphorylase

Answer 37

rate limiting enzyme of glycogenolysis; phosphorylases break bonds using an inorganic phosphate instead of water; breaks alpha-1,4 glycosidic bonds, releasing glucose 1-phosphate from the periphery of the grandule. cannot break alpha-1,6 bonds. Actived by glucagon in the liver and AMP and epi in skeletal muscle. Inhibited by ATP

Question 38

Debranching Enzyme

Answer 38

in glycogenolysis - a two-enzyme complex that deconstructs the branches in glycogen that have been exposed by glycogen phosphorylase;

• It breaks an alpha-1,4 bond adjacent to the branch point and moves the small oligoglucose chain that is released to the exposed end of the other chain.
• It forms a new alpha-1,4 bond
• It hydrolyzes the alpha-1,6-bond, releasing the single residue at the branch point as free glucose which represents only free glucose produced directly in glycogenolysis

Question 39

Isoforms

Answer 39

slightly different versions of the same protein

Question 40

gluconeogenesis

Answer 40

liver carries out and kidneys on a smaller level; produces acetyl-CoA from beta oxidation of fatty acids; starts with glucose; most steps represent a reversal of glycolysis. stimulated by glucagon lowering F2,6-BP and inhibited by insulin increasing F2,6-BP

Question 41

Important substrates for gluconeogenesis

Answer 41

glycerol 3-phosphate (from stored fats, or triacylglycerols, in adipose tissue); lactate (from anaerobic glycolysis); and glucogenic amino acids (from muscle proteins)

Question 42

glucogenic amino acids

Answer 42

all except leucine and lysine - can be converted into intermediates that feed into gluconeogenesis

Question 43

ketogenic amino acids

Answer 43

can be converted into ketone bodies, which can be used as alternative fuel

Question 44

pyruvate carboxylase

Answer 44

gluconeogenesis - one of the important enzymes required to catalyze rxns that circumvent irreversible steps of glycolysis. a mitochondrial enzyme that is activated by acetyl-CoA (from beta-oxidation). Product is oxaloacetate (OAA) - citric acid cycle intermediate which cannot leave the mitochondrion so it is reduced to malate which can which is then reoxidized back to OAA. replaces pyruvate kinase

Question 45

Phosphoenolpyruvate Carboxykinase (PEPCK)

Answer 45

important enzyme in gluconeogenesis. induced by glucagon and cortisol which generally act to raise blood sugar levels; converts OAA to PEP in a rxn that requires GTP. Combination of this and pyruvate carboxylase is used to circumvent the action of pyruvate kinase by converting pyruvate back into PEP

Question 46

Fructose-1,6-Bisphosphatase

Answer 46

important enzyme in gluconeogenesis. in cytoplasm, key control point of gluconeogenesis, represents rate-limiting step of the process. Reverses action of phosphofructokinase-1 (rate limiting step of glycolysis) by removing phosphate from fructose 1,6-bisphosphate to produce fructose 6-phosphate. activated ATP and inhibited by AMP and fructose 2,6-bisphosphate.

Question 47

What opposes kinases

Answer 47

phosphatases

Question 48

Glucose-6-phosphatase

Answer 48

important enzyme in gluconeogenesis; found only in the lumen of the ER in liver cells; used to circumvent glucokinase and hexokinase which convert glucose to glucose-6-phosphate

Question 49

four important enzymes for gluconeogenesis and what irreversible glycolytic enzymes they replace

Answer 49

pyruvate carboxylase replaces pyruvate kinase; phosphoenolpyruvate carboxykinase (PEPCK) replaces pyruvate kinase; Fructose-1,6-bisphosphatase replaces phosphofructokinase-1, glucose-6-phosphatase replaces glucokinase

Question 50

pentose phosphate pathway (PPP)

Answer 50

also called hexose monophosphate (HMP) shunt; occurs in cytoplasm of all cells. Two functions: production of NADPH and serving as a source of ribose 5-phosphate for nucleotide synthesis; first part begins with glucose 6-phosphate, ends with ribulose 5-phosphate and is irreversible; produces ribulose 5-phosphate which is isomerized to ribose 5-phosphate, the backbone of nucleic acids

Question 51

glucose-6-phosphate dehydrogenase (G6PD)

Answer 51

important rate-limiting enzyme of PPP

Question 52

functions of NADPH

Answer 52

primarily e- donor in many biochemical reactions; reducing agent which is oxidized in the process; cells require it for biosynthesis, mainly of fatty acids and cholesterol, assisting in cellular bleach production in certain WBCs which contributes to bactericidal activity, maintenance of a supply of reduced glutathione to protect against reactive oxygen species (acting as the body’s natural antioxidant)

Question 53

Glutathione

Answer 53

reducing agent that can help reverse radical formation before damage is done to the cell