BIOCHEM chap 9 mcat

GLUT 2

Glucose transporter found in the liver (glucose storage) and pancreatic beta-islet cells (part of the glucose sensor); high Km

GLUT 4

Glucose transporter found in adipose tissue and muscle; stimulated by insulin and has a low Km

Glycolysis

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

Glucokinase

- converts glucose to glucose-6-phosphate

- present in pancreatic beta-islet cells as part of glucose sensor

- responsive to insulin in the liver

beta-islet

Glucokinase and GLUT 2 are located in pancreatic ________ cells.

Hexokinase

glucose to glucose-6-phosphate in peripheral tissues

Phosphofructokinase-1 (PFK-1)

- phosphorylate fructose-6-phosphate to fructose 1,6-bisphosphate

- rate limiting step of glycolysis

- activated by AMP and fructose 2,6-bisphosphate (F2,6-BP)

- inhibited by ATP and citrate

ATP; citrate

Phosphofructokinase-1 is inhibited by _____ and ______.

AMP; fructose 2,6-bisphosphate

Phosphofructokinase-1 is activated by ________ and ________.

Phosphofructokinase-2 (PFK-2)

produces F2,6-BP, which activates PFK-1

Activated by insulin

Inhibited by glucagon

insulin; glucagon

Phosphofructokinase-2 is activated by _______ and inhibited by _______.

Glyceraldehyde-3-phosphate dehydrogenase

produces NADH which can feed into the electron transport chain

3-phosphoglycerate kinase and pyruvate kinase

Each perform substrate-level phosphorylation, placing an inorganic phosphate (Pi) onto ADP to form ATP.

glucokinase/hexokinase, PFK-1, pyruvate kinase

enzymes that catalyze irreversible reactions

mitochondrial ETC

The NADH produced in glycolysis is oxidized by the ___________ when oxygen is present.

cytoplasmic lactate dehydrogenase

oxidizes NADH produced by glycolysis when oxygen or mitochondria are absent

Ex: red blood cells, skeletal muscle (short intense bursts of exercise), and any cell deprived of oxygen

Galactose

a monosaccharide that comes from lactose in milk

Trapped in the cell by galactokinase and converted to glucose 1-phosphate via galactose-1-phosphate uridyltransferase and an epimerase

Fructose

a monosaccharide that comes from honey, fruit, and sucrose (common table sugar)

Trapped in the cell by Fructokinase and then cleaved by aldolase B to form glyceraldehyde and DHAP

pyruvate dehydrogenase

Complex of enzymes that convert pyruvate -> acetyl-CoA

stimulated by insulin

inhibited by acetyl-CoA

Glycogenesis

glycogen synthesis- is the production of glycogen using two 2 main enzymes:

Glycogen synthase and Branching enzyme

glycogen synthase

- creates alpha-1,4 glycosidic links between glucose molecules

- activated by insulin in liver and muscle

branching enzyme

- 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

Glycogenolysis

breakdown of glycogen using 2 main enzymes:

Glycogen phosphorylase and Debranching enzyme

glycogen phosphorylase

- removes single glucose 1-phosphate molecule by breaking alpha-1,4-glycosidic links

- activated in liver by glucagon to prevent low blood sugar

- activated in exercising skeletal muscle by epinephrine and AMP to provide glucose to the muscle itself

debranching enzyme

moves a block of oligoglucose from one branch and connects it to the chain using an alpha-1,4 glycosidic link.

Removes the branchpoint which is connected via an alpha-1,6 glycosidic link, releasing a free glucose molecule

Gluconeogenesis

simply the reverse of glycolysis, using most of the same enzymes; occurs in both cytoplasm and mitochondria, predominantly in the liver; small contribution from the kidneys; the three irreversible steps of glycolysis are bypassed by pyruvate carboxylase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase

pyruvate carboxylase

Converts pyruvate to oxaloacetate; works with phosphoenolpyruvate carboxykinase (PEPCK) to bypass pyruvate kinase

phosphoenolpyruvate carboxykinase (PEPCK)

converts oxaloacetate to phosphoenolpyruvate (PEP); works with pyruvate carboxylase to bypass pyruvate kinase

acetyl-CoA

Pyruvate carboxylase is activated by _______ from beta-oxidation.

glucagon; cortisol

PEPCK is activated by _______ and _______.

Fructose-1,6-bisphosphatase

- converts fructose 1,6-bisphosphate to fructose 6-phosphate

- bypass of phosphofructokinase-1 (PFK-1) in gluconeogenesis

- rate limiting for gluconeogenesis

AMP; insulin

Fructose-1,6-bisphosphatase is inhibited directly by ______ and indirectly (via increased fructose-2,6-bisphosphate) by _______.

ATP; glucagon

Fructose-1,6-bisphosphatase is activated directly by ______ and indirectly (via decreased fructose-2,6-bisphosphate) by ________.

Glucose-6-phosphatase

- converts glucose 6-phosphate to free glucose

- bypasses glucokinase for gluconeogenesis

endoplasmic reticulum

Glucose-6-phosphatase is found only in the _______________ of the liver.

Pentose Phosphate Pathway (PPP)

aka hexose monophosphate (HMP) shunt; generates NADPH and sugars for biosynthesis (derived from ribulose 5-phosphate)

cytoplasm

The PPP occurs in the _______ of most cells.

Glucose-6-phosphate dehydrogenase

rate-limiting enzyme of PPP; activated by NADP+ and insulin; inhibited by NADPH

NADP+; insulin; NADPH

Glucose-6-phosphate dehydrogenase is activated by ______ and _______, but inhibited by ______.