Ch. 9 Carbohydrate metabolism Pt. 1

GLUT ___ is found in the liver (for glucose storage) and pancreatic β-islet cells (as part of the glucose sensor). It has a high K_m

2

GLUT ___ is found in adipose tissues and muscle and is stimulated by insulin. It has a low K_m

4

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

glycolysis

important glycolytic enzymes

• Glucokinase

• Hexokinase

• Phosphofructokinase-1 (PFK-1)

• Phosphofructokinase-2 (PFK-2)

• Glyceraldehyde-3-phosphate dehydrogenase

• 3-phosphoglyercate kinase

• pyruvate kinase

Glucokinase

converts glucose to glucose 6-phosphate. It is present in the pancreatic β-islet cells as part of the glucose sensor and is responsive to insulin in the liver

Hexokinase

converts glucose to glucose 6-phosphate in peripheral tissues

Phosphofructokinase-1 (PFK-1)

phosphorylates fructose 6-phospahte to fructose 1,6-biphosphate in the rate-limiting step pf glycolysis. PFK-1 is activated by AMP and fructose 2,6-biphosphate (F2,6-BP) and is inhibited by ATP and citrate

Phosphofructokinase-2 (PFK-2)

produces the FN,6-BP that activates PFK-1. It is activated by insulin and inhibited by glucagon

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 (P_i) onto ADP to form ATP

enzymes that catalyze irreversible reactions

• glucokinase/hexokinase

• PFK-1

• pyruvate kinase

the _____ produced in glycolysis is oxidized by the mitochondrial electron transport chain when oxygen is present

NADH

if oxygen or mitochondria are absent, the NADH produced in glycolysis is oxidized by cytoplasmic ___ ____. examples include red blood cells, skeletal muscle (during exercise) and any cell deprived of oxygen

lactate dehydrogenase

Galactose comes from lactose milk. It is trapped in the cell by _____ and converted to glucose 1-phosphate via galactose-1-phosphate uridyltransferase and an epimerase

galactokinase

Fructose comes from honey, fruit, and sucrose. It is trapped in the cell by ____ and then cleaved by ____ to form glyceraldehyde and DHAP

fructokinase, aldolase B

____ ____ refers toa complex of enzymes that convert pyruvate to acetyl-CoA

pyruvate dehydrogenase

pyruvate dehydrogenase is stimulated by ___ and inhibited by ____

insulin, acetyl-CoA

the production of glycogen using two main enzymes

glycogenesis

the main 2 enzymes in glycogenesis

• glycogen synthase

• branching enzyme

glycogen synthase does what

creates a α-1,3 glycosidic links between glucose molecules. It is activated by insulin in liver and muscle

branching enzyme does what

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

the breakdown of glycogen using two main enzymes

glycogenolysis

two main enzymes in glycogenolysis

• glycogen phosphorylase

• debranching enzyme

glycogen phosphorylase does what

removes single glucose 1-phosphate molecules by breaking α-1,4 glycosidic links.

In the liver glycogen phosphorylase isa activated by ___ to prevent low blood sugar; in exercising skeletal muscle, it is activated by ____ and ____ to provide glucose for the muscle itself

glucagon, epinephrine, AMP

debranching enzyme does what

moves a block of oligoglucose from one branch and connects it to the chain using an α-1,4 glycosidic link. It also removes a branchpoint, which is connected via an α-1,6 glycosidic link, releasing a free glucose molecule

gluconeogenesis occurs in

the cytoplasm and mitochondria, predominantly in the liver. There is a small contribution form the kidneys

most of gluconeogenesis is simply the reverse of _____, using the same enzymes

glycolysis

the three irreversible steps of glycolysis must be bypassed for gluconeogenesis to occur, what are the 3 new enzymes that aid in bypassing these steps

• pyruvate carboxylase

• fructose-1,6-biphosphatase

• glucose-6-phosphatase

pyruvate carboxylase converts pyruvate into _______(OAA), which is converted to phosphoenolpyruvate by _____ _______ (PEPCK). Together, these two enzymes bypass pyruvate kinase.

oxaloacetate, phosphoenolpyruvate carboxykinase

pyruvate carboxylase is activated by ______ from β-oxidation; PEPCK is activated by ____ and ____

acetyl-CoA, glucagon, cortisol

Fructose-1,6-biphospahte does what

converts fructose 1,6-biphosphate to fructose 6-phosphate, bypassing phosphofructokinase-1. This is the rate-limiting step of gluconeogenesis.

Fructose-1,6-biphosphate is activated by ___ directly and ___ indirectly(via decreased levels of fructose 2,6-biphosphate). It is inhibited by ____ directly and ___ indirectly (via increased levels of fructose 2,6-biphosphate

ATP, glucagon, AMP, insulin

glucose-6-phosphatase does what

converts glucose 6-phosphate to free glucose, bypassing glucokinase. It is found ONLY in the endoplasmic reticulum of the liver

the pentose phosphate pathway (PPP), also known as the hexose monophosphate (HMP) shunt occurs in the cytoplasm of most cells, generating _____ and sugars for biosynthesis (derived from ribulose 5-phosphate)

NADPH

the rate-limiting enzyme in the PPP is

glucose-6-phosphate dehydrogenase

glucose-6-phosphate dehydrogenase is activated by ____ and ____ and inhibited by _____

NADP⁺, insulin, NADPH