BIOL 304 - Chapter 16 Glycolysis and Gluconeogenesis

glycolysis

the sequence of reactions that metabolizes one molecule of glucose to two molecules of pyruvate with the net production of two molecules of ATP

gluconeogensis

the process by which metabolic products are salvaged to synthesize glucose

How are foods/sugars broken down to glucose to use for glycolysis?

complex carbs (starch and glycogen) are broken down

alpha-amylase

pancreatic enzyme that digests starch and glycogen

cleaves α-1,4 bonds

α-glucosidase (maltase)

intestinal surface enzyme breaks maltose, maltotriose, and other α-1,4-linked oligosaccharides into glucose molecules

sucrase

intestinal surface enzyme that degrades ingested sucrose to fructose and glucose

How are monosaccharides transported into the bloodstream?

transported into epithelial cells lining intestine by active transporters

Glucose moves passively down its concentration gradient into the bloodstream and passively into cells
that will utilize it

glucose transporters (GLUT1 or GLUT5)

mediate the thermodynamically downhill movement of glucose across animal plasma membranes

stage 1 of glycolysis

traps glucose in the cell and modifies it so that it can be cleaved into phosphorylated three-carbon units

doesn’t generate ATP

stage 2 of glycolysis

oxidizes three-carbon unit to pyruvate

generates 2 molecules of ATP

hexokinase

uses ATP to phosphorylate glucose to glucose 6-phosphate

phosphoglucose isomerase

catalyzes the isomerization of G-6P to the fructose 6-phosphate

phosphofructokinase

uses ATP to phosphorylate F-6P to fructose 1,6-bisphosphate

allosteric enzyme that sets the pace of glycolysis

aldose

cleaves F-l,6-BP into glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP)

readily reversible

triose phosphate isomerase

catalyzes the isomerization of DHAP to GAP

readily reversible

Glyceraldehyde 3-phosphate dehydrogenase

catalyzes the conversion of GAP into 1,3-bisphosphoglycerate

phosphoglycerate mutase

catalyzes the conversion of 3-phosphoglycerate to 2-phosphoglycerate

enolase

catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate

pyruvate kinase

catalyzes the transfer of a phosphoryl group from PEP to ADP, generating pyruvate and ATP

net reaction in glycolysis

Glucose + 2P + 2ADP + 2NAD+ —→ 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H2O

why is glucose trapped in the cytoplasm?

G-6P is negatively charged and is not a substrate for glucose transporters

ethanol fermentation

the conversion of glucose into ethanol in anaerobic conditions

step 1 of ethanol fermentation

pyruvate decarboxylase catalyzes the decarboxylation of pyruvate

requires the coenzyme thiamine pyrophosphate that is derived from the vitamin thiamine (B1)

step 2 of ethanol fermentation

alcohol dehydrogenase catalyzes the reduction of acetaldehyde to ethanol by NADH

regenerates NAD+

lactic acid fermentation

the conversion of glucose into lactate in anaerobic conditions

Lactase dehydrogenase catalyzes the reduction of pyruvate by NADH to lactate

fructose entering glycolytic pathway

fructose 1-phosphate pathway metabolizes fructose in the liver

converted by fructokinase

galactose–glucose interconversion pathway

converts galactose to glucose 6-phosphate

the primary control of muscle glycolysis

ratio of ATP to AMP

high levels of ATP (muscle tissue)

allosterically inhibit phosphofructokinase

Binding to a specific regulatory site lowers the enzyme's affinity for F-6P

AMP (muscle tissue)

reverses the inhibitory action of ATP

a decrease in pH

inhibits phosphofructokinase activity by augmenting the inhibitory effect of ATP

occurs when muscle is functioning anaerobically and producing excessive lactic acid

prevents muscle damage

hexokinase is inhibited by

its product, G-6P

Inhibition of phosphofructokinase leads to the inhibition of hexokinase

pyruvate kinase is allosterically inhibited 

by ATP

regulation of phosphofructokinase in the liver

can be regulated by ATP

can be regulated by pH

inhibited by citrate, enhancing the inhibitory effect of ATP

pyruvate kinase regulation in the liver

phosphorylated makes it less active

major precursors of gluconeogenesis

1. lactate

2. amino acids

3. glycerol

pyruvate carboxylase

catalyzes the carboxylation of pyruvate to oxaloacetate using a molecule of ATP

malate dehydrogenase

catalyzes the reduction of oxaloacetate to malate

Phosphoenolpyruvate carboxykinase (PEPCK)

decarboxylates and phosphorylates oxaloacetate to phosphoenolpyruvate

free glucose is a

control point

only formed in some tissues (like the liver)

glycolysis and gluconeogenesis are

reciprocally regulated

glycolysis dominates when

energy or glycolytic intermediates are needed

gluconeogenesis dominates when

there is a surplus of energy and glucose precursors

Cori cycle

a series of reactions carried out by cooperation between the liver and muscle

Lactate produced by active muscle is released into the blood and converted to glucose by the liver

Nitrogens from amino acids used by muscle for fuel are transferred to pyruvate to form alanine