Chapter 18 (Entry of other Sugars) Flashcards
Entry of Fructose via the muscle
Very similar to metabolism of Glucose
Hexokinase is used to convert Fructose into Fructose-6-P
Fructokinase
The phosphorylation of Fructose into Fructose-1-P (which is not a metabolite in glycolysis)
Irreversible
Found in the liver
Fructose-1-P Aldolase
Catalyzes the cleavage of Fructose-1-P into Glyceraldehyde and DHAP
Reversible
Found in the liver
Glyceraldehyde has two pathways
Short: direct to GAP via the enzyme Glyceraldehyde Kinase
Long: to GAP (creating Glycerol-3-P in the process) via several steps
Glyceraldehyde (Short Path)
converted to GAP via the enzyme Glyceraldehyde Kinase
Requires 1 ATP (to phosphorylate)
Irreversible
Alcohol Dehydrogenase
Changing the aldehyde on Glyceraldehyde into a alcohol to create Glycerol
Consumes a NADH (oxidation)
Glycerol Kinase
Adds a phosphate group onto the C#3 of glycerol to create a Glycerol-3-P
Only found in the liver
Requires ATP
Irreversible
Glycerol Phosphate Dehydrogenase
Oxidizes the -OH on C#2 of Glycerol-3-P into a ketone to create a DHAP
Consumes NAD+ (reduced)
What is the purpose of the longer Glyceraldehyde pathway?
It is used to create Glycerol-3-P which is used as a backbone for triacylglycerols (and other compounds)
Entry of Fructose via the liver
Glucokinase has a low affinity for fructose to fructokinase is used
Entry of Glycerol
Occurs only in the liver
Same steps used during the long pathway to convert Glycerol to DHAP in the Fructose pathway
Entry of Galactose
Galactose is the C4 epimer of Glucose
Only metabolized in the liver
Galactokinase is used
Galactokinase
Adds a phosphate group onto the C#1 of Galactose to create Galactose-1-P
Irreversible
Requires ATP
Galactose-1-P Uridylyltransferase
Transfers a phosphate group from UDP-Glucose to create a UDP-Galactose
Glucose-1-P continues on down the pathway
Phosphoglucomutase
Transfers the phosphate group from C#1 to C#6 to create Glucose-6-P (which continues down glycolysis)
What enzyme epimerizes UDP-Galactose back into UDP-Glucose and vice versa
UDP-Galactose-4-epimerase
If an infant has a galactose problem
they cannot drink their mothers milk or it will lead to mental retardation
Dr. Luis Leloir
won noble prize for galactose metabolism
Entry of Mannose
Mannose is C#2 epimer of Glucose Can use the enzyme hexokinase (muscle) Adds phosphate group to create Mannose-6-P Requires ATP Irreversible
Phosphomannose Isomerase
Turns Mannose-6-P into Fructose-6-P
Mannose is primarily seen as
Mannose-1-P (via phosphomannose mutase)
Aldose Reductase
Reduces aldoses to their corresponding sugar alcohols
Reduces Glucose to Sorbitol
Requires NADPH
Aldose Reductase has a (high/low) Km for aldoses which means a lot of substrate is needed
High
High aldolase = increased activity
Sorbitol Dehydrogenase
Converts Sorbitol into Fructose
Requires NAD+
Found in sperm (and several tissues)
Low concentrations can lead to damage similar to that of Type 1 Diabetes
At which point in the glycolysis pathway does arsenate poisoning become a problem
During the conversion of GAP to 1,3-BPG via GAPDH
GAP mistakes arsenate as a phosphate group and creates 1-Arseno-3PG
The arsenate is then hydrolized off (no enzyme needed)
Why is arsenate poisoning so dangerous
Because this causes the GADPH step to be skipped which leads to a net yield of 0 ATP through glycolysis
Bisphosphoglycerate Mutase
Converts 1,3-BPG to 2,3-BPG via the movement of a phosphate group
Irreversible
Significance of 2,3-BPG
it is a allosteric effector for hemoglobin
2,3-BPG Phosphatase
Hydrolyzes the phosphate off of 2,3-BPG to create 3-PG
Phosphate group comes off as Pi
Irreversible
The effect of Hexokinase deficiency on blood oxygen
Low Hexokinase = low 1,3-BPG Low 1,3-BPG = low 2,3-BFG Low 2,3-BFG = higher affinity for O2 Blood can carry less O2 than normal Curve shifts to the left
The effect of Pyruvate Kinase deficiency on blood oxygen
Low Pyruvate Kinases = more 1,3-BPG (everything backs up) More 1,3-BPG = more 2,3-BPG More 2,3-BPG = lower affinity for O2 Blood can carry more O2 than normal Curve shifts to the right
