Week 4 - Fructose Pathway Flashcards
What is fructose?
Isomer of glucose (same chemical fomula)
Main sources are honey and high fructose corn syrup
Sucrose to glucose and fructose.
What is the enzyme?
The enzyme is sucrase / isomaltase
Polyol Pathway
what does it do?
Where in body?
Generates fructose from glucose
This pathway occurs normally in seminal vesicles, producing fructose as an energy source for sperm
- first reaction is a reduction of aldehyde to alcohol
- second reaction is oxidation of the second alcohol carbon to ketone
Cataracts disease
- In cases of uncontrolled diabetes (hyperglycemia), the polyol pathway can lead to elevated sorbitol and fructose levels in the eye. This leads to water influx (osmotic pressure) and high fluid pressure.
- The excess sugar also results in non-enymatic glycosylation of lens proteins. The high pressure and glycolsylated lens proteins make an opaque spot on the lens
- So much of glucose around, make sorbital and fructose.. this causes cataracts
- Aldose Reductase & Sorbitol dehydrogenase
How is fructose transported into cells?
What happens inside cell?
Primarily IN LIVER
GLUT V transporter
First step is phosphorylization to trap fructose in cell (it cant diffuse back out into blood through a channel)
Enzyme fructokinase* (ATP to ADP)
What does Aldolase do in glycolysis?
Converts 16BPG toDihydroxyacetone phosphate and glyceraldehyce
What does Aldolase B do?
All forms of aldolase can cleave Fructoser 1,6 bis phosphate but only Aldolase B cleave fructose 1 phosphate (6 carbons)
IN LIVER ONLY*
The same products ensue (one more step for the phosphoylation), dihydroxyacetone phosphate and glyceraldehyde (triose kinase phosphorylates it to G3P) (both 3 carbon molecules*)
G3P is a glycolysis intermediate. dihydroxyacetone phosphate can be converted to G3P via triose phosphate isomerase and then move into glycolysis, but IN LIVER G3P is usually used in glycogenesis
Glycolysis
What is the glycerol 3-phosphate shuttle? and dihydroxyacetone phosphate
Converts dihydroxyacetone phosphate to G3P using enzyme G3P dehydrogenase…
The reverse reaction uses the reducdtion of FAD to FADH2 to power oxidative phosphorylation in the mitochondria to produce ATP (Electron transport chain..
Look at overview of fructose metabolism.. and glucose metabloism..
awhere does fructose enter into glycolysis.. ?
used to make glycogen, in beta oxidation etc.. review.
REVIEW
Is the metabolism of fructose usually in FED or FASTED state?
Where does the carbon from fructose go?
Metabolism of fructose is associated with FED state so usually it is only in FED state metabolism, it will go to glycogen pathway.. usually not the energy producing pathway to pyruvate… *
It will go through to gluconeogenesis and then to glycogenogenolysis
Fructose to G3P then to Glucose 6 phosphate then to glycogen..
What major regulatory step in glycolysis is skipped when fructose is the substrate instead of glucose?
PFK1
major regulatory step in glycolysis is PFK 1…
Fructose 6 phosphate to fructose 1,6 bis-phosphate
Significance - if feeding someone intraveiniously… lactic acidosis could ensue. no way to shut off glycolysis (PFK1 will be active) No energy demand so they produce lactate from pyruvate instead of going through TCA.
What is fructose converted to in muscle?
In muscle, fructose can also be phosphorylated directly by hexokinase (although glucose is its preferred substrate) producing fructose 6-phosphate
This skips gluconeogensis and goes directly into glycogenogolysis…
In muscle PFK1 is NOT skipped..
What is PFK inhibited and activated by?
Inhibited by ATP and low pH and glucagon
it is activated by AMP
One way fructose related to obesity
can store carbon from fructose as fatty acids..
drink lots of pop with fructose OR glucose will produce a lot of fat… no real difference.
