Carbohydrates (Lecture 4) Flashcards
what are the metabolic consequences of hexokinase and glucokianse?
since hexokinase has a higher affinity for glucose than glucokinase, hexokinase will bind to most of the glucose to promote glycolysis and glucokinase ill onlyinteract with the remaining circulating glucose, which will be converted into glycogen in the liver.
what is the 4th step in glycolysis?
F1,6BP will be converted into GAP and DHAP via the enzyme,e aldolase.
it is important tot note that GAP and DHAP can be interconverted
how are GAP and DHAP related?
these two molecules are interconvertible with the enzyme Triose phosphate isomerase
the consumption of GAP during glycolysis pushes the generation of more GAP
in cells, there’s a higher concentration fo DHAP, since GAP is quickly con summed by glycolysis
how many GAP molecules are generated per glucose?
2
what’s the 6th step of glycolysis?
GAP is converted to 1,3 Bisphosphoglycerate via GAPDH
this step generates NADH
the product is a high energy intermediate due to the presence of two Pi groups
what’s the 7th step of glycolysis?
1,3 BPG is converted to 3PG via PGK
this step generates ATP
why does substrate channeling occur and which compounds are involved in this process?
1,3 bisphosphoglycerate is a high energy intermediate with a short half life, it is rapidly passed along 2 enzymes to ensure that it does not accumulate.
which step is involved with PGM, phsophoglycerate mutase.
the 8th step, where 3PG is converted into 2PG
transfer of the phosphate form position 3 to position 2
how is PEP formed?
2PG is transformed into PEP via an enolase, which dehydrates the 2PG to phsophoenolpyruvate
explain the importance of PEP
PEP is a high energy intermediate
before making ATP , PEP is hydrolyzed to supply energy
how is pyruvate kinase involedi n glycolysis?
PK is the last step of glycolysis, as for it generates pyruvate
this step generates two ATPs per glucose molecule
how is pyruvate kinase activated?
allosterically via a feed forward mechanism with Fructose 1,6 bisphosphate
how is pyruvate kinase inhibited?
allosterically via ATP
hormonally via glucagon (cAMP and PKA)
PKA phosphorylates PK to inactivate the enzyme
name the 3 committed steps:
- HK
- PFK1
- PK
how man y ATPare generated from aerobic glycolysis?
7 ATP
how man y ATPare generated from anaerobic glycolysis?
2 ATP
what is the importance of NAD+?
NAD+ must be regenerated for glycolysis
how is NAD+ replenished?
in the citric acid cycle (need for O2)
homolactic fermentation, NADH is invested to generate lactate ans NAD+
alcoholic fermentation generated CO2 and ethanol via the investment of NADH
homolactic fermentation
does not require O2
lactate dehydrogenase (LDH) converts NADH and pyruvate into lactate
the NAD+ will then refill the pool for glycolysis
explain the Cori cycle
lactate in the muscle will be transported into the liver via this cycle
when exercising, pyruvate is converted into lactate
lactate circulates the blood, but when blood glucose levels are low, gluconeogenesis will occur with the lactate supplied by the muscle to the liver
the glucose synthesized will be released not the blood
when there is an accumulation of glucose, it will be converted into glycogen for storage in the muscle
to access the glucose, glycogen will be transformed into lactate
why is fructose metabolized in the liver?
the fructose consumed by our diets will enter the hepatic vein, thus it will pass through the liver before reaching any other organ
what is the role of fructokinase?
once fructose enters the liver, it will be converted into fructose-1-phospahte via fructokinase
this process requires the investment of 1 ATP
the ring structure is opened to form a chain
how is glyceraldehyde formed?
F1P chain is converted into glyceraldehyde via fructose-1-phospahte aldolase
what are the two options for glyceraldehyde?
1.
glyceraldehydeis converted into glyceraldehyde-3-phosphate (GAP) via glyceraldehyde kinase
thuis requires 1 ATP
G3P can then be used for glycolysis
- with the investment of 1 NADH, glyceraldehyde can generate glycerol via alcohol dehydrogenase
glycerol is then converted into glycerol-3-phosphate via glycerol kinase
glycerol-3-phosphate is used as a precursor for glycerophospholipids
with NAD+, DHAP is generated via glycerol phosphate DH
this supplies 1 NADH
DHAP is converted to GAP via TIM to enter glycolysis
why are there 2 options for glyceraldehyde, if both processes se the same amount of energy?
the pathway converting glyceraldehyde into glycerol is favoured when the liver needs to synthesize glycophospholipids
when this is not the case, glyceraldehyde will be directly converted into GAP
when does fructose enter the muscle tissue and that happens?
only about 5% of the time
in the muscle, hexokinase converts fructose into F6P with the investment of 1ATP
F6P is converted to F1,6BP via PFK
F1,6BP is converter to GAP/DHAP to enter glycolysis via fructose-1,6-phosphate aldolase
how does fructose enter glycolysis in the liver?
GAP
how does fructose enter glycolysis in the muscle ?
F6P
fructose metabolism in the fed state:
bypasses the PFK 1 control in anabolic liver and it is converted into fatty acids instead (via accumulation of acetyl coa)
explain the Leloir pathway
with the investment of 1 ATP, galactose can be converted into galactose-1-phosphate via galactokianse
galactose-1-phosphate is converted unto a UDP-galactose with a free floating UDP
UDP galactose is converted into UDP glucose via UDP-galactose-4-epimerase
the UDP glucose is then converted into G1P (via galactose-1-phosphate uridylyl transferase) and then further into G6P via phosphoglucomutase
the G6P will then be used for glycolysis
what causes galactosemia?
mutations in enzymes that prevent the conversion of galactose into G6P
this causes patients to not be able to consume dairy products
type 1 galactosemia
mutation in uridylyl transferase
UDP glucose -> G1P
type 2 galactosemia
mutation in galactokinase
galactose -> galactose-1-phosphate
type 3 galactosemia
epimerase
UDP galactose -> UDP glucose
mannose is a substrate for ____ and converted into___
hexokinase, mannose-6-phosphate
how is mannose-6-phosphate converted into F6P?
phosphomannose isomerase
