Lecture 6 Flashcards
What are the six processes that involve carbohydrate metabolism?
- glycogenesis
- glycogenolysis
- glycolysis
- hexose monophosphate shunt
- Krebs cycle
- gluconeogenesis
What are the three fates of glucose in the cell?
First turned into G6P and then:
1. glycogenesis - energy storage
2. glycolysis - energy production
3. hexose monophosphate shunt (pentose phosphate pathway) - generates precursors for biogenesis
Where does glycogenesis occur in the body?
Liver or muscles
When does glycogenesis come into effect?
when we are eating
What is the purpose of glycogenesis?
Store excess glucose for use by the body at a later time
What is glycogenin?
An enzyme that serves as a scaffold on which to attach glucose molecules to build glycogen. A primer, it initially attaches glucose molecules to itself before glycogen synthase takes over and adds glucose to the growing glycogen store, this process requires energy
What are the brief steps to glycogenesis?
- Glucose (blood) enters the liver/muscles through insulin stimulation
- Glucose (blood) uses ATP and hexokinase/glucokinase to form G6P
- G6P is converted into G1P by hexo/glucokinase and then converted into glycogen through glycogen synthase (which is activated by insulin)
What are the brief steps to glycogenolysis?
- Glycogen is broken down into G1P by glycogen phosphorylase (breaks alpha 1-4 glycosidic bonds) which is initiated by glucagon
- G1P is converted to G6P which has two possible fates: can be turned into fructose 6 phosphate which can be used for glycolysis OR can be converted into glucose by glucose 6 phosphatase for the blood-stream
What is the function of glycogenolysis
breaking down glycogen when starving
Which organ is the only organ that can release glucose back into the blood?
the liver
What is insulin resistance?
When the body produces insulin, but the cell doesn’t know it exists
What is the function of insulin?
Insulin reduces blood sugar, it helps package glucose in the liver, it is an anabolic hormone
What is the process of blood sugar levels and the pancreas and liver?
High blood sugar:
- promotes insulin release in the pancreas which goes to the liver to stimulate glucose -> glycogen which lowers blood sugar
Low blood sugar:
- promotes glucagon release in the pancreas which goes to the liver to stimulate glycogen -> glucose which raises blood sugar
Where does glycolysis take place?
cytoplasm
What are the brief steps to glycolysis?
Investment phase:
Glucose -> G6P -> Fructose-1,6-bisphosphate using glucokinase and hexokinase (step 1) and phosphofructokinase (step 3)
Generation phase:
Fructose-1,6-bisphosphate -> 2x Glyceraldehyde-3 phosphate -> 2x pyruvate
What is phosphofructokinase inhibited by?
ATP and glucagon (in the liver) therefore, if enough ATP is being produced by glycolysis, it inhibits phosphofructokinase so that it doesn’t continue glycolysis and keeps glucose for storage
What is the ATP and NADH usage and gained in glycolysis?
Investment: 2 ATP
Generation: 4 ATP and 2 NADH
Net energy yield from 1 glucose:
2 NADH + 2ATP
(1 NADH = 3 ATP)
2 NADH (3) + 2 ATP
8 ATP
What are the two fates of pyruvate?
Aerobic: Krebs cycle
Anaerobic: Lactate
What are the steps to anaerobic metabolism of glucose?
Pyruvic acid -> (pyruvate decarboxylase) acetaldehyde -> (alcohol dehydrogenase, NADH) ethanol
Pyruvic acid -> (lactate dehydrogenase, NADH) Lactic acid
When does lactic acid production occur?
In anaerobic conditions after glycolysis
- occurs in muscle during prolonged exercise and in RBC
- pyruvate converted to lactate in cytosol
- regenerates NAD+ which allows glycolysis to cont
- net of 2 ATP produced when glucose -> lactate
When does ethanol production occur?
In anaerobic conditions:
- doesn’t happen in the body
- basis of fermentation
- yeast breaks down pyruvate into CO2 and ethanol
- regenerates NAD+ to allow glycolysis to cont
What is the cori cycle steps?
In the liver:
2 lactate -> 2 pyruvate (reversible) -> glucose (gluconeogenesis) using 6 ATP
In the muscle:
Glucose -> 2 pyruvate using 2 ATP (glycolysis) -> 2 lactate (reversible)
glucose can be transported from liver-> muscles through blood and reverse
Where does the hexose monophosphate shunt occur?
cytoplasm
What is the hexose monophosphate shunt important for?
Important for NADPH production and ribose synthesis
What are the steps to the hexose monophosphate shunt?
- Glucose → G6P, enters pathway
- 2 phases can be entered depending on requirements of cell
- Oxidative phase is unidirectional
- Nonoxidative phase (regenerative phase) is bidirectional
Which cells use non-oxidative phase vs oxidative phase?
All cells use the nonoxidative phase (for R5P), but only cells that perform biosynthesis (making fatty acids, antioxidant activity) will use the oxidative phase
What is the oxidative phase of the hexose monophosphate shunt?
G6P-> glucose-6-phosphate dehydrogenase 6PG-> 6PG lactone-> Ribulose-5-phosphate-> ribose-5-phosphate which synthesizes nucleotides
produces 2 NADPH
can be converted back to G6P
What is the non-oxidative phase of the hexose monophosphate shunt?
G6P-> F6P-> intermediates -> ribose-5-phosphate which synthesizes nucleotides
What are the NADH produced in the oxidative phase of the hexose monophosphate shunt used for?
biosynthesis of fatty acids, role in anti-oxidant production
What inhibits glucose-6-phosphate dehydrogenase?
NADH produced in the oxidative phase of the hexose monophosphate shunt
What are the vitamins that are cofactors the pyruvate dehydrogenase complex?
thiamine
niacin
riboflavin
pantothenic acid
What is the net energy yield of pyruvate dehydrogenase?
about 6 ATP
What are the steps of the Krebs cycle?
acetyl coA-> citrate-> isocitrate-> alpha ketoglutarate-> succinyl coA-> succinate-> fumarate-> malate-> oxaloacetate
Which products of the krebs cycle can amino acids form?
succinyl-CoA: amino acids can be converted into succinyl CoA
Oxaloacetate: amino acids can be converted into pyruvate and oxaloacetate
What does acetyl-CoA activate?
Activates pyruvate carboxylase which catalyzes the conversion of pyruvate to oxaloacetate to initiate the krebs cycle
What is the total energy yield for the krebs cycle?
energy yield from 1 acetyl CoA
3NADH, 1FADH2, 1GTP
equivalent to 12 ATP
How much energy do you get from one molecule of glucose?
Glycolysis, pyruvate dehydrogenation, krebs cycle
38 ATP
What is the conversion of other energy molecules to atp?
1 NADH = 3 ATP
1 FADH2 = 2 ATP
1 GTP = 1 ATP
What is gluconeogenesis used for?
pathway that is active when glucose is needed by the body
Where does gluconeogenesis occur?
very active in the liver but can also happen in the kidney during starvation
What parts of the body lack enzymes for gluconeogenesis?
Muscle and adipose tissue
What are the enzymes involved in gluconeogenesis?
- Pyruvate carboxylase and PEP carboxylase
- Fructose-1,6-bisphosphatase
- Glucose-6-phosphatase
enzymes to bypass irreversible steps and allow gluconeogenesis
