Carbohydrate Metabolism and dietary handling Flashcards
What are the 5 portions of metabolism of carbohydrates?
- Glycolysis
- TCA cycle
- Gluconeogenesis
- Glycogen synthesis and breakdown
- Pentose Phosphate Pathway
What is oxidation?
- Transfer of electrons from a reduced molecult ot an acceptor molecule
- The acceptor molecule gains the electron and becomes reduced
- Donor molecule loses an electron and becomes oxidized
What is the Km?
- The concentration at which the reaction is half maximal is the Km
- If a reaction has a low Km it suggests that the substrates have a strong affinity for the enzyme and the reaction will go at low substrate concentrations
What is Vmax?
- Maximal rate
* High Vmax means there will be a ton of product in a short period of time
Why is conversion of glucose to G6P a key step in glycolysis?
- This traps the glucose inside the cytoplasm
* It’s also an ATP investment step
What is the rate-limiting step in glycolysis?
- The conversion of fructose 6P to fructose-1,6-bis-P is rate-limiting and very important
- The bottleneck of glycolysis
- Performed by PFK1
- Second investment of ATP step
Pyruvate is important why?
• Energy source for other pathways after glycolysis
• This is the end-result of glycolysis
• With oxygen and mitochondria, TCA cycle and electron transport
• It can also be put into formation of fatty acids
*when no mitochondria like in sperm and RBCs, conversion to lactate and export form the cell helps regenerate the NAD from NADH to allow glycolysis to continue (since this reaction by itself can produce some ATP)
NADH and FADH2 are important for what cycle?
• Electron transport chain
What is the overall goal of TCA cycle?
• Acetyl-CoA to CO2 w ith the release of a good deal of energy stored in GTP, NADH and FADH2
What is nice about the variety and flexibility in the carbon skeletons being used in the TCA cycle?
• You can get, in times of excess, acetyl-coa that goes in but gets kicked out for other reactions like the synthesis of fatty acids
What is the electron transport system?
- The inner membrane of the mitochondria
* NADH and FADH2 must get into the mitochondria to participate in the electron tansport chain
What’s up with gluconeogenesis?
• Glucose for the brain is super important
• In fasting state the liver and kidney produce glucose
• Sources for carbon in this processare typically lactate, amino acids from muscle, or glycerol from triglyceride breakdown in adipose tissue
• These substrates are transported through the blood stream to the liver where they are converted to glucose (gluconeogenesis)
• Amino acis must first lose nitrogen (urea cycle)
• Essentially gluconeogenesis is glycolysis in reverse but it uses different enzymes
○ These enzymes are regulated in the reverse manner
• Also, this uses energy, it doesn’t create energy
• THREE KEY REGULATED STEPS here
○ Same as in glycolysis
• First - conversion of pyruvate to phosphoenol pyruvate
○ Through oxaloacetate and malate and is catalyzed by pyruvate carboxylase and phospho-enol-pyruvate carboxy-kinase (PEPCK)
○ Second - producto of F6P from F16BP
§ By F16bisphosphatase
What’s up with glycogen?
- Rapidly available source of glucose for acute energy needs
- Stored within tissues, though only liver stores are available for direct brain use
- Synthesized from glucose-6-P
- Important hub in glucose metabolism because it either goes down glycolysis pathway or gets stored as glycogen
- First step is conversion of G6P to glucose-1-P then converting glucose-1-P to UDP-glucose
- Committed step in synthesis of glycogen thus KEY
- Second key step is catalyzed by the enzyme glycogen synthase which adds UDP-glucose to growing glycogen molecule
- Glycogen phosphorylase is the enzyme that removes glucose from glycogen and another key enzyme highly regulated
- Once activated, glycogen is made by glucose being added in a 1-4 orientation
- Highly brached polymer for the rapid release of many glucose molecules at a time
- Super important for STEP as inborn glycogen storage diseases result in poor release and poor storage of glucose
- Glycogen breakdown is from the ends until a branch-point is reached then another enzyme takes off the branch point
Describe in general the pentose phosphate pathway
• AKA hexose monophosphate shunt
• Modest amount of lgucose in the liver glycolysis is the preferred pathway
• When more glucose is available, glycogen storage becomes a site for glucose disposal
• When glucose is abundant, directed to pentose phosphate pathway with two main functions
○ Generation of NADPH, the source of energy for synthesis reactions
§ Fatty acid, cholesterol, defense against oxidative stress, WBC function
○ Generation of five cabon sugars for nucleotide synthesis
• KEY STEP = oxidation of glucose-6-phosphate to 6-phosphogluconolactone
○ By G6PD
○ Glucose-6-phosphate dehydrogenase
Insulin is balanced by counter-regulatory hormones. What are these?
• Glucagon • Catecholamines • Growth hormone • Cortisol ○ In particular, catecholamines and glucagon bind to membrane associated receptors that alter intracellular signaling pathways typically by changing the levels of cAMP and kinases that phosphorylate key enzymes in tehse pathways in a manner that alters activities