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Flashcards in Overview of Carbohydrate Metabolism Deck (19)
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Main metabolic pathways of carbohydrates

1. Glycolysis

2. The TCA cycle 

3. Gluconeogenesis 

4. Glycogen synthesis and breakdown

5. The Pentose Phosphate Pathway


Oxidation definiction

  • transfer of electrons from a reduced molecule to an acceptor
  • reduced molecule = contains lots of potential chemical energy (electrons)
    • e.g. glucose


Fxn of "linked enzyme pathways"

  • facilitate the stepwise modification of the parent molecule in an energy-efficient manner
  • e.g. glycolysis


Km definition

  • concentration of substrate @ which the rxn proceeds at half maximal rate
  • low Km = substrates have strong affinity for enzyme; rxn will proceed @ low substrate concentrations


Vmax definition

  • Vmax = maximal rate of rxn
  • high Vmax = rxn will produce a lot of product over a short period of time


Characteristics of/clues to "key steps" in biochemical pathways

  • step in which important molecule changes location
    • e.g. enters/leaves cell, mitochondria,etc
  • step in which energy is used to transition from one molecular state to another
    • e.g. addition of phosphate group
  • rate limiting step


Key steps in glycolysis

  1. Glucose ==> cytoplasm via glucose transporter
  2. Glucose + ATP ==> Glucose-6-P  
    1. G6P is now trapped in cell
  3. G6P + Phospho-fructo-kinase (PFK)  + ATP ==> Fructose 1,6-biphosphate
    1. activating step
  4. Fructose 1,6-bis-P ======> Pyruvate + ATP + NADH
    1. pyruvate = 3 Carbon compound


Pyruvate destination: oxygen vs. no oxygen

  • if O2 + mitochondria present:
    • pyruvate ==> mitochondria
    • pyruvate ==> acetyl CoA ==> TCA cycle
    • ==> complete oxidation to CO2 & generation of NADH & FADH2
    • energy of NADH & FADH2 ==> electron transport chain @ wall of mitochondria ==> O2 ==> production of ATP & H2O
  • if NO O2 or mitochondria:
    • pyruvate ==> lactate ==> exported from cell


Steps of TCA cycle

  • pyruvate ==> mitochondria ==> acetyl-CoA
  • acetyl-CoA ==> TCA cycle = circle of linked enzymatic reaction
  • ==> CO2 production + NADH + FADH2 + GTP ("energy carrier" molecules)
  • NADH & FADH2 ==> electron transport chain


Primary fxn of glycolysis

The purpose of glycolysis is the generation of energy and useful chemical intermediates from the breakdown of glucose


Primary fxn of gluconeogenesis

  • @ negative energy balance, the liver and to a lesser extent the kidney produce glucose.
  • Lactate, or glycerol, or AA → liver → glucose
  • The process of gluconeogenesis is essentially glycolysis in reverse except that it uses different enzymes and is regulated in an opposite manner.


Primary fxn of glycogen synthesis/breakdown

  • Glycogen synthesis: A rapidly available source of glucose for oxidative needs when the body needed energy is glucose stored within tissues as glycogen. This polymer of glucose is synthesized from glucose-6-P.
  • Glycogen breakdown:  glycogen is a highly branched polymer. This allows the rapid release of many glucose molecules when needed.


Primary fxn of TCA cycle

  • TCA cycle is at the heart of oxidative metabolism
  • circle of linked enzymatic reactions
  • TCA cycle provides a flexible circuit (like a roundabout at an intersection) into which a variety of carbon skeletons can enter and be modified while always replenishing substrates within the cycle.  


Primary fxn of electron transport system

  • linked reactions of the electron transport system are separate from the TCA cycle
  • occur @ inner mitochondrial membrane and the space between the inner and outer mitochondrial membranes
  • ==>  conversion of O2 to H2O and ADP to ATP in a process known as oxidative phosphorylation


Glucose levels impact on metabolic pathways

  • When glucose is abundant, glycolysis predominates.
  • When glucose is in short supply, gluconeogenesis predominates.
  • When more glucose is available, glycogen storage becomes a site for glucose disposal.
  • When glucose is particularly abundant, it is directed to the pentose phosphate pathway


Positive vs. negative energy balance impact on carbohydrate metabolic

  • Glycogen synthesis is stimulated when the liver/skeletal muscle is in positive energy balance
  • glycogen breakdown is activated when the body is in negative energy balance


General factos that impact fluctuation of metabolic pathways (enzymatic factors)

  • More substrate → more product
  • More enzymes for a given pathway → more flux down this pathway
  • Allosteric regulation
  • Covalent modification of a key enzyme
  • Hormones can regulate flux through these pathways as well


Major forms of enzyme modification

  • Allosteric regulation: a molecule/product can alter activity of a key enzyme in a pathway
  • Covalent modification of a key enzyme: enzyme activity can be changed if phosphorylated or otherwise modified