Overview of Carbohydrate Metabolism

Question 1

Main metabolic pathways of carbohydrates

Answer 1

1. Glycolysis
2. The TCA cycle
3. Gluconeogenesis
4. Glycogen synthesis and breakdown
5. The Pentose Phosphate Pathway

Question 2

Oxidation definiction

Answer 2

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

Question 3

Fxn of “linked enzyme pathways”

Answer 3

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

Question 4

Km definition

Answer 4

• 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

Question 5

Vmax definition

Answer 5

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

Question 6

Characteristics of/clues to “key steps” in biochemical pathways

Answer 6

• 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

Question 7

Key steps in glycolysis

Answer 7

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

Question 8

Pyruvate destination: oxygen vs. no oxygen

Answer 8

• 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

Question 9

Steps of TCA cycle

Answer 9

• 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

Question 10

Primary fxn of glycolysis

Answer 10

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

Question 11

Primary fxn of gluconeogenesis

Answer 11

• @ 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.

Question 12

Primary fxn of glycogen synthesis/breakdown

Answer 12

• 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.

Question 13

Primary fxn of TCA cycle

Answer 13

• 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.

Question 14

Primary fxn of electron transport system

Answer 14

• 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

Question 15

Glucose levels impact on metabolic pathways

Answer 15

• 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

Question 16

Positive vs. negative energy balance impact on carbohydrate metabolic

Answer 16

• 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

Question 17

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

Answer 17

• 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

Question 18

Major forms of enzyme modification

Answer 18

• 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