Biochemistry- an overview of metabolism

Question 1

What are the organs that most influence whole body metabolism

Answer 1

Brain: uses glucose as a metabolic fuel (during starvation can also use ketone bodies).
o Liver: when glucose is plentiful, it stores glycogen for its own use.
o Adipose tissue: when glucose is plentiful, makes glycogen and fat. Releases fuel during fasting and exercise.
o Muscle: when fat is plentiful, stores fat. Releases fat during fasting and exercise.

Question 2

Describe the effect of fed, fasting, starvation and exercise on the brain, skeletal muscle, liver, adipose tissue and digestive system

Answer 2

Fed state- 
Skeletal muscle: Glycolysis ↑ Fatty acid breakdown ↓
Liver: Receives portal blood first
Digestion products as fuel and
stores as glycogen or fat
Adipose tissue: Glucose used for fat synthesis
Digestive tissue: Sugars and amino acids
(hydrophilic molecules) enter the
portal vein to the liver
Fat (hydrophobic) enters lymph

Fasting state-
Skeletal muscle:Fat instead of
glucose
Liver: Glycogen → ↑
glucose Gluconeogenesis →
↑ glucose
Adipose tissue: Release fatty acids into blood for other tissues to use

Starvation-
Brain: Uses ketone bodies
Skeletal muscle: Fat and ketone bodies
Releases some amino acids
Liver: ↓ Gluconeogenesis
Fatty acids converted to
ketone bodies and released
Adipose tissue: Release fatty acids into blood for other tissues to use

Exercise
Brain uses glucose
Skeletal muscle: ↑ use of stored fuel
Uptake of
glucose/fat depends
on exercise
Adipose tissue: Release fatty acids into blood for other tissues to use

Question 3

What are the key metabolic pathways and their purpose

Answer 3

Glycolysis: only ATP producing pathway that occurs in the
cytosol, and does not need oxygen.
 Fatty acid oxidation: can produce large amounts of ATP, but
relatively slowly.
 Fatty acid synthesis: mostly exclusive to liver and adipose tissue.
 Gluconeogenesis: only occurs in liver, small amounts in kidney.
 Urea cycle: only occurs in liver.
 Citric acid cycle: intermediates are important for biosynthesis.
 Electron transport chain: produces most of the ATP.

Question 4

What is Gibbs free energy

Answer 4

Standard Gibbs free energy: 1 molar concentration of every reactant and product.
 For a given reaction: A+ B ⇌ C +D

The reaction can go forwards, backwards or form an equilibrium.
o A reaction that favours the product has a −∆G.
o A reaction that favours the reactants has a +∆G.

The standard ∆G can be used to calculate the ∆G for a reaction that is not in standard conditions:
∆G = ∆G
0 + RTln
[C] × [D]
[A] × [B]

Question 5

Describe 4 key conezymes: ATP, NAD(P)H, FADH and coenzyme A

Answer 5

1. ATP: adenosine triphosphate
    Much of the energy released during catabolism is temporarily stored making these
   high energy phosphodiester bonds.
    Many anabolic reactions are driven using the energy released by hydrolysis of the
   phosphodiester bonds.
2. NAD(P)H: nicotinamide adenine dinucleotide phosphate
    Coenzyme is reduced during catabolism and then can go onto reduce other compounds during
   biosynthetic reactions (NADPH) or take part in the electron transport chain (NADH).
3. FADH: Flavin adenine dinucleotide
    Reduced during catabolism.
4. Coenzyme A
    Enzyme involved in fatty acid metabolism as it can use the binding energy
   released by interactions with this coenzyme to drive catalysis.