Fuel storage

Fuels and fuel stores in animals
1. Major fuel stores include: glycogen (muscle and liver), triacylglycerol (adipose in liver and muscle), protein (muscle and other cells)
2. Animals cannot convert most fatty acids into carbohydrate
3. Total E store: triacylglycerol>> protein>> glycogen
  Annotations:
  - Protein can be used as a fuel but normally conserved until triacylglycerol in adipoose tissue is depleted.
Fate of major fuels during feeding
1. CARBOHYDRATE: converted to glucose (and other monosaccharides in the gut). Travels via blood to tissues, is then converted to CO2 (muscle), GLYCOGEN (liver, muscle) and FATTY ACIDS (--> TAG - liver and fat).
2. PROTEIN: converted to a.a in gut. Travels via blood to tissues, then converted to GLUCOSE (liver ---> glycogen), FA (-->TAG-liver,fat & --> CO2-muscle) and PROTEIN (muscle)
3. FAT: converted to FA and GLYCEROL in gut. Travels in blood as TAG (CMs) to tissues, then converted to FA (--> TAG- liver,fat) and CO2 (muscle)
Glycogen
1. Linear chain of glucose molecules joined by a-1,4 links with a-1,6 branch points.
Glycogen synthesis
1. GS incorporates glucose into glycogen with a-1,4 linkages. a-1,6 added by BRANCHING ENZYME
  Annotations:
  - GS- glycogen synthase
2. GS can only add glucose to existing chain.
  1. PRIMER required to intiate glycogen synthesis, GLYCOGENIN, a SELF GLYCOSYLATING protein.
    1. Glucose --> glucose 6-P --> glucose 1-P --> UDP glucose (added to glycogen using GS) --> glycogen strand (n+1).
      Annotations:
      - UDP : uridine diphosphate UTP : uridine triphosphate -equivalents to ATP and ADP
Glycogen breakdown (glycogenolysis)
1. Glycogen strand (n)
  1. glucose 1-P
    1. Glucose 6-P
      1. Glucose
      2. Glycolysis (muscle)
      3. glucose 6-phophatase (liver)
    2. phophoglucomutase
  2. +Pi, glycogen phophorylase
  3. Glycogen strand (n-1) +
2. Debranching enzymes break down the a-1,6 branch points
Regulation of glycogen metabolism
1. Key regulatory enzymes: GS, Phos
  Annotations:
  - GS- glycogen synthase Phos - Glycogen phosphorylase
  1. Allosteric control
    Annotations:
    - allosteric - alteration of the activity of an enzyme by means of a conformational change induced by a different molecule.
    1. LIVER: Glucose and Glc 6-P allosterically activate GS but inhibit Phos
    2. MUSCLE: Phos activated by Ca2+ and AMP
2. Hormonal control
  1. INSULIN: activates GS and inhibit Phos
    1. Effects brought about by changes in phosphorylation of GS and Phos
  2. ADRENALIN and GLUCAGON: inhibit GS and activate Phos.
Glycogen storage diseases
1. Rare genetic defects in glycogen metabolism affect a number of species.
2. TYPE 1: Glc 6-P deficiency. Hypoglycaemia.
3. Type 3: debranching enzyme defects.
4. Type 5: muscle phosphorylase defiency.
Fat storage
1. Fat - primarily stored as triacylglycerol in adipose tissue, small amounts stored in liver and muscle.
2. Types of adipose tissue
  1. WAP: main lipid storage. Metabolic priority = TAG storage, lipid synthesis and lipolysis
    Annotations:
    - WAP - white adipose tissue Histologically- contains large lipid droplet (80% of molecule), large nucleus and few mitochondria.
  2. BAP: Thermogenesis.
    Annotations:
    - BAP - Brown adipose tissue Histologically - few small lipid droplets (10% of molecule), several mitochondria and nucleus.
Brown adipose tissue
1. Many mitochondria and many small TAG droplets
2. IMPORTANT in new-born and cold adapted individuals
  Annotations:
  - because is required for thermogenesis
3. Inner mitochondrial membrane contains large amounts of uncoupler protein, thermogenin (UCP1)
  1. Thermogenin acts as proton channel and short-circuits the PROTON GRADIENT - energy released as HEAT= non shivering thermogenesis.
    1. stimulated by NORADRENALIN
      Annotations:
      - Noradrenaline is the main neurotransmitter of the sympathetic nerves in the cardiovascular system
Heat production in brown adipose tissue
1. Noradrenalin converts TAG --> FA using hormone sensitive lipase. This opens thermogenin proton channel creating a shorter circuit and allowing more rapid respiration and heat production.
Summary of TAG metabolism in adipose tissue
1. Glucose is continually added to adpiose cells. Glucose is used to make TAG which is lipolysed into glycerol and FA. FA leaves and binds to albumin in circulation and glycerol is converted to glucose in the liver.
  Annotations:
  - Glucose, pyruvate and acetyl co-A are activated by INSULIN. HSL - hormone sensitive lipase is activated by GLUCAGON and AFRENALIN. DHAP = dihyrdroxyacetone phosphate.

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Fuel storage

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	Created by Florence Papworth about 9 years ago