Pii: Outline protein metabolism, including the effects of starvation

Definitions

  • Proteins are large polypeptides of amino acids linked through peptide linkages

Protein Ingestion

  • Almost all protein is digested & absorbed as amino-acids
  • 20 a-acids, 10 of which are essential and cannot be synthesised in the body in sufficient quantities and must be obtained from food
  • [Amino acid] in blood = 35 – 65mg/dL
  • But [amino acid] in blood is only slight because protein digestion occurs over 2 – 3hrs & amino acids are rapidly absorbed by cells, especially the liver
  • Transport of amino acids into cells is by carrier mechanism (because they’re so large)

Fate of Absorbed Amino Acids

  1. Storage
  2. Used for function
  3. Used for energy

1) Storage

  • Enters cells → combines with other peptide linkages
  • Directed by mRNA & ribosomes to form cell proteins

2) Functional

  • Liver forms albumin, fibrinogen, plasma proteins
  • Liver can form 30g/day of plasma proteins

3) Energy

  • When cells replenish their protein stores, additional amino acids are degraded for E & stored as fat/glycogen

↓

Degradation process

  • Deamination = removal of amino group from a-acid → in this process, NH3 is formed
  • Urea formation by liver = all NH3 released into blood is converted to urea
  • Oxidation of deaminated a-acids = once deaminated, resulting ketoacids can be oxidised for metabolism
  • Gluconeogenesis & ketogenesis

Obligatory protein degradation = 30g/day

  • Body protein is degraded, deaminated & oxidised
  • ∴To prevent loss needs ingestion of 60g/day

Protein metabolism influenced by hormones:

Protein Synthesis

GH

Insulin

Testosterone

Protein Breakdown

Glucocorticoids

  • Thyroxine overall ↑ all aspects metabolism

Starvation

  • Starvation = state of inadequate E supply
  • During fasting BGL needs to be maintained at 4 – 6mmol/L → required as 1° E source of brain, RBC
  • Body obtains E from endogenous sources:
    1. GLYCOGEN
    2. GLUCONEOGENESIS
    3. FFAs

Glycogenolysis

  • Main stores glycogen are liver 100g & muscle 400g
  • ↓BGL → ↑glucagon → stimulates glycogenolysis
  • Also stimulated adrenaline (in fight or flight, not starvation)
\( \textbf{GLYCOGEN } \underrightarrow{\text{Glycogen phosphorylase}} \textbf{ GLUCOSE 1 PHOSPHATE } \underrightarrow{\text{Phosphoglucomutase }} \textbf{ GLUCOSE 6 PHOSPHATE } \underrightarrow{\text{G-6-phosphatase }} \textbf{  GLUCOSE }  \)
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\( \textbf{GLYCOGEN } \underrightarrow{\text{Glycogen phosphorylase}} \textbf{ GLUCOSE 1 PHOSPHATE } \underrightarrow{\text{Phosphoglucomutase }} \textbf{ GLUCOSE 6 PHOSPHATE } \underrightarrow{\text{G-6-phosphatase }} \textbf{  GLUCOSE }  \)
  • Glycogen stores are depleted in 24hrs
Glycogenolysis in Liver

Gluconeogenesis

  • Synthesis of glucose from non-carb precursors
  • Major non-carb precursors are amino-acids, lactate & glycerol
  • Major site of gluconeogenesis is liver, some can also occur on cortex of kidney
  • Lactate & amino acids enter as pyruvate
  • Glycerol enters as dihydroxyacetone phosphate
Gluconeogenesis

FFAs

  • FAs can be oxidised by β-oxidation into Acetyl-CoA → Kreb’s
FFAs