Fate of proteins

Question 1

What are the functions of proteins in the body?

Answer 1

transporters for hydrophobic compounds in blood
cell adhesion molecules
hormones
ion channels
enzymes

Question 2

what is the primary structure of a protein?

Answer 2

amino acids sequence

Question 3

Describe protein metabolism after eating

Answer 3

Ingested proteins digested to amino acids in stomach and SI
AAs transported to liver via hepatic portal vein
AAs used to synthesise new proteins (biosynthesis) or converted to energy sources (gluconeogenesis)
AAs also enter general circulation and go into tissues to be synthesised into new proteins

Question 4

What happens to amino acids in the liver?

Answer 4

Synthesis of serum proteins
biosynthesis of nitrogen-containing compounds that need AA precursors e.g., non-essential AAs, haeme, hormones, neurotransmitters, DNA
Oxidizes AAs to produce energy via Krebs cycle in the fed state

Question 5

Describe how the balance of AAs in the blood is maintained

Answer 5

Proteins undergo turnover, constantly being synthesised and degraded, especially in muscle tisse
AAs released by protein breakdown enter the same pool of free AAs in the blood from the diet

Question 6

Describe protein metabolism in the fed state

Answer 6

AAs released from dietary proteins sent to liver for synthesis of proteins
Excess AAs converted to glucose or triacylglycerols
Triacylglycerols packaged and secreted in VLDL
Glucose stored as glycogen or released into blood if blood glucose levels are low
AAs that pass through the liver are converted to proteins in the cells of other tissues

Question 7

Describe protein metabolism during fasting

Answer 7

AAs released from muscle protein
Some enter blood
Some are partially oxidised and the nitrogen is stored as alanine and glutamine which enter the blood (gluconeogenic AAs)
AAs enter the liver, nitrogen is converted to urea (excreted in urine) and carbons converted to glucose and ketone bodies which are oxidised for energy