Amino Acid Metabolism

Question 1

What is it important to remember about the half lives of individual proteins?

Answer 1

All enzymes and proteins turnover at different rates, e.g adult collagen is relatively inert and has a half life of 300 days, but lipoprotein lipase has a half life of only 1 hour.

Question 2

What are essential amino acids?

Answer 2

Amino acids that can’t be synthesised by the body by transamination, so have to be obtained from the diet in order to maintain the nitrogen balance in the body.

Question 3

What are non-essential amino acids?

Answer 3

Amino acids that can be synthesised in the body from common metabolic intermediates by transamination.

Question 4

What are semi-essential amino acids?

Answer 4

Amino acids that are synthesised in the body by transamination from essential amino acid precursors, so are essential in the diet to stop essential amino acids being used up. They become essential under conditions of metabolic stress.

Question 5

What do you call amino acids without the amino group?

Answer 5

Carbon skeletons.

Question 6

What is deamination?

Answer 6

Transferring the amino group to form glutamate by transamination, then glutamate being oxidised (oxidative transamination by glutamate dehydrogenase) to produce ammonia which is excreted as urea.

Question 7

What is the co-factor for transaminase enzymes?

Answer 7

Vitamin B6.

Question 8

How does transamination work?

Answer 8

Amine group of amino acid is transferred to transaminase enzyme. Transaminase enzyme transfers amine group onto alpha keto-acid (e.g pyruvate) to produce corresponding amino acid. Remnants of original amino acid are alpha keto-acid (carbon skeleton).

Question 9

Name the three main transaminase enzymes in the liver.

Answer 9

Alanine aminotransaminase (ALT).
Aspartate aminotransferase (AST).
Glutamate aminotransferase.
ALT and AST are used in liver biochemistry tests, as damage to hepatocytes causes them to leak out into the blood.

Question 10

What does hypoalbuminaemia show?

Answer 10

Advanced chronic liver disease, or severe acute liver damage.

Question 11

What do increases in blood alkaline phosphatase (ALP) show?

Answer 11

Increased synthesis by bile cannaliculi, usually due to cholestasis. (Gamma glutamyl transpeptidase [GGT] also raised in cholestasis).

Question 12

What can happen to the carbon skeletons of the amino acids?

Answer 12

Glucogenic: converted to pyruvate, then glucose and can be oxidised or stored as glycogen.
Ketogenic: converted to acetyl coA, then oxidised, or converted to fatty acids and stored as TGA, or converted to the ketone bodies acetoacetate and butyrate (used to fuel CNS during starvation).

Question 13

Where is the concentration of ammonia high?

Answer 13

In the liver, because it is produced by oxidative deamination of glutamate, and in hepatic portal venous blood because it is produced by bacteria in the colon from urea and recycled.

Question 14

How is urea formed?

Answer 14

2 ammonia + carbon dioxide ==> urea
One ammonia comes from glutamate by glutamate dehydrogenase, the other from aspartate.
The urea cycle only happens in hepatocytes.

Question 15

What happens to urea?

Answer 15

It diffuses into the gut lumen, is hydrolysed by bacteria to produce ammonia which is transported back to the liver via the portal vein to produce amino acids. It is also excreted in urine.

Question 16

How is the glucose level in the blood regulated?

Answer 16

There is coarse autoregulation by the liver (an isolated liver could maintain the correct level of glucose by glycogenolysis and glycolysis without any nervous or hormonal stimulation).
Superimposed on that there is hormonal control by insulin and glucagon.
Superimposed on that there is stimulation by adrenaline (which can cause immediate glycogenolysis in the liver, and slower glycogenolysis in muscles).

Question 17

What does insulin stimulate?

Answer 17

Increased glucose transport into cells, increased storage as glycogen, increased protein synthesis and lipogenesis. Decreases glycogenolysis, ketogenesis, lipolysis.

Question 18

What does glucagon stimulate?

Answer 18

Increased glycogenolysis, gluconeogenesis, and ketogenesis in hepatocytes.

Question 19

Why is there a big cost of protein in repair of trauma?

Answer 19

A lot of protein is lost so has to be replaced.

Question 20

Why does the guy use up a lot of protein itself?

Answer 20

To produce enterocytes, enzymes, mucus.

Question 21

What are the two types of amino acids?

Answer 21

Ketogenic - converted to acetyl coA

Glucogenic - converted to glucose

Question 22

What do you call amino acids without the amino group?

Answer 22

Carbon skeleton

Question 23

What happens in the “fed” stage where there is high insulin?

Answer 23

Proteins hydrolysed to amino acids in duodenum and jejunum.
Amino acids transported to the liver in the portal vein.
Liver carries out protein synthesis, and stores excess glucogenic amino acids as glycogen, and ketogenic amino acids as triacylglycerol.
Liver also transports amino acids to extra hepatic tissues like the muscles for protein synthesis.

Question 24

What happens in the “fasting” stage where there is high glucagon?

Answer 24

White adipose tissue releases fatty acids which are oxidised in the liver to ketones.
Muscles release amino acids to the liver, which the liver converts to pyruvate and acetyl coA.

Question 25

What are four uses of amino acids that are not in excess?

Answer 25

Growth and repair of cells, plasma proteins, hormones, enzymes.

Question 26

What are three uses of amino acids that are in excess?

Answer 26

Gluconeogensis
Lipogenesis
Deamination (glutamate donates amino group to urea cycle in liver)

Question 27

What is an alpha keto-acid?

Answer 27

What is formed when an amino acid is deaminated, or what recipes the amino group in transamination to form an amino acid.

Question 28

In the first stage of deamination, which enzyme transfers the amino group to produce glutamate and an alpha-keto acid?

Answer 28

Glutamate aminotransferase

Question 29

Which reaction is catalysed by glutamate dehydrogenase?

Answer 29

Oxidative transamination - the deamination of glutamate to produce ammonia (to combine with carbon dioxide to produce urea) and alpha-ketoglutamate.

Question 30

What does autoregulation by the liver mean?

Answer 30

An isolated liver can autoregulate the glucose concentration in the fluid flowing through it, by changing the rates of glycogenolysis and glycogenesis.
This coarse autoregulation is superimposed by hormonal and nervous stimulation.