Amino Acid Metabolism

Front 1

What is amino acid metabolism?

Back 1

Protein break down through sequential enzyme digestion

Front 2

What is a protease?

Back 2

Enzyme that catalyses the hydrolytic breakdown (hydrolysis - chemical breakdown due to reaction with water) of proteins into peptides or amino acids (proteolysis)

Front 3

What is an endopeptidase?

Back 3

An enzyme that breaks peptide bonds other than terminal ones in a peptide chain

Front 4

What is an exopeptidase?

Back 4

A peptidase that catalyses the removal of the last (carboxypeptidases) or first (aminopeptidases) amino acid from a peptide chain.

Front 5

What enzyme does the stomach secrete?

Back 5

Pepsin (an endopeptidase) in its inactive form (zymogen) pepsinogen

Front 6

How is pepsinogen activated?

Back 6

Cleavage (by HCl) of a peptide fragment from its amino terminus

Front 7

When does this activation occur?

Back 7

Autoactivated by active pepsin

Also, when pH of stomach lumen <5 - acidity denatures proteins so they are more susceptible to hydrolysis

Front 8

What are serine proteases(/endopeptidases)?

Back 8

Enzymes that cleave peptide bonds in proteins in which serine is one of the amino acids in the enzyme's active site.

Front 9

What enzymes does the pancreas secrete?

Back 9

Serine endopeptidases - Trypsin, chymotrypsin and elastase.

Exopeptidases - carboxypeptidases A&B

Front 10

Where do these enzymes act?

Back 10

Work in the lumen of the small intestine

Neutral conditions due to secretion of bicarbonate rich pancreatic juice

Front 11

In what form are all pancreatic proteases secreted in?

Back 11

Inactive precursors - (trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase A&B)

Front 12

How is trypsinogen activated?

Back 12

By enteropeptidase (also called enterokinase)

By active trypsin (autocatalytic process)

Front 13

What releases enteropeptidase?

Back 13

Secreted from enterocytes (the epithelial cells of the small intestine)

Front 14

How are the other inactive precursors secreted by the pancreas activated?

Back 14

Cleavage with trypsin

Front 15

What is left after digestion by pancreatic enzymes?

Back 15

Mix of amino acids and small peptides up to 6 amino acids long (oligopeptides)

Front 16

Diseases which interfere with pancreatic secretion... (2) - associated problems

Back 16

Pancreatitis and CFTR - prevent proper protein digestion resulting in protein malabsorption leading to malnutrition

Front 17

What is the treatment for reduced pancreatic secretion?

Back 17

Supply either extra exogenous pancreatic enzymes or dietary supplements of easily digested proteins

Front 18

After pancreas where do digestive enzymes come from and at?

Back 18

Brush-border membrane of enterocytes

Lumen of small intestine

Front 19

What enzymes does the brush-border contain?

Back 19

Endopeptidases, aminopeptidases and dipeptidases

Front 20

After brush-border enzyme digestion, what is left?

Back 20

Mixture of dipeptides, tripeptides and amino acids

Front 21

What takes up di- and tripeptides into enterocytes?

CLINICAL: What else is this transporter responsible for?

Back 21

Proton-coupled transporter

Absorption of beta-lactam antibiotics

Front 22

What absorbs amino acids into enterocytes?

Back 22

A number of mainly sodium-coupled transport systems

Front 23

What happens to di- and tripeptides once in enterocytes?

Back 23

Cleaved by intracellular peptidases into free amino acids

Front 24

What happens to amino acids in enterocytes?

Back 24

Released via the basolateral membrane and enter the circulation

Front 25

What are essential amino acids? Name them

Back 25

Those that the body cannot make - alanine, aspartate, asparagine, cysteine, glutamate, glutamine, glycine, prolein, serine and tyrosine.

Front 26

Uses of amino acids

Back 26

Protein synthesis, hormone production e.g. adrenaline, neurotransmitter synthesis.

Deaminated - remaining carbon skeleton can then be either oxidised via TCA cycle, converted into glucose via gluconeogenesis or turned into fatty acids

(not all are capable of all these fates)

Front 27

Which amino acids are glucogenic?

Back 27

Those which can be degraded to pyruvate or TCA cycle intermediates

Front 28

Which amino acids are ketogenic?

Name the two which are solely ketogenic

Back 28

Those which are convert into acteyl-CoA or acetoacetyl-CoA

Leucine and lysine

Front 29

What is the term for amino acids which are both gluco and ketogenic?

Back 29

Mixed

Front 30

Points of entry of amino-acid carbon skeletons into the citric acid cycle and into ketone body synthesis...

Back 30

Front 31

Where are excess amino acids stored?

Back 31

They cannot be stored

Front 32

First step of amino acid oxidation

Back 32

Remove amino group (deamination) - excreted as urea

Front 33

How is the amino group removed?

Back 33

Transamination - catalysed by aminotransferases (transaminases)

Each amino acid has its own specific aminotransferase

Front 34

What is the overall reaction for transamination?

Two features of the reaction

Back 34

Amine group of an amino acid and keto group of a keto acid are exchanged

Easily reversible, requires no energy input

Front 35

What is the co-factor of all aminotransferases?

Back 35

Pyridoxal phosphate (vitamin B6 derivative)

Front 36

Most common amino group acceptor?

What does this form and how is the product used?

Back 36

Alpha-ketoglutarate

Glutamate - provides pool of amino groups for non-essential amino acid synthesis and deamination.

Front 37

Two other amino group acceptors and what they form as a result

Back 37

Pyruvate --> alanine

Oxaloacetate --> aspartate

Front 38

What deaminates glutamate

Back 38

Glutamate dehydrogenase

Front 39

What is the benefit of pooling excess amino groups into glutamate?

Back 39

Only one deamination pathway is required

Front 40

Overall deamination reaction and where it takes place

Back 40

Removal of amine group

Mitochondria of liver cells

Front 41

Main fate of ammonium (NH4+) from deamination reaction?

Back 41

Incorporation into urea for excretion

Front 42

What allosterically regulates glutamate dehydrogenase and why?

Back 42

Increases in ADP and GDP - these compounds signal that amino acids need to be used as an energy source.

Front 43

Other sites of NH4+ production (3)

Back 43

Brain, muscle, intestinal cells

Front 44

How are ammonium ions produced in the brain?

Back 44

Inactivation (through breakdown) of neurotransmitter GABA into succinate and NH4+

Front 45

How do ammonium ions produced in the brain enter the urea cycle?

Back 45

One NH4+ combined with alpha-ketoglutarate to produce glutamate and then another incorporated to form glutamine --> transported to liver where it is deaminated and enters urea cycle

Front 46

How are ammonium ions produced in muscle? (3)

Back 46

Natural protein turnover
Muscle catabolism during starvation
Breakdown of excess ADP during extreme exercise -
2ADP ----> ATP + IMP + NH4+

Front 47

What happens to the NH4+ produced in muscle?

Back 47

Combines with alpha-ketoglutarate to form glutamate - used to transaminate pyruvate, forming alanine and regenerating alpha-ketoglutarate.

Front 48

What is the fate of alanine produced from the transamination of pyruvate in muscle cells?

Back 48

Released into the bloodstream and taken up by the liver.
Deaminated, producing pyruvate which can either be oxidised in the TCA cycle or used for gluconeogenesis.

Front 49

What happens to the NH4+ produced in intestinal cells (enterocytes)?

Back 49

Same as brain, NH4+ combined with alpha-ketoglutarate to produce glutamate and then another incorporated to form glutamine - this serves as an energy source for the cell.

Front 50

What are the two forms in which excess nitrogen can be excreted?

Back 50

Urea or ammonium ions

Front 51

Where in the body is urea generated and why?

Back 51

In the liver as a soluble, non-toxic way of eliminating excess ammonia

Front 52

What else can deaminate glutamate?

Back 52

The renal cortex

Front 53

How is ammonium used in the renal cortex?

Back 53

Ammonium is used to assist with acidifying urine.

Front 54

Why is the mechanism for deamination of glutamate in the renal cortex beneficial?

Back 54

Mechanism conserves HCO3- which would otherwise need to be used in urea synthesis - would exacerbate any acidosis.

Front 55

What happens if protein intake is greater than need?

Back 55

Cannot store excess amino acids - carbon skeletons are used/stored and unwanted amino groups are excreted.

Front 56

What happens if protein intake is less than need?

Back 56

Protein catabolism to free carbon skeletons for energy - the amino groups produced need to be excreted

Front 57

Why do blood ammonia levels need to be kept low?

What is the normal value?

Back 57

Toxic

(25-40 μM - micro molar)

Front 58

What happens if ammonium ion levels rise?

Back 58

NH4+ reacts with alpha-ketoglutarate to form glutamate

Front 59

What are the consequences for the brain of raised NH4+ levels?

Back 59

Resulting raised glutamate levels reduce the rate at which ATP can be formed, starving brain cells of energy

Front 60

What is 'nitrogen balance'

Back 60

State of healthy adults - around 80% of excess nitrogen excreted as urea (remainder in free ammonium ions and creatine)

Front 61

Where is most urea synthesised? (Organ)

Why is rate of synthesis strictly controlled?

Back 61

In the periportal cells (those surrounding the portal vein) or the liver.

To prevent ammonia build up

Front 62

The urea cycle

Back 62

Formation of urea from one free ammonium ion and one donated from aspartate

Front 63

Where does the urea cycle take place? (Cell)

Back 63

Partly in the mitochondrial matrix, partly in the cytoplasm

Front 64

What two amino acids does the urea cycle include that are not found in proteins?

Back 64

Ornithine and citrulline

Front 65

What are the two levels of urea cycle control?

Back 65

Acute and chronic

Front 66

What enzyme controls acute regulation of the urea cycle and what is this regulated by?

Back 66

Carbamoyl-phosphate synthetase is regulated by the concentration of the allosteric activator N-acetyl-glutamate

Front 67

What forms N-acetyl-glutamate and what stimulates this?

Back 67

N-acetyl-glutamate synthase - activity stimulated by arginine (intermediate of the urea cycle)

Front 68

What controls chronic regulation of the urea cycle? Time period?

Back 68

Induction of urea cycle enzymes over 24-36 hours

Front 69

What triggers chronic regulation?

Back 69

Increased levels of ammonia in liver cells

Front 70

What are the consequences for enzyme synthesis of prolonged or severe starvation?

Back 70

Amino acids deaminated to use carbon back bone for energy - this may cause protein (and so enzyme synthesis) to be compromised

Front 71

What is the treatment if control of the urea cycle is not functioning correctly?

Back 71

Reduce protein level in diet and give a compound which aids nitrogen excretion (either through stimulating urea cycle or another compensatory pathway)

Front 72

EXTRA What is the most common urea cycle disorder?

Back 72

Ornithine transcarbamoylase deficiency

Front 73

EXTRA Which gender is most severely affected by ornithine transcarbamoylase deficiency and why?

Back 73

Generally males because the disease is X-linked

Front 74

EXTRA What are the symptoms of ornithine transcarbamoylase deficiency and what do these cause?

Back 74

Raised ammonia and amino acid levels, high blood or ororatic acid levels.
Mental retardation and can cause dead.

Front 75

EXTRA What is the treatment for ornithine transcarbamoylase deficiency and why?

Back 75

Large quantities of benzoate and phenylacetate:

Benzoly-CoA reacts with glycine to form hippurate
Phenylacetyl-CoA reacts with glutamine to form phenylacetylgluatime

These products act as excretable substitutes for urea in the disposal of nitrogen.

Front 76

Which organ is the main site of deamination (amino acid degradation) and urea synthesis?

Back 76

The liver

Front 77

Which organ is the main site of gluconeogenesis during fasting? What fuels this process?

Back 77

The liver

Carbon skeletons from amino acids

Front 78

What is the function of glutamate reductase?

Back 78

Synthetic enzyme for citrulline

Front 79

Which are the only cells to contain glutamate reductase?

Back 79

Enterocytes

Front 80

What is the fate of citrulline produced in the gut?

Back 80

Metabolised to arginine in the liver - this is then converted to ornithine to increase the capacity of the urea cycle during times of increased protein intake.

Front 81

What happens to muscle protein during fasting and starvation?

Back 81

Broken down to provide carbon skeletons for gluconeogenesis in the liver

Front 82

What are the main amino acids released from muscle protein breakdown?

Back 82

Alanine and glutamine

Front 83

What is the fate of alanine released from muscle protein catabolism?

Back 83

Transported by the circulation to the liver for deamination and gluconeogenesis

Front 84

What is the fate of glutamine released from muscle protein catabolism?

Back 84

Transported to small intestine by circulation, taken up by enterocytes for energy and released as alanine

Front 85

Glucose-alanine cycle

Back 85

Transport nitrogen to liver as alanine and glucose back to muscles