Lecture 28 - Amino Acids as Fuel Molecules

Question 1

What does protein digestion involve?

Answer 1

Hydrolysis of peptide bonds, performed by several different proteases and peptidases

Question 2

What do endopeptidases do?

Answer 2

attack (break) peptide bonds within the protein (peptide) polymer

Question 3

What do exopeptidases do?

Answer 3

attack (break) the last peptide bond near the end of protein (peptide) polymer

Question 4

What are some examples of Endopeptidases?

Answer 4

Pepsin, Trypsin, Chymotrypsin

Question 5

What is endopeptidase specificity determined by?

Answer 5

adjacent amino acid side chains in protein substrate

Question 6

What best describes how endopeptidases work?

Answer 6

Sequential (each round produces smaller peptides)

Question 7

What are some examples of exopeptidases?

Answer 7

Carboxypeptidase, Aminopeptidase

Question 8

What is the order in which proteases digest proteins?

Answer 8

1. Pepsin
2. Trypsin
3. Chymotrypsin
4. Carboxypeptidases
5. Aminopeptidases

Question 9

Where does a carboxypeptidase cut the peptide?

Answer 9

Near the carboxy-terminal residue

Question 10

Where does an aminopeptidase cut the peptide?

Answer 10

Near the amino-terminal residue

Question 11

What do exopeptidases release?

Answer 11

Amino acids, di- and tri- peptides

Question 12

How are proteases produced?

Answer 12

As zymogens

Question 13

Why are proteases produced as zymogens?

Answer 13

Need to prevent protease activity before reaching the
gastrointestinal tract

Question 14

What are zymogen or proenzymes?

Answer 14

Proteases secreted as inactive forms

Question 15

How are zymogens activated?

Answer 15

by cleavage of peptides from their structure

Question 16

What is the autolytic activation of pepsinogen?

Answer 16

activation by the low stomach pH

Question 17

What is the catalytic activation of pepsinogen?

Answer 17

When the activated pepsin interacts with pepsinogen, activating it

Question 18

How is trypsinogen activated into trypsin?

Answer 18

Membrane-bound enterokinase (enteropeptidase)

Question 19

How is chymotrypsinogen activated?

Answer 19

By trypsin, and chymotrypsin activating other chymotrypinogens

Question 20

How are procarboxypeptidases activated into carboxypeptidases?

Answer 20

By interaction with Trypsin

Question 21

How much absorption is there of peptides 4 or more amino acids long?

Answer 21

Very little

Question 22

How are di, and tri peptides absorbed into the intestine?

Answer 22

By co transport with H+ ions via a membrane transporter

Question 23

How are absorbed di and tri peptides further digested?

Answer 23

After absorption they are further digested into individual amino acids by cytoplasmic peptidases

Question 24

How do the amino acids enter the blood?

Answer 24

They pass through a facilitative transporter into the interstitial fluid and then into the blood

Question 25

What mechanism is used for the absorption of amino acids in the small intestine?

Answer 25

Amino acids are absorbed via Na⁺-dependent transport into epithelial cells, where sodium and amino acids move together

Question 26

How is the concentration gradient for Na⁺ maintained in epithelial cells during amino acid absorption?

Answer 26

The Na⁺/K⁺-ATPase pump actively transports Na⁺ out of the cell and K⁺ in, using ATP to maintain a low intracellular Na⁺ concentration.

Question 27

What type of transport moves amino acids from the epithelial cells into the blood?

Answer 27

Amino acids are transported into the blood via facilitated diffusion (which doesn't require energy but relies on transport proteins).

Question 28

How many different Na⁺-dependent amino acid carriers exist, and what do they transport?

Answer 28

There are at least six (semi specific) Na⁺-dependent carriers that transport specific amino acid types.

Question 29

What is the role of Na⁺ in amino acid absorption in the small intestine?

Answer 29

Na⁺ moves down its concentration gradient into epithelial cells, and this movement powers the symport of amino acids against their concentration gradient.

Question 30

What does deamination generate?

Answer 30

a carbon skeleton, and a free amino group

Question 31

What can the carbon skeleton be used for?

Answer 31

energy capture

Question 32

What happens to the free amino group?

Answer 32

usually excreted

Question 33

What are some ways that deamination occurs?

Answer 33

1. Release of amino groups to solution. 2. Transfer of amino group to a keto acid.

Question 34

What is transamination?

Answer 34

When amino acids are deaminated by transferring their amino group to a keto acid

Question 35

What catalyses transamination?

Answer 35

Aminotransferase enzymes

Question 36

What is Pyridoxal phosphate (PLP)?

Answer 36

a co-enzyme required for transamination reactions

Question 37

What is PLP derived from?

Answer 37

Vitamin B6

Question 38

What does PLP do during transamination reactions?

Answer 38

Carries amino group (from the amino acid to the keto acid)

Question 39

What are the two forms of PLP?

Answer 39

- Pyridoxal phosphate (no amino group) - Pyridoxamine phosphate (with amino group)

Question 40

How many steps are there to transamination?

Answer 40

2

Question 41

What is the first step of transamination?

Answer 41

Amino group is transferred from the amino acid to the pyridoxal phosphate (becomes pyridoxamine phosphate)

Question 42

What is the second step of transamination?

Answer 42

Amino group is transferred pyridoxamine phosphate (becomes pyridoxal phosphate) to the keto acid

Question 43

What are some common amino acid/ keto acid pairs in metabolism

Answer 43

Amino acid. Keto acid Glutamate. -> α-ketoglutarate Aspartate. -> Oxaloacetate Alanine. -> Pyruvate

Question 44

What can happen to Keto acids?

Answer 44

They can be fed into the metabolic pathways

Question 45

Can Keto acids directly enter metabolic pathways?

Answer 45

Some can, but some others must be modified beforehand

Question 46

What else are Transamination reactions important for?

Answer 46

Removing excess nitrogen from cells

Question 47

What are the steps of removing excess ammonia from muscle cells?

Answer 47

1. Ammonia attaches to keto acid in muscle forming glutamate 2. Transamination occurs with addition of pyruvate to glutamate forming alanine which can then enter the blood. 3. When the Alanine reaches the liver it undergoes transamination again losing a pyruvate to form glutamate 4. Deamination of the glutamate occurs and ammonia forms Urea, which is non-toxic and is removed by the kidney

Question 48

Why is it important to remove this excess ammonia?

Answer 48

It is toxic in cells