Lecture 28 - Amino Acids as Fuel Molecules Flashcards
What does protein digestion involve?
Hydrolysis of peptide bonds, performed by several different proteases and peptidases
What do endopeptidases do?
attack (break) peptide bonds within the protein (peptide) polymer
What do exopeptidases do?
attack (break) the last peptide bond near the end of protein (peptide) polymer
What are some examples of Endopeptidases?
Pepsin, Trypsin, Chymotrypsin
What is endopeptidase specificity determined by?
adjacent amino acid side chains in protein substrate
What best describes how endopeptidases work?
Sequential (each round produces smaller peptides)
What are some examples of exopeptidases?
Carboxypeptidase, Aminopeptidase
What is the order in which proteases digest proteins?
- Pepsin
- Trypsin
- Chymotrypsin
- Carboxypeptidases
- Aminopeptidases
Where does a carboxypeptidase cut the peptide?
Near the carboxy-terminal residue
Where does an aminopeptidase cut the peptide?
Near the amino-terminal residue
What do exopeptidases release?
Amino acids, di- and tri- peptides
How are proteases produced?
As zymogens
Why are proteases produced as zymogens?
Need to prevent protease activity before reaching the
gastrointestinal tract
What are zymogen or proenzymes?
Proteases secreted as inactive forms
How are zymogens activated?
by cleavage of peptides from their structure
What is the autolytic activation of pepsinogen?
activation by the low stomach pH
What is the catalytic activation of pepsinogen?
When the activated pepsin interacts with pepsinogen, activating it
How is trypsinogen activated into trypsin?
Membrane-bound enterokinase (enteropeptidase)
How is chymotrypsinogen activated?
By trypsin, and chymotrypsin activating other chymotrypinogens
How are procarboxypeptidases activated into carboxypeptidases?
By interaction with Trypsin
How much absorption is there of peptides 4 or more amino acids long?
Very little
How are di, and tri peptides absorbed into the intestine?
By co transport with H+ ions via a membrane transporter
How are absorbed di and tri peptides further digested?
After absorption they are further digested into individual amino acids by cytoplasmic peptidases
How do the amino acids enter the blood?
They pass through a facilitative transporter into the interstitial fluid and then into the blood
What mechanism is used for the absorption of amino acids in the small intestine?
Amino acids are absorbed via Na⁺-dependent transport into epithelial cells, where sodium and amino acids move together
How is the concentration gradient for Na⁺ maintained in epithelial cells during amino acid absorption?
The Na⁺/K⁺-ATPase pump actively transports Na⁺ out of the cell and K⁺ in, using ATP to maintain a low intracellular Na⁺ concentration.
What type of transport moves amino acids from the epithelial cells into the blood?
Amino acids are transported into the blood via facilitated diffusion (which doesn’t require energy but relies on transport proteins).
How many different Na⁺-dependent amino acid carriers exist, and what do they transport?
There are at least six (semi specific) Na⁺-dependent carriers that transport specific amino acid types.
What is the role of Na⁺ in amino acid absorption in the small intestine?
Na⁺ moves down its concentration gradient into epithelial cells, and this movement powers the symport of amino acids against their concentration gradient.
What does deamination generate?
a carbon skeleton, and a free amino group
What can the carbon skeleton be used for?
energy capture
What happens to the free amino group?
usually excreted
What are some ways that deamination occurs?
- Release of amino groups to solution.
- Transfer of amino group to a keto acid.
What is transamination?
When amino acids are deaminated by transferring their amino group to a keto acid
What catalyses transamination?
Aminotransferase enzymes
What is Pyridoxal phosphate (PLP)?
a co-enzyme required for transamination reactions
What is PLP derived from?
Vitamin B6
What does PLP do during transamination reactions?
Carries amino group (from the amino acid to the keto acid)
What are the two forms of PLP?
- Pyridoxal phosphate (no amino group)
- Pyridoxamine phosphate (with amino group)
How many steps are there to transamination?
2
What is the first step of transamination?
Amino group is transferred from the amino acid to the
pyridoxal phosphate (becomes pyridoxamine phosphate)
What is the second step of transamination?
Amino group is transferred pyridoxamine phosphate
(becomes pyridoxal phosphate) to the keto acid
What are some common amino acid/ keto
acid pairs in metabolism
Amino acid. Keto acid
Glutamate. -> α-ketoglutarate
Aspartate. -> Oxaloacetate
Alanine. -> Pyruvate
What can happen to Keto acids?
They can be fed into the metabolic pathways
Can Keto acids directly enter metabolic pathways?
Some can, but some others must be modified beforehand
What else are Transamination reactions important for?
Removing excess nitrogen from cells
What are the steps of removing excess ammonia from muscle cells?
- Ammonia attaches to keto acid in muscle forming glutamate
- Transamination occurs with addition of pyruvate to glutamate forming alanine which can then enter the blood.
- When the Alanine reaches the liver it undergoes transamination again losing a pyruvate to form glutamate
- Deamination of the glutamate occurs and ammonia forms Urea, which is non-toxic and is removed by the kidney
Why is it important to remove this excess ammonia?
It is toxic in cells
