Amino Acids as Fuel Molecules Flashcards
describe the types of enzymes involved in protein digestion
- involves hydrolysis of peptide bonds
- performed by several different proteases/peptidases
Endopeptidases: attack (break) peptide bonds within the protein (peptide) polymer
Exopeptidases: attack (break) the last peptide bond (of last two or three) near the end of protein (peptide) polymer
describe protein digestion:the sequential hydrolysis by proteases
Endopeptidases (start with pepsin, then trypsin and chymotrypsin) sequentially break down the peptides, each round producing smaller peptides
- endopeptidase specificity is determined by adjacent amino acid side chains in protein substrate
After endopeptidases have made the peptides much smaller
Exopeptidases (carboxipeptidases - cuts carboxy- terminal residue and amino peptidases - cuts amino- terminal residue)
- releases amino acids, di- and tri- peptides
describe how proteins are produced as zymogens
- need to prevent protease activity before reaching the gastrointestinal tract
- proteases secreted as inactive forms (called zymogens or proenzymes)
- activated by cleavage of peptides from their structure
describe the process of how pepsinogen is activated
- made as pepsinogen (translated in cells which have neutral pH, inactive form) - has a tail which inhibits the catalytic domain
- secreted into stomach (GIT) where there is a low pH
- undergoes autolytic activation (by self cleavage) where the catalytic domain gets uninhibited by the low pH as the tail inhibiting it is removed/cleaved
OR - undergoes catalytic activation where the active form of pepsin cleaves the inhibiting tail bit off and makes the pepsinogen into pepsin
describe how the activation of proteases works in terms of the sequence they act on the protein chain
- pepsinogen to pepsin (autolytic and catalytic)
- trypsinogen to trypsin (by membrane bound enterokinase [actually a enteropeptidase])
- chymotrypsinogen to chymotrypsin (trypsin activates it)
- procarboxypeptidases to carboxypeptidases (activated by trypsin as well)
describe the absorption of peptides from the intestine
- very little absorption of peptides four amino acids or longer
- absorption of di- and tri- peptides in the small intestine by co-transport with H+ ions via a membrane transporter
- absorbed di- and tri- peptides are further digested into individual amino acids by cytoplasmic peptidases
- amino acids pass through a facilitative transporter into the interstitial fluid and then into the blood
what is the process of absorption of amino acids into the cells
Uptake into epithelial cells by a transporter similar to the SGLT for glucose
- Na+ dependent
- Amino acid moving to a higher concentration linked with Na+ moving to a lower concentration
- At least six different Na+-dependent carriers (semi-specific):
eg. neutral AA, proline and hydroxyproline, acidic AA, basic AA and cysteine
- Facilitative transport of amino acids into interstitial fluid then blood
- gradient for Na+ created by K+/Na+ ATPase cotransporter taking Na+ out of the cell constantly and moving K+ in
what does the deamination of amino acids produce?
- a carbon skeleton (alpha carbon with R group, carboxylic acid group and double bonded O on it)
- a free amino group (which is generally excreted)
describe how some amino acids can be deaminated by releasing their amino groups to solution
eg. glutamate deamination catalyses by glutamate dehydrogenase (does redox, but in this case the important bit is that the NH3+ is getting cleaved and related into solution)
describe how some amino acids are deaminated by transferring their amino group to a keto acid
- known as transamination
- catalysed by aminotranferase enzymes (also called transaminases)
reaction:
amino acid + keto acid (an alpha carbon with an R group, double bonded O and carboxyl group on it) —> keto acid + amino acid (the amino group from the original AA goes onto the keto acid and they basically switch names)
eg. glutamate + alpha-keto acid –> alpha-ketoglutarate + alpha-amino acid
what is pyridoxal phosphate (PLP)
- PLP is a coenzyme required for transamination reactions
- derived from vitamin B6
- carrier: amino group (from the amino acid to the keto acid)
exits in two forms: - pyridoxal phosphate (no amino group)
- pyridoxamine phosphate (with amino group)
- transamination reactions involve two steps
what are the two steps of transamination?
- amino group is transferred from the amino acid to the pyridoxal phosphate (becomes pyridoxine phosphate)
- amino group is transferred from pyradoxamine phosphate (becomes pyridoxal phosphate) to the keto acid
what are some common amino acid/keto acid pairs in metabolism?
glutamate <–> alpha-ketoglutarate (CAC and todays lecture)
aspartate <–> oxaloacetate (CAC)
alanine <–> pyruvate (end point of glycolysis)
describe how keto acids can be fed into the metabolic pathways
some keto acids can directly enter metabolic pathways, some keto acids require modification first
- key point is that amino acid skeletons can be fed into metabolic pathways
describe how transamination reactions are also required to remove excess nitrogen via the liver
muscle:
- amino acids
- NH4+ (ammonia) excreted. is highly toxic
- to rectify this we attach it to a keto acid, which produces glutamate
- which undergoes transamination to alanine (uses pyruvate to do so and makes alpha-ketoglutarate as well as the alanine)
- alanine enters the blood
- and then the liver
- where it undergoes transamination again - alpha-keotglutarate + alanine = glutamate (the pyruvate that is a product of this reaction goes to glucose and then back to muscle cells where it can be used for this process again)
- and then deamination of glutamate (which produces the alpha-ketoglutarate again)
- to make ammonia
- which is made into urea in the liver and then excreted out of the body (in urine)
