Protein Flashcards
Digestion
1- it starts in the stomach by the action of HCL that kills bacteria and activate pepsinogen into pepsin
2- pepsin digest protein into large polypeptides
3- entering small intestine , pancreatic proteases ( which are exopeptidases and endo ) eg : trypsin, chymotrypsin , elastases, collagenases
Digest the large polypeptides into oligopeptides and free a.a
4- there is also enzymes called amino peptidases which are exopeptidases that cleave the N terminal of oligopeptides to make it even smaller and release a.a
5- free a.a are absorbed to enterocytes through sodium dependant secondary active transport via carrier called SLC and tri and bipeptides are also absorbed by proton linked peptide transporter and then hydrolyzed to a.a and all are transported to portal circulation then
What is amino acid pool
It is amino acid plasma
1- it is not constant through 24h but it has a normal range between 4-8 mg/dl plasma
2- it contain a.a that is absorbed from diet , from hydrolysis of body protein , synthesised in body to form the pool finally
Fate of a.a
Anabolic fate
- formation of protein : hormones, enzymes
- formation of nitrogenous substances: purine , pyrimdine , glutathione
Catabolic fate
- cleavage of them into ammonia and carbon skeletons , ammonia is mostly converted to urea and excreted in urine , little ammonia is also excreted in urine, carbon skeletons are either glucogenic or ketogenic or both or maybe oxidized to co2 and water
Nitrogen group and a.a
The presence of a amino group keeps the acid away from being effected by oxiditive breakdown , the removal of the group make an obligatory step of oxidative breakdown
Transamination
1- The first step in the catabolism of
most amino acids is the transfer of
their a-amino group to a-
ketoglutarate, producing an a-keto
acid (derived from the original
amino acid and glutamate )
a-ketoglutarate accepts a amino groups and is converted to glutamate That can be oxidatively deaminated or used as amino group donor
This process is catalyzt by enzymes named transaminases or aminotransferases that uses PLP as co enzyme
There areTwo important transaminases reactions , one catalyzd by alanine aminotransferase ( ALT) and aspartate aminotransferase ( ACT)
Alanine aminotransferase
Aspartate aminotransferase
Their reactions are reversible however during catabolism direction of the reactions goes to glutamate synthesis ( alanine breakdown )
Oxidative deamination
Libration of a amino group from glutamate forming a ketoglutarate and ammonia which converts to urea and excreted and this process is done by glutamate dehydrogenase in liver and kidney mainly ,
Talk about glutamate dehydrogenase
1- unique as the glutamate is the only a.a to be oxidatively deaminated
2- it is mitochondrial enzyme
3- a keto glutarate ~> glutamate ( NADPH/ NADP)
Glutamate ~> a ketoglutarate ( NAD/ NADH)
4- Allostericly inhibited by guonseine triphosphate and activated by ADP ( low level of energy)
Fate of carbon skeletons
1- glucogenic
Their catabolism gives arise to intermediates of TCA cycle that can undergo gluconeogensis and give glucose
2- ketogenic
Their catabolism gives arise to acetoacetate or any precursor of ketogenisis ( acetyl coa or acetoacetyl coa) to give ketone bodies
There are two : leusine and lysine
3- both
Their carbon skeleton after deamination gives arise to glucose and ketone bodies , they are tyrosine , tryptophan, phenylalanine and isoleucine
Talk about glycine
1- glucogenic as it can convert to serine by serine hydroxymethyl transferase and then serine converts to pyruvaic acid by serine dehydratase
2- its non essential a.a
Metabolic functions and derivatives of glycine
1- serine synthesis
2- pyrine synthesis
Carbon no . 4,5 and nitrogen no .7 derivatives from glycine
3- creatine synthesis
As it consists if 3 a.a ( glycine, arginine, methionine)
4- heme formation
As glycine reacts with succinyl CoA giving aminoleuvlinc acid ( ALA) that later convert to heme
5- glycine betiane : methyl donor
6- bile salt formation : glycine is conjugated with bile acids to give bile salts that help in emulsification of fat
7- glutathione synthesis
That is formed from glycine, cysteine and glutamate
Talk about glutathione
There is two forms oxidized ( GS SG) and reduced ( GSH )
Cysteine and glutamate get together by cysteinyl glutamate synthatase and then glycine is added to form glutathione by glutathione synthatase
It has important roles as
1- a.a transporter through mucosal membrane ( glytamyl cycle )
2- antioxidant ( mentioned before)
3- GSH as cofactor in many processes like PGs
synthesis and Aconitase in TCA cycle
4- detoxification of arommatic compounds like bromobenzenes
Nitrogen metabolism
Nitrogen intake and output or can be protein anabolism and catabolism as most nitrogen intake are in protein and most of nitrogen output results from protein breakdown
What is + nitrogen balance and - nitrogen balance
Normally there is balance between nitrogen intake and output , it occurs in normal healthy adults
+ : intake > output and this happens during synthesis of new tissues as in growth and pregnancy and during musclar training
- : output > intake it can be due to
Loss of protein eg chronic haemorrhage
Increased protein Catabolism eg DM
little protein intake eg starvation
Synthesis of phenylalanine and tyrosine
• Phenylalanine is an essential
amino acid.
• Tyrosine is a non-essential amino
acid, as it is synthesized from
phenylalanine by hydroxylation.
This reaction is catalyzed by
phenylalanine hydroxylase and BH4 as coenzyme ( tetrahydrobiopterin) that is converted to BH2 ( dihydrobiopterin) in the end and is regenerated by dihydrobiopterin reductase that needs NADPH+H or NADH and any deficiency in these substances hencing phenylalaline to convert to tyrosine results in PKU
