recommended amt of protein in the human diet?
= 300-400 Kcal of energy/day
= 15-20% of our caloric intake
where does protein digestion occur
begins in the stomach, is completed in the GI tract
essential vs non essential amino acids?
essential: those that can't be synthesized by us and most mammals; must be provided in the diet
nonessential: can be synthesized by us, from carbohydrate precursors and a source of nitrogen - ammonia or alpha amino group of other amino acids
name the essential amino acids
PVT TIM HALL
A: Arginine (children/growth!)
name the non-essential amino acids
Tyr* (comes from Phe)
Cys* (comes frm Met & Ser)
what is nitrogen balance?
if amount of nitrogen consumed, mainly in form of protein = amount of nitrogen excreted, mainly as urea
when does positive vs negative nitrogen balance occur?
POSITIVE balance: Dietary N > Excreted N; growth - childhood, pregnancy
NEGATIVE balance: Dietary N < Excreted N; illness, infection, catabolic stress, inadequate intake of protein in the diet, or intake of protein diet deficient in essential amino acids
what's unique about Tyr and Cys?
they're nonessential amino acids made from essential amino acids
Tyr is from Phe
Cys is from Met & Ser
functions of amino acids?
1. synthesis of new protein for growth and repair
2. replace damaged protein
3. synthesize regulatory enzymes per metabolic conditions
what sorts of metabolites do amino acids produce?
near all nitrogenous metabolites found in the body, including:
purine and pyrimidine rings of the ntds
what do amino acids in excess provide/become?
per degredation to pyruvate, acetyl CoA, TCA cycle intermediates, ketone bodies
how do amino acids interact w/ gluconeogenesis and FA metabolism?
aa are a critical source of carbon for gluconeogenesis
aa can be used to produce fatty acids
do we have a storage form of aa?
what is result of this?
excess aa are made into C atoms which acn be stored as triglycerides or glycogen
however aa also have Nitrogen, amino, group - which become NH4+, ammonia, which is v. toxic
thus must get rid of ammonia, which is via urea, excreted in urine via urea cycle
function of transamination rxns?
allow removal of the alpha amino group from an amino acid, or allow transfer of an alpha amino group from an amino acid to an alpha-keto acceptor
result: production of a new aa from the alpha keto acceptor & a new alpha keto acceptor from the original aa
what is the GOT reaction
aspartate aminotransferase reaction aka glutamic-oxalacetic transamination reaction
glutamate + oxalacetate -> a-KG + aspartate
what is the GPT reaction?
alanine aminotransferase or glutamic pyruvate transaminase reaction
glutamate + pyruvate -> a-KG + alanine
nature of transamination rxns?
most use glutamic acid/a-KG as 1 of the exchange partners
can use non-essential amino acids as long as glutamic acid is available, along w/ C sources (OAA, pyruvate for ex)
what would happen if found GOT in a patient's bloodstream?
would know there is tissue damage b/c GOT was released from tissue -> bloodstream
what is PLP?
what's it derived from?
cofactor of all transaminases, and many other enzymes involved in amino acid metabolism
it's derived from Vitamin B6, Pyridoxine
method of a transamination reaction?
2 half reactions:
1st half reaction: a-amino group from donor aa is transferred to the PLP cofactor, which is at active site of the transaminase, forming a-keto acid and pryidoxamine P. cofactor changes from an aldehyde to an amine
2nd half reaction: 1st a-keto acid product dissociates away from the active site, & is replaced by a new a-ketoc acid; pyridoxamine cofactor transfers the amino group to pyruvate, which becomes alanine; PLP form of cofactor is regenerated, stays attached to ransaminase for another cycle
alanine product dissociates away from active site, new substrate aa, glutamate, enters to repeat rxn
what is a schiff base linkage?
linkage produced between alpha amino group of an amino acid + PLP cofactor at the acive site of enzymes metabolizing amino acids
what reactions produce schiff base linkages?
reactions involving aa metabolism - transamination, decarboxylation, racemization, modifications of the side chains of aa
what is nature of the linkages surrounding a schiff base
all can be broken depending on the reaction + the enzyme
what happens to schiff base in a transamination
bond between the alpha amino group and the alpha carbon of the incoming amino acid is eventually hydrolyzed
this results in amino group hanging on to the cofactor as pyridoxamine P and formation of the alpha keto acid
what happens to schiff base in a decarboxylase reaction?
eg histdine -> histamine
bond between the alpha carbon and the alpha carboxyl group is broken, resulting in release of Co2 and decarboxylated former amino acid, which is not an amine
cofactor: vitamin B6
cofactor of carboxylases?
what happens in racemase reaction w/ a schiff base?
bond between alpha carbon and alpha hydrogen is labilized - results in a change of congiuration around the alpha carbon - so from D-aa to L-aa or vice versa by moving alpha C from 1 side to the other
what happens in a dealdolase reaction re: schiff base?
bond between the alpha carbon and R group is labilized, result in release of R group
i.e. serine to glycine by taking off CH2OH
how can aa be used as a source of energy
AA can be used as a source of energy if they are in excess; in such conditions, AA are
degraded to pyruvate, acetyl CoA, TCA cycle intermediates, and ketone bodies. AA also
are a source of C for gluconeogenesis and can be used for FA synthesis.
why do proteins have such diff turnover rates
s the rate at which old proteins are degraded
and new proteins are made to replace them. The question is very broad, but just asks
you to think about why some proteins need to be replaced daily and others may last for
weeks or years.
what is transamination
why is it important
what is significant of GOT or GPT in clinical medicine or in diagnosis
Transamination is a reaction that removes the a-amino group from an AA.
involves an a-amino acid and a-keto acid with a cofactor including PLP, whereby the
amino group from the a-amino acid is transferred to an alpha keto acid acceptor,
producing a new AA from the a-keto acceptor and a new a-keto acceptor from the
GOT or GPT in the blood would indicate tissue damage
4 basic reactions invovling amino acids which require pyridoxal phosphate as a cofactor?
what is fate of aa group in each?
4 basic reactions involving AA that require PLP as a cofactor are: racemase;
decarboxylase; modifications to amino acid side chains; and transamination.
a. Transamination: alpha amino group ends up on the cofactor
b. Decarboxylase: alpha amino group stays attached to the a-carbon, producing a
decarboxylated former AA, now amine
c. Racemase: Change in configuration from D to L; amino group thus remains attached
to the alpha C of the original AA
d. Dealdolase: Alpha amino group remains attached to AA alpha carbon