Lecture 9 - Protein Metabolism Flashcards
What are three functions of proteins?
- provides AA for protein synthesis
- source of energy (if needed)
- substrate for glucose synthesis
What do we need from protein?
not the protein per se, but rather the AA in the protein
How many proteinogenic AA are there in humans?
21, all but selenocysteine are part of the genetic code
What is a proteinogenic AA?
refers to an AA that is incorporated into a protein during translation
What is the average protein percent consumption of daily calories in NA?
16%
What is health canada’s recommended protein intake?
10-30%
Are non-proteinogenic AA used to make protein?
no
How many AA are considered essential?
9
What is the water% of various tissues in the body?
- adipose - 30
- blood (RBC) - 64
- connective tissue - 60
- eye lens - 64
- skeletal mm - 74
- cortical bone - 12
- skin - 65
What is the lipid% of various tissues in the body?
- adipose - 62
- blood (RBC) - 0.5
- connective tissue - 1
- eye lens - 2
- skeletal mm - 4
- cortical bone - –
- skin - 10
What is the protein% for various tissues
- adipose - 8
- blood (RBC) - 35
- connective tissue - 37
- eye lens - 34
- skeletal mm - 20
- cortical bone - 25
- skin - 24
What is the % other of various tissues?
- adipose - –
- blood (RBC) - 0.1
- connective tissue - 0.9
- eye lens - –
- skeletal mm - 1
- cortical bone - 5
- skin - –
What %minerals are in various tissues?
- adipose - <0.1
- blood (RBC) - 0.4
- connective tissue - 1.1
- eye lens - <0.1
- skeletal mm - 1
- cortical bone - 58
- skin - 1
What is the whole body average percentage for water, fat, minerals protein and CHO?
water - 60%
fat - 20-25%
minerals - 2%
protein - 15%
CHO - 0.2%
do animal derived foods contain more protein than plant derived foods?
yes
What is the general structure of an AA?
Amino terminal - N
alpha carbon
side chain - variable composition
carboxyl terminal - COOH
What is the only thing in the amino acid structure that differs between AA?
the side chain
What are entantiomers?
- mirror images
- D vs L enantiomers
What AA exist as enantiomers?
- all standard AA exist as enantiomers except for glycine
Which enantiomer of AA is naturally occurring?
The L configuration
D configuration is made through post-translational modifications
What are zwitterions?
a molecule or ion having separate positively and negatively charged groups
- for AA - the carboxyl group has a negative charge and the amine group has a positive charge, but resulting in a net charge of 0
When are AA ionized?
at physiological pH
- protonated amine
- deprotonated carboxyl
- this increases polarity
How are AAs connected?
peptide bonds (amide bonds)
What type of bond is a peptide bond?
covalent
How do AA form a peptide bond?
the carboxyl group of one AA reacts with amino group of another AA, releasing H2O (condensation reaction)
How do you break a peptide bond?
add water
What is the difference between a peptide and a protein?
A peptide has no function until it is folded into a 3D structure via chaperone proteins
What is the primary protein structure?
- determined by DNA sequence
- polypeptide chain of AA
- held together by peptide bonds (tln helped by chaperones)
- polypeptide chain has a amino and carboxyl terminus
What is the protein secondary structure?
- bonds dont involve SC. only backbone
two types - alpha helix: amino group makes a hydrogen bond with a carboxyl group 4 AA down the chain, creating a helical shape in the polypeptide
- beta pleated sheets: an amino group makes a hydrogen bond with a carboxyl group in the folded back polypeptide chain, can be parallel or anti-parallel
What are secondary structures determined by?
hydrogen bonds that create a more stable structure
What are the proteins tertiary structure?
- arrangement of the secondary structure in 3D space
- one polypeptide chain
involves interactions between AA S.C. - Hydrophobic AA are in the center and hydrophilic AA are on the outside
What is the quaternary structure of proteins?
- combination of 2+ tertiary structures
- tertiary structures are called subunits
- forms multi-subunit complex which contributes to the overall function of the protein
What is a native vs denatured protein?
native - corresponds to the protein in its normal 3D conformation
denatured - when proteins unfold and therefore lose their function, loses its bioactivity
How can proteins be denatured?
- heat, salt, detergents, acidity
What type of structures does denaturation effect?
2, 3, 4 but not 1
How is the egg protein denatured?
egg protein is albumin
native albumin is transparent and liquid
when cooked, the albumin becomes opaque and hard - now denatured egg whites
What are conditionally essential AA?
not normally required in the diet in a healthy individual, but become essential under specific contexts
What is phenylketonuria?
an inborn error of metabolism whereby a person is unable to breakdown phe into tyr
- a build-up of phe in the body causes intellectual disability
- the solution is to limit phe in the diet and supplement with tyr
What is the relationship between liver disease and AA?
impairs phe and met catabolism
- tyr and cys are synthesized from phe and met respectively
- try and cys become indispensable in this context
What are the 7 AA classifications?
- basic
- acidic
- neutral
- branched chain
- hydroxylated
- sulphur-containing
- aromatic
What are the basic AAs?
- polar
- basic aa (positive charge on NH3 group on s.c. enables DNA binding)
- important in histone proteins, which interact with DNA
Lysine - essential, simple straight chain, absent from grain products
Arginine - non essential in healthy adults, absent in newborns, polar
Histidine - essential, ring structure, used to produce histamine, basic, has immune function in children
What are the acidic AAs?
- acidic AA (-ve charge on side chain carboxyl group)
- acidic AAs are polar
Aspartate - non essential, transaminated to oxaloacetate (krebs), polar, protein catabolism
Glutamate - non essential, transaminated to alpha ketoglutarate (krebs), used to produce GABA, polar, get rid of nitrogen groups in the body
Asparagine - non essential
Glutamine - non essential, important in AA catabolism because it is a carrier of nitrogen (to live/kidney), interorgan nitrogen transfer - aspartate and glutamate react with a basic amino group to produce neutral amides such as asparagine and glutamine
What are neutral AAs?
- no charge on s.c.
- non polar
- aliphatic (C and H atoms joined in straight or branched chains)
Glycine - non essential, no enantiomers, used primarily to produce porphorin (component of heme, which is found in hemoglobin)
Alanine - non essential, important in AA catabolism because it is a carrier of nitrogen (to liver/kidney), important role in the glucose alanine cycle
What are branched chain AAs?
- neutral aliphatic aa (no s.c. charge)
- non polar
- all are branched
Leucine
Isoleucine
Valine - all are essential, not catabolized in the liver (no enzymes to break it down), so high levels found in circulation
what are hydroxylated AAs?
- OH group on s.c. is important for protein phosphorylation
- polar AA
- tyrosine can also be classified as hydroxylated but is group with aromatic AA
Serine - non essential
Threonine - essential
What are sulfur-containing AAs?
- contain a sulfur group
- non polar
Cysteine - non essential, made from methionine, spares methionine when cysteine consumed in the diet, used to form disulfide bonds, used in glutathione synthesis
Methionine - essential, methionine is limiting in legumes, first step in the synthesis of all proteins
What are aromatic AAs?
- contain aromatic rings
- non polar (except tyrosine because of OH group in its s.c.)
Phenylalanine - essential, used to make tyrosine
Tyrosine - non essential, spares phe, used to synthesize neurotransmitters
Tryptophan - essential, used to make serotinin, used for niacin (B3) synthesis
Proline - non essential, important for collagen production, aliphatic s.c., atypical ring, s.c. attached to backbone
