BIOCHEM - Protein Metabolism Flashcards
Describe the primary, secondary, tertiary & quaternary structure of proteins.
Primary structure:
Peptide bonds form between amino group and carboxylic group of two amino acids forming a polypeptide chain.
Secondary structure:
Polypeptide chain containing regions of amino acid chains that are stabilised by hydrogen bonds from the polypeptide backbone.
These hydrogen bonds create either:
a-helix
b-pleated sheets
Tertiary structure:
The overall three-dimensional arrangement of a polypeptide chain.
Stabilised by outside polar hydrophilic hydrogen and ionic bond interactions, and internal hydrophobic interactions between non-polar amino acid side chains.
Tertiary structure is primarily due to interactions between the R groups of the amino acids that make up the protein.
Quaternary structure:
Some proteins are made up of multiple polypeptide chains, also known as subunits. When these subunits come together, they give the protein its quaternary structure. Can contain many different structures.
Why is the 3D structure of a protein important?
- give an example of what can happen when this goes wrong.
The 3D structure of an amino acid chain gives the protein its structure and function.
E.g., sickle cell disease
Alteration of the primary structure (polypeptide chain) of haemoglobin leads to altered Hb shape, Hb arrangement and eventually misshaped red blood cells that do not function well.
Describe how dietary protein is digested and absorbed.
Dietary protein is ingested and digested into dipeptides and tripeptides.
Transport proteins move the products of protein digestion (dipetides & tripeptides) into the mucosal cells in the lumen of the small intestine - they are broken down into single amino acids (monomers).
Single amino acids move into the bloodstream and travel to the liver. Here they become part of the amino acid pool.
What is the fate of ingested protein?
- briefly describe protein synthesis
Amino acids from the digestion of protein join the amino acid pool in the bloodstream.
Protein synthesis consists of two phases:
transcription & translation.
Transcription:
* DNA double helix ‘unzips’
* Code for a specific protein is copied from a section of the exposed DNA molecule into a molecule of messenger RNA (mRNA).
Translation:
* mRNA takes the genetic information from the nucleus to the ribosomes in the cytosol
* at the ribosomes, transfer RNA (tRNA) reads the genetic information and delivers the correct amino acids to the ribosome to make the polypeptide chain.
Define the term ‘limiting amino acid’ - what are the common limiting amino acids?
Biochem angle:
Essential amino acids that are ‘missing’ in the ribosome – not available for tRNA during translation:
- Lysine
- Threonine
- Methionine
- Tryptophan
Dietetic/food angle:
The term “limiting amino acid” is used to describe the essential amino acids present in the lowest quantity in a food protein relative to a reference food protein.
Why is the ammonia ion toxic?
Causes depletion of a-ketoglutarate in citric acid cycle:
→ Less ATP production → death
Ammonia concentration rises = increased formation of glutamate from ketoglutarate which causes depletion of the mitochondrial pool of a-ketoglutarate (a key intermediate in the citric acid cycle).
How does oedema occur in children with Kwashiorkor?
Oedema = movement and retention of fluid from the vasculature into interstitial spaces.
Protein deficiency causes hypoalbuminaemia (decreased serum albumin level), which causes intravascular hypovolaemia (decreased volume of fluid in the vascular system).
Water moves into extracellular spaces - legs & abdominal cavity.
Protein deficiency:
- reduced albumin synthesis in the liver
- reduced albumin content in blood (hypoalbuminaemia)
- intravascular hypovalaemia (decreased fluid volume in blood)
- reduced oncotic pressure
- increased hydrostatic pressure
- movement of interstitial fluid in the abdominal cavity.
What is the relationship between our DNA and the proteins in our body?
DNA contains the genetic code for the synthesis of proteins (transcription and translation) proteins (DNA is the template).
‘Proteins are the machines to make happen what is written in the DNA’
DNA codes for all proteins in our body.
DNA – mRNA – amino acid chains – proteins.
Describe the function of the urea cycle
Protein metabolism produces toxic by-product: ammonia.
The urea cycle mediates the removal of ammonia as urea, which is excreted as urine.
Occurs in the liver!!
In healthy adults the urea cycle removes 10-20g urea per day.
Occurs in two stages:
* Mitochondrial
* Cytosolic
Urea cycle requires ATP
25% energy from protein is used to detoxify protein.
(synthesis of urea is energy consuming process).
What is protein quality?
- how can protein quality be optimised?
Humans have 21 proteinogenic amino acids.
we can synthesise 12 of them - 9 need to come from diet. These are the ‘essential amino acids’.
* Histidine
* Isoleucine
* Leucine
* Lysine
* Methionine
* Phenylalanine
* Threonine
* Tryptophan
* Valine
Whether or not dietary protein sources contain the essential amino acids is referred to as its ‘protein quality’ - animal protein & eggs best protein quality.
Protein digestibility/availability must be considered when assessing the protein quality of foods.
protein quality of meals can be optimised by clever food combinations e.g., rice (met & cys) & beans (lys)
Describe how ATP is generated from protein.
Amino acids can be used as a source of energy in times of starvation.
ATP generated from AA’s in Citric Acid Cycle.
- Deamination:
- The amine group on the amino acid is removed. (α-keto-acids are the deaminated form of AA’s)
- α-keto-acids are metabolised
- The removed amino group is converted into a toxic by-product - ammonium ion.
- Ammonium ion is combined with CO2 in the liver which creates urea (less toxic) which is excreted by the kidneys in urine.
- Different AA’s = different metabolizable products:
- glucogenic AA’s create 3-C molecules used to generate glucose or intermediates in the citric acid cycle to generate glucose
- ketogenic amino acids create molecules that can be used to generate Acetyl CoA.
- used to synthesise fatty acids when in positive energy balance.
- ETC:
- High energy electrons amino acid breakdown are transferred to the ETC & used to generate ATP from ADP.
How are new amino acids synthesised?
‘transamination’
the process by which an amino group from an amino acid is transferred to a carbon skeleton to create a new amino acid.
