Protein Metabolism Flashcards
What are the three main functions of proteins
Enzymes, hormones and structural
How many essential AA are there
9
How many amino acids are not essential (can be made from other AA in the diet)
11
What are complete proteins
Proteins that contain 9 essential AA e.g. whey
What are the 9 essential AA
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
What is the basic structure of an AA
Central carbon atom (alpha carbon [Cα]) linked to:
Amino group (positive charge)
Carboxylic acid group (negative charge)
Hydrogen
Distinctive side chain (R)
Where does Branched Chain AA metabolism occur
skeletal muscle
What are the three Branched Chain Amino Acids (BCAA)
Leucine, isoleucine, valine
Which amino acid can stimulate synthesis of new muscle proteins
Leucine
Which organ lacks the first two enzymes in the pathway that breaks down BCAAs and hence cannot metabolise BCAAs
Liver
Where are amino acids primarily metabolised
Liver and skeletal muscle
Can amino acids be stored in the body
No, freely circulate in the amino acid pool in the blood, can be incorporated into tissue/ used in the body or excreted
What is the rate of amino acid turnover
300g/day
What is the free amino acid pool
The free amino acid pool represents the AAs in the circulation and extracellular fluids
What is the free amino acids pool composed of
Comprised of: AA from food, AA degraded in tissues (all – not just muscle), AA metabolized in liver in a constant flux throughout the day
How much of protein turnover is directed towards muscle (muscle protein turnover)
20-30% towards muscle (1-2% per day)
What is transanimation
The process by which other AAs are formed by transferring an α-amino group (NH3) to a keto acid (an amino acid without its amino group) to form a new AA
What are the enzymes that catalyse transanimation reactions
transaminases and aminotranferases
Are the transanimation reactions irreversible or reversible
Reversible
What happens to the amino acid that loses its amino group in transanimation
frees up the keto acids (carbon backbones) to have roles in other metabolic systems, as the nitrogen in the amino group cannot be used for energy production
Describe the transanimation process
An amino acid transfers is NH3 group (α amino group) to an alpha-keto acid.
This reaction is catalysed by an amino transferase enzyme.
This leads to the formation of a new amino acid.
When the amino group is transferred, a carbon skeleton is left and this forms a new keto acid that can be used in the TCA cycle.
Where does transanimation occur
Both cytosol and mitochondria
Describe the specificity of enzymes involved in transanimation
These enzymes are specific to each group – e.g., alanine aminotranferase
What is the most common transanimation reaction and why
This reaction most commonly occurs with the amino group being transferred to Αlpha Ketoglutarate and forming the AA glutamate.
Why? Pyruvic acid is then the keto acid left over. Transamination reactions tend to form products that can be used to produce energy for the TCA or gluconeogenesis.
What else can transamination be used for apart from energy production
Produce ketone bodies
Which two amino acids are ketogenic
Leucine and lysine
What products can be formed in the process of transanimation
Pyruvate, acetyl-CoA, fumarate, succincyl CoA, alpha ketoglutarate, Oxaloacetate, acetoacetyl CoA
What is the glucose alanine cycle
Converting alanine into glucose for energy
Why is the glucose alanine cycle useful
The glucose-alanine cycle allows us to form glucose from proteins in the muscle. So when we are starving, fasting or during very long duration exercise and when energy stores are running low we can use this cycle to form glucose for energy.
Describe the glucose alanine cycle
Glutamate and pyruvate can react together and form the AA, alanine (transamination)
This is catalysed by the enzyme alanine transferase
Alanine can then be shuttled via the blood stream to the liver
In the liver it can be converted back to pyruvate in the same reaction, just in the reverse manner.
Pyruvate can then be used to produce glucose in the liver (gluconeogenesis)
This glucose can be shunted back across to the muscles to be used to fuel energy production
What is oxidative deaminatiom
Oxidative deamination is when the amino group is taken from the newly formed AA e.g., glutamate
This leaves a keto acid e.g., alpha keto-glutarate that can be used in the TCA cycle (or for further transamination)
What enzyme catalyses oxidative deamination
The enzyme that catalyses this is known as glutamate dehydrogenase (specific to glutamate)
When is ketogluterate production upregulated (oxidative deamination)
When ADP is high (low energy status)
What are the products of oxidative deamination
Ketogluterate and ammonia
When does oxidative deamination occur and why
This occurs when energy is low – so very hard long exercise or when starved. We are essentially oxidizing our proteins to produce energy.
What is ketogluterate
carbon skeleton, which can go on to be used in the TCA cycle – or be used for more transanimation
What is the urea cycle
Ammonia is quickly converted to urea in the liver and the excreted via the kidneys- the removal of urea
How many steps is the urea cycle
5 steps
What is the starting product of the urea cycle
Ammonia
Why do we need the urea cycle
Any AAs that the body does not use are removed via the urea cycle in the liver
Degrading AAs leads to an accumulation of a highly toxic product, ammonia.
How many g of urea will a normal individual excrete per day
35-55g
