Nutrition and metabolism of proteins Flashcards
What is a indispensable/essential a.a?
Indispensable/essential – cannot be synthesised by the organism out of materials ordinarily available to the cells at a speed that covers the demands for normal growth.
Essential amino acids
Valine, leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan and Histidine
What is a conditionally indispensable/essential a.a?
Conditionally indispensable/essential – can be synthesized by the organism but materials or speed of synthesis not always sufficient for the demands of normal growth
Conditionally essential amino acids. Rate limiting steps (e.g. precursors or limited number of synthesizing organs) mean that diet is an important source of these amino acids.
Glycine, Arginine, Glutamine, proline, cystine, tyrosine, taurine (bile acid synthesis), ornithine (part of the urea cycle), citrulline (part of the urea cycle)
What is a dispensable/non-essential a.a?
Dispensable/non-essential – can be synthesised by the organism from a non-amino acid source (e.g. NH4+) and a carbon source (e.g. glucose or CO2)
Glutamic acid/glutamate!, glutamine, alanine, serine, aspartic acid, asparagine
How is protein and nitrogen connected?
Protein is the main source of nitrogen in the body, protein content is often approximated using total N measurements: kjeldahl method (old method but still standard), total Nx6.25 = protein (meat, eggs, corn), exact correction factors available for different foods.
Nitrogen balance
The body requires nitrogen for many processes, through amino acid syntheses is the main use.
Describe the recycling of a.a.
The free amino acids are in the body used for protein synthesis and protein breakdown. In the environment the intake and extraction is the gains/losses of FAA.
The twin cycles of nitrogen balance: the cycle of input and output and the cycle of protein syntheses and breakdown, connecting through the free amino acid pool. Most N from protein breakdown is recycled.
What is the food sources of a.a?
Food sources of amino acids
Western diet: animal protein: 60-70%
Developing country diet: plant proteins: 60-80%, mainly from cereals
Limiting indispensable AA, proteins vary greatly in their amino acid composition, as do food. Animal protein sources are the most complete, they contain proportionally higher amounts of indispensable amino acids than plant foods. Lysine is the first limiting amino acid in wheat.
Methionine is the first limiting amino acid in pulses/legumes.
How is protein digested?
Stomach
– gastric HCl, denatures protein structure, activation of pepsinogen to pepsin, ideal pH
- Pepsin
Small intestine
- trypsin: hydrolyses at basic amino acids
- Chymotrypsin: hydrolyses at aromatic amino acids
- Elastase: hydrolyses at Gly, Ala and Ser (e.g. collagen)
- carboxypeptidase: hydrolyses terminal amino acid at – COOH end
- Aminopeptidase, dipeptidase and tripeptidase digest peptides to free amino acids.
What are some inhibitors for protein digestions?
Plant protein often has lower digestibility: many plants have protease inhibitors that need to be inactivated – e-g- trypsin inhibitors, traditional methods of cooking/soaking have often evolved to remove/destroy protease inhibitors.
Describe the absorption of peptides and a,.a.
Proteins in the intestinal lumen is degraded by pancreatic protease to oligo-peptides,
Oligo-peptides in then further degraded to di-tri-peptides.
The smaller peptides kan be transported through the membrane Peptide transport. after transport the peptides is degraded på cytosolic peptidases to AA.
free AA can transport by single transport.
Describe AA transport
Amino acids require active transport into the cell against a concentration gradient.
Amino acid transporters recognise amino acids based on shape and chemical properties (e.g. neutral, basic or anionic). Energy dependent process 1-3 ATP molecules/amino acids. Two types of amino acid transporters: Na dependent and Na independent, still relatively poorly understood.
Only amino acids or small peptides are absorbed (in the blood stream)
Expect new born babies may be able to absorb intact proteins by pinocytosis – the cell membrane formin a vesicle around the protein. But small fragments of some proteins can be absorbed intact by some people, leading to allergic reactions. Food allergies are generally mediated by protein fragments. NB- allergy = anaphylactic shock, not just discomfort(=intolerance). Digestion is not always perfect.
Describe Gluconeogenesis.
The process of converting amino acids into glucose for energy. Transamination of glutamate + oxaloacetate to make alfa-ketoglutarate + aspartate: alfa-ketoglutarate can then be converted directly into energy via the TCA cycle, branch chain amino acids (leu, ile and val) can also be transaminated to make alfa-ketoglutarate – unique among indispensable amino acids, glutamate is the primary mediator of N transfer for use of amino acids as energy.
The gluconeogenesis is approximately energy neutral process, 6 ATP equivalents used from pyruvate- glucose, glucose yields net 36 ATP.
Can one have to much proteins?
No upper limit for protein intake as no clear negative effects. However, excess dietary protein is removed mainly as urea via the kidneys (over long periods this can overtax the kidneys and intiate renal disease), increased urinary calcium excretion (promoting of loss of bone mass & formation of kidney stones), increased plasma homocysteine (risk factor for cardiovascular disease).
How is nitrogen excreted?
Urine: urea (80-90%), creatinine (3-4%), ammonia (2.5-4.5%), uric acid (1-2%), amino acids (1-2%)
Faeces: protein and free amino acids, most come from intestinal cells and bacteria.
What happens when the body gets too little protein?
Lower total body protein turnover – greater chance of protein damage? Accelerated loss of muscle mass. Sarcopenia/muscle wasting.
Extreme case: Kwashiorkor: energy needs met, but not enough protein. Lack of albumin synthesis=oedema, liver damage, immune deficiency.
What is muscle wasting?
Unintentional loss of body weight (5-10%) due to accelerated muscle proteolysis. Low food intake/high energy expenditure may be reasons. Anorexia and starvation lead to increased protein degradation and decreased synthesis. Muscle wasting is increased in many disease: diabetes, renal and liver failure, HIV/AIDS, cancer.
