Fundamentals of Human Nutrition/Synthesis< Fundamentals of Human Nutrition
The process of protein synthesis can be broken up into two main steps: transcription and translation. Transcription is the process of creating mRNA from DNA, and translation is the process of creating a protein from the mRNA.
The DNA in our cells holds all the necessary information for protein synthesis. In the nucleus of a cell, RNA polymerase unzips the DNA to begin the formation of the RNA. C and G nucleotides match up and A and T nucleotides match up. However, whenever an A is found on the DNA, the RNA polymerase places a U on the RNA. For example, if the sequence of amino acids on the DNA were ATCG, the sequence on the mRNA would be UAGC. This is the process of transcription to messenger RNA, or mRNA. The mRNA is carried outside the nucleus into the cytoplasm (Dobson, 2000).
It is important to note that the mRNA will not code for the entire DNA, it will code for a portion of the DNA strand that is necessary to create the protein needed at that time. (Teacher’s Pet, 2014) Once the new mRNA is formed, transcription will occur. Transcription can be broken down into three main steps: initiation, elongation, and termination.
Once the mRNA is outside of the nucleus and inside the cytoplasm of the cell, two ribosomal subunits attach to the mRNA. Ribosomes can be found on the rough endoplasmic reticulum (Farabee, 2007). They can be thought of as protein making machines; they themselves are made up of protein and RNA. This linking step between the mRNA and ribosomes is called initiation.
The ribosome reads the nucleotides from the mRNA in groups of threes, known as a codon. It begins at a start codon, which is usually AUG. Transfer RNA, or tRNA, delivers the necessary amino acids in order to add the amino acids to the chain and create a protein. The codon of the mRNA is matched to three nucleotides on the tRNA, known as an anticodon. The anticodon is on one side of the tRNA while the amino acid that will be added to the chain is on the other. In elongation, the ribosome has three tRNA binding sites, known as the acceptor (A) site, the peptidyl (P) site, and the exit (E) site. The tRNA that has the chain of amino acids attached to it binds to the P site, and the new tRNA with an anticodon binds to the A site to be read and checked that it has the correct next anticodon. When the two tRNAs are next to each other, a bond forms between the amino group of one amino acid, and the acid group of the other. One of the tRNAs continues to the E site where it exits the ribosome and then goes back to the cytoplasm to find new amino acids. The protein grows until a stop codon is reached (Farabee, 2007).
The bonds between amino acids are called peptide bonds, and a strand of 10 or more amino acids is known as a polypeptide. Once the chain of amino acids is complete, termination takes place and the new protein is released from the ribosome (Teacher’s Pet, 2014), ready to perform one of the many functions of proteins which will be described in the next section.
Dobson CM (2000). "The nature and significance of protein folding". In Pain RH (ed.). Mechanisms of Protein Folding. Oxford, Oxfordshire: Oxford University Press. pp. 1–28. ISBN 0-19-963789-X. Retrieved November 10, 2015.
Farabee, M.J. (2007). Protein Synthesis. Retrieved November 9, 2015.
[Teacher’s Pet]. (2014, December 7). Protein Synthesis. [Video File]. Retrieved November 9, 2015 from https://www.youtube.com/watch?v=2zAGAmTkZNY.
5.3.1 Protein turnoverEdit
Protein turnover is defined as the continuous degradation and synthesis of proteins in our body. This process is important in human and animal cells because it allows them to grow and build the protein we need to survive. The organs in our body make use of this process to repair and build tissues and regulate metabolic pathways. When we obtain proteins from the food we eat, our body digests that food in our small intestines and breaks down the nutrient into smaller parts known as amino acids, which are essentially the building blocks of proteins (W H Freeman, 2002). When proteins are broken down into nitrogen containing amino acids, they are added into the amino acid pool, which is an accumulation of amino acids that are stored in the body for the future. The amount of protein we need in our body from our diet depends on nitrogen balance, which is the balance between the amounts of nitrogen digested versus the amount of nitrogen excreted. In normal cases, the rate of protein degradation equals the rate of protein synthesis. During an anabolic state, organisms or cells are growing therefore the rate of protein synthesis is higher than the rate of degradation. During a catabolic state, the rate of degradation is higher than that of protein synthesis (Doherty & Whitfield, 2011). This constant balancing of metabolic pathways allows our bodies to react to different cellular situations and stay in homeostasis.
There are a lot of factors that influence the rate of protein synthesis and degradation. In protein synthesis, the initiation of transcription as well as the activity of ribosomes has an effect. In protein degradation, the half-lives of proteins in a cell are not always constant and may vary (Cooper, 2000). In some cases degraded proteins are used as regulatory molecules, like in transcription, therefore they must be broken down rapidly. In other cases, proteins are degraded in response to certain signals so they can act as a mechanism in regulation depending on the intracellular environment (Cooper, 2000). In any case protein turnover must be swift in order to change their levels and react to external stimuli.
Protein turnover is a dynamic process that requires certain techniques in order for it to be measured. Some ways protein turnover can be measured include measuring the amount of RNA per DNA or protein, the state of aggregation of ribosomes (i.e. the polyribosome index), the abundance of mRNA for particular proteins, and the enzymatic activity of proteins such as proteases, ribonuclease, etc. (Smith & Rennie, 1996). However, the more common and efficient method of measuring protein turnover involves using a proteomic machine to set a precursor into a protein or amino acid so that quantifiable data can be obtain throughout the process (Doherty & Whitfield, 2011).
Berg JM, Tymoczko JL, Stryer L, W. H. Freeman and Company. (2002). “Chapter 23: Protein Turnover and Amino Acid Catabolism.” Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK21182/
Mary K Doherty; Phillip D Whitfield. (2011). “Proteomics Moves From Expression to Turnover.” Retrieved from http://www.medscape.com/viewarticle/745130_2
Geoffrey M Cooper. (2000). “Protein Degradation.” Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK9957/
Smith K, Rennie MJ. (1996). “The measurement of tissue protein turnover.” Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/9022947
5.3.2 Amino acid poolEdit
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5.3.3 Nitrogen balanceEdit
Nitrogen balance is the relationship between the intake of nitrogen into the body (through protein rich foods) and the output of nitrogen excreted from the body (through urine and feces). The majority of the nitrogen in our bodies comes from protein. (Segen's Medical Dictionary, 2011).
Achieving nitrogen balance
In order to measure nitrogen balance and to determine nutritional competence, track the amount of protein that the body loses in the urine within twenty-four hours. To calculate this protein lost, measure the nitrogen products which are excreted through urine. On average, 0.5g/kg of protein from the diet is enough to ensure a zero nitrogen balance (Segen's Medical Dictionary, 2011). Our bodies require nitrogen in order to complete tissue protein synthesis and to produce nitrogen-containing compounds which are key in several vital functions; some of these important functions include hormones, immune competence, peroxidative defenses, and neurotransmitters (Tomé & Bos, 2000). Our bodies lose nitrogen in several ways, also. Mainly, nitrogen is lost by means of urea, creatinine, and ammonia in the urine; in addition, nitrogen is lost in fecal matter and other miscellaneous losses (Tomé & Bos, 2000).
When the intake of nitrogen through the diet is the same as the output of nitrogen through urine and feces, total body protein does not change; the body is at nitrogen equilibrium/balance which is the normal state in a healthy adult. When intake in nitrogen is greater than the output of nitrogen, total body protein increases; people who experience a positive nitrogen balance are those who are pregnant, growing, or recovering from the body’s protein loss from undernutrition or trauma. When intake of nitrogen is less than the output of nitrogen, total body protein endures a net loss; the body is at a negative nitrogen balance which is abnormal and unhealthy. A negative nitrogen balance is the body’s response to infection, trauma, or the inadequate intake of nutrients to aid in replacing tissue proteins which are turning over (Bender, 2006).
On average, healthy adults consume 80 grams of protein in one day, and the body intakes 70 more grams of protein through digestive enzymes in the gut, intestinal enzymes, and shed intestinal cells. Our bodies hydrolyze the majority of this protein to dipeptides and amino acids, and the dipeptides are then hydrolyzed. The body loses on average ten grams of protein each day through fecal matter; protein that is lost through feces is often indigestible protein from the diet, bacterial protein, or mucin (mucus’s main protein which resists hydrolysis from enzymes). Proteins that are absorbed are utilized for protein synthesis which occurs in recovering adults and growing children. Their bodies must synthesize proteins in order to replace the proteins that are turning over. From the amino acid pool, some of the amino acids are reused, but most are deaminated; amino acids that are newly absorbed in excess for protein synthesis use are also deaminated. The carbon skeletons that remain are utilized for gluconeogenesis, metabolic fuels, or fatty acid synthesis. The amino groups from the amino acids form urea which is the primary nitrogen-containing compound which we excrete through urine (Bender, 2006).
Bender, D. (2006). Nitrogen balance and protein requirements. Retrieved from http://www.ucl.ac.uk/~ucbcdab/simulations.htm
Nitrogen balance. (n.d.). Segen's Medical Dictionary. (2011). Retrieved from http://medical-dictionary.thefreedictionary.com/nitrogen+balance
Tomé, D., & Bos, C. (2000). Dietary Protein and Nitrogen Utilization. Journal of Nutrition, 130, 1868S-73S. Retrieved from http://jn.nutrition.org/content/130/7/1868S.long