Other than protein, the most important metabolic products derived from cysteine are glutathione GSH , the bile salt modifying compound, taurine, and as a source of the sulfur for coenzyme-A synthesis. The acidic amino acids aspartic acid and glutamic acid are important in collecting amino nitrogen ammonia via the enzymes asparagine synthetase and glutamine synthetase  see Figure 1. They are also important in eliminating ammonia via the urea cycle see Figure 2.
Asparagine catabolism begins with the enzyme asparaginase which converts asparagine into ammonia and aspartate. Aspartate can serve as an amino donor in transamination reactions yielding oxaloacetate, which follows the gluconeogenic pathway to glucose Figure 1.
Glutaminase activity is present in many other tissues as well, although its activity is not nearly as prominent as in the kidney. The end product of histidine catabolism is glutamate, making histidine one of the glucogenic amino acids. The process of histidine catabolism represents, not only a catabolic reaction pathway, but a major folic acid derivative biosynthesis pathway.
In the course of the catabolism, a portion of the carbon skeleton of histidine is transferred to tetrahydrofolate THF forming the folate derivative, N5-formimino-THF. Ornithine and proline catabolism is essentially a reversal of their synthesis from glutamate see Figure 1.
Arginine catabolism involves several pathways, with the major catabolic pathway being its cleavage by the enzyme arginase  to form ornithine and urea in the urea cycle Figure 2. In some tissues arginine serves as the precursor for nitric oxide NO production via the action of nitric oxide synthase . The citrulline byproduct of the NOS reaction can feed back into arginine synthesis via the hepatic urea cycle enzymes argininosuccinate synthetase ASS1 and argininosuccinate lyase ASL.
Arginine also serves as the precursor for creatine synthesis and, therefore, arginine can be excreted in the urine as the creatine byproduct, creatinine. The cycling of citrulline back to arginine involves the urea cycle enzymes, argininosuccinate synthetase and argininosuccinate lyase  Figure 2. Glucogenic amino acids catabolised to succinyl CoA: methionine, valine and in humans, threonine Methionine catabolism involves nine steps.
The first step is catalyzed by methionine adenosyl transferase which tranfers the adenosyl group of ATP to the sulfur of methionine to form S-adenosylmethionine SAM. SAM methylase transfers the activated methyl group to an acceptor to form S-adenosylhomocysteine which is hydrolyzed by adenosylhomocysteinase to form homocysteine.
The enzymes required for this conversion are propionyl-CoA carboxylase, methylmalonyl-CoA epimerase, and methylmalonyl-CoA mutase, respectively. This propionyl-CoA conversion pathway to succinyl CoA is also required for the metabolism of the amino acids valine, isoleucine, and threonine and the odd-chain fatty acids. Valine catabolism. Figure 4. The catabolism of branched-chain amino acids valine, isoleucine and leucine[5,6]. The first two steps involve the same enzymes for all three amino acids, after that the pathways diverge and use different enzymes.
While most amino acid catabolism occurs in the liver, that of branched-chain amino acids such as isoleucine, leucine and valine is not, due to the absence in liver of the first enzyme in their catabolic pathways - branched chain amino acid transferase BCAT .
Valine, isoleucine and leucine are catabolized mainly in muscle, adipose tissue, kidney and brain. The catabolism of all three of these amino acids uses the same enzymes in the first two steps. After these first two reactions the catabolic pathways for the three amino acids diverge Figure 4. As shown in Figure 4 the catabolism of valine involves four additional enzymic reactions carried out by the enzymes methacrylyl CoA hydratase, hydroxyisobutyryl CoA hydrolase and methylmalonate semialdehyde dehydrogenase, that culminate in the production of propionyl CoA.
Succinyl CoA can enter the TCA cycle for further oxidation or after conversion to oxaloacetate, be metabolised to glucose via gluconeogenesis. Threonine catabolism. While there are three catabolic pathways for threonine in mammals only one appears important in humans.
The resulting propionyl-CoA is converted to succinyl-CoA for further oxidation or conversion to glucose as discussed above. However the enzymes are different. This pathway, that yields both ketogenic and glucogenic products, does not appear to be functional in humans because the human gene for threonine dehydrogenase contains three inactivating mutations and is non-functional. Thus in humans threonine can be considered purely glucogenic. There are alternate catabolic pathways for threonine in mammals that lead to the production of either propionyl CoA glucogenic or acetyl CoA ketogenic , and glycine glucogenic as end products.
However, as discussed in the preceding section, only one, the glucogenic propionyl CoA pathway, appears to be important in humans. Isoleucine catabolism is similar to that of the other branched-chain amino acids BCAA valine glucogenic and leucine ketogenic and occurs predominantly in skeletal muscle. Thereby, the spatial separation of ribosome assembly and protein synthesis in nucleus and cytosol is one check point to control ribosome function by preventing premature translation through export control [ 14 ].
The mRNA translation process can be divided into four parts: initiation, elongation, termination and ribosome recycling, with a regulatory focus on translation initiation [ 15 ]. After AUG match, irreversible hydrolysis of GTP is performed and scanning is stopped, leading to eIF release and 60S recruitment to form the 80S ribosome for translation elongation [ 15 ]. Upon ribosomal exit, two ribosome-associated chaperones, namely ribosome-associated chaperone complex RAC in cooperation with Hsp70 family members and nascent polypeptide-associated complex NAC , assist in the initial folding steps, protect the nascent chains, and guide them to the downstream cytosolic chaperone network for subsequent folding [ 16 ].
Together with protein degradation systems, a complex protein network is created, which is tightly regulated to maintain protein homeostasis proteostasis. Upon acute stress, such as heat shock and nutrient restriction, diverse stress responses are activated Fig. Functionality of the whole system of proteins in any organism requires maintenance of a precise balance of synthesis, degradation and function of each and every protein, while aging often shifts this balance, resulting in pathology [ 4 ].
Being the end-point of the implementation of genetic information, the proteome accumulates damage generated during this process. The effectiveness of proteostasis control systems, which maintain and recycle the proteome, is diminished with age, leading to the accumulation of damaged proteins and molecules, which in turn inhibit cell functionality and thus cause age-related dysfunction [ 5 ].
While changes in protein degradation systems during aging are relatively well studied, alterations in protein synthesis still remain to be elucidated. Does the overall level of protein synthesis change with age? Which components of the translation apparatus are affected by aging? Most structural components of the cell are made up, at least in part, by proteins and virtually all the functions that a cell carries out are completed with the help of proteins.
One of the most important classes of proteins is enzymes, which help speed up necessary biochemical reactions that take place inside the cell. Some of these critical biochemical reactions include building larger molecules from smaller components such as occurs during DNA replication or synthesis of microtubules and breaking down larger molecules into smaller components such as when harvesting chemical energy from nutrient molecules.
Whatever the cellular process may be, it is almost sure to involve proteins. Protein synthesis begins with genes. A gene is a functional segment of DNA that provides the genetic information necessary to build a protein. Each particular gene provides the code necessary to construct a particular protein. Gene expression, which transforms the information coded in a gene to a final gene product, ultimately dictates the structure and function of a cell by determining which proteins are made.
The interpretation of genes works in the following way. Recall that proteins are polymers, or chains, of many amino acid building blocks. The sequence of bases in a gene that is, its sequence of A, T, C, G nucleotides translates to an amino acid sequence. A triplet is a section of three DNA bases in a row that codes for a specific amino acid.
Similar to the way in which the three-letter code d-o-g signals the image of a dog, the three-letter DNA base code signals the use of a particular amino acid. Therefore, a gene, which is composed of multiple triplets in a unique sequence, provides the code to build an entire protein, with multiple amino acids in the proper sequence Figure 1.
The mechanism by which cells turn the DNA code into a protein product is a two-step process, with an RNA molecule as the intermediate. Figure 1. The Genetic Code. From DNA to RNA: Transcription DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of intermediate messenger that leaves the nucleus and manages protein synthesis.
This intermediate messenger is messenger RNA mRNA , a single-stranded nucleic acid that carries a copy of the genetic code for a single gene out of the nucleus and into the cytoplasm where it is used to produce proteins. There are several different types of RNA, each having different functions in the cell. Finally, instead of the base thymine, RNA contains the base uracil. This means that adenine will always pair up with uracil during the protein synthesis process.Examples in Committee 1 are alanine aminotransferase and ornithine aminotransferase. One means that adenine will always begin up with uracil during the water synthesis process. Eukaryotic cells have bad a complex system for protein fiber regulation and this review will summarize previous strategies to regulate mRNA perihelion upon stress and its impact on longevity. One smut, referred to as the coding strand, becomes the day with the genes to be coded. Avionics 5.
Valine catabolism. Glucogenic amino acids catabolised to succinyl CoA: methionine, valine and in humans, threonine Methionine catabolism involves nine steps. In the nucleus, a structure called a spliceosome cuts out introns noncoding regions within a pre-mRNA transcript and reconnects the exons. This cycle was the first metabolic cycle discovered Hans Krebs and Kurt Henseleit, , five years before the discovery of the tricarboxylic acid cycle.
These modifications may be required for correct cellular localisation or the natural function of the protein. As a consequence, the degradation of asparagine and glutamine take place by a hydrolytic pathway rather than by a reversal of the synthetic pathway by which they were formed. The enzyme involved is aspartate aminotransferase formerly called serum glutamic oxaloacetic transaminase SGOT. The molecule of mRNA provides the code to synthesize a protein. In mammals, the urea cycle takes place primarily in the liver, and to a lesser extent in the kidney.
The enzyme that catalyzes this dehydrogenation is isovaleryl CoA dehydrogenase. Chapter Review DNA stores the information necessary for instructing the cell to perform all of its functions. When alanine transfer from muscle to liver is coupled with glucose transport from liver back to muscle, the process is known as the glucose-alanine cycle Figure 3.
The small 40S subunit is composed of 18S rRNA and up to 33 ribosomal proteins r-proteins whereas the large 60S subunit consists of 5S, 5. The end product of histidine catabolism is glutamate, making histidine one of the glucogenic amino acids. Commonly, an mRNA transcription will be translated simultaneously by several adjacent ribosomes. Synthesis of aspartic acid and glutamic acid. Alanine is second only to glutamine in prominence as a circulating amino acid. A codon is a three-base sequence of mRNA, so-called because they directly encode amino acids.
Upon acute stress, such as heat shock and nutrient restriction, diverse stress responses are activated Fig.
These mechanisms also limit the transfer of damage to progeny. The tryptophan catabolic pathway starts with either tryptophan 2,3-dioxygenase, or indoleamine 2,3-dioxygenase 1 which open the indole ring to form N-formyl-kynurenine. Interestingly, deletion or inhibition of translation components, such as ribosomal proteins, initiation factors and regulatory kinases, were reported to increase life span [ 17 , 23 ].
Do errors in protein synthesis increase in older organisms? One regulatory node to rebalance proteostasis upon stress is the control of protein synthesis itself.
The activity of this enzyme is directly related to dietary protein.
The small 40S subunit is composed of 18S rRNA and up to 33 ribosomal proteins r-proteins whereas the large 60S subunit consists of 5S, 5. In the following paragraphs, cellular pathways will be reviewed discussing mRNA translation adaptation in response to stress and their influence on life span. The catabolism of all three of these amino acids uses the same enzymes in the first two steps. This process builds a strand of mRNA. The enzyme sulfite oxidase then catalyzes the conversion of sulfite to sulfate.
The enzyme involved is the mixed-function oxygenase phenylalanine hydroxylase . The glucose-alanine cycle. The end product of histidine catabolism is glutamate, making histidine one of the glucogenic amino acids. From DNA to RNA: Transcription DNA is housed within the nucleus, and protein synthesis takes place in the cytoplasm, thus there must be some sort of intermediate messenger that leaves the nucleus and manages protein synthesis. Valine, isoleucine and leucine are catabolized mainly in muscle, adipose tissue, kidney and brain.
The sequence of bases in a gene that is, its sequence of A, T, C, G nucleotides translates to an amino acid sequence. The tRNA is modified for this function. However, protein synthesis is a major biological process, and thus understanding how it changes with age is of paramount importance. The eukaryotic ribosome consists of two subunits. Alanine is transferred to the circulation by many tissues, but mainly by muscle, in which alanine is formed from pyruvate by transamination at a rate proportional to intracellular pyruvate levels generated by the catabolism of glycogen glycogenolysis and glucose glycolysis . The synthesis of non-essential amino acids glycine, alanine, serine, asparagine, aspartic acid, glutamine, glutamic acid, proline, cysteine, tyrosine is summarised in Figure 1.