Proteins are organic compounds comprised of conjugated amino acids in proportions that are characteristic of each protein. This nutrient always contains carbon, hydrogen oxygen and nitrogen; however ,some proteins also contain sulphur,phosphorus and iron.
Proteins and Amino acids
Since most proteins contain about 16% nitrogen , crude protein of a product can be obtained by determining the nitrogen content and then multiplying that value by 6,25 (100%:16%= 6,25).
Plants generally contain their protein in actively growing portions ,such as leaves and seeds , while animals tend to distribute protein in a wider variety of tissues, such as bones ,skin,organs,feathers ,scales and muscle.
Body protein is constantly undergoing two processes: protein synthesis (anabolism) and prοtein breakdown (catabolism).
Protein deposition (or loss)equals the amount of protein synthesis minus the amount of protein breakdown.
The basic structural component of proteins consists of amino acids. Ten amino acids cannot be synthesized by vertebrates,including fish , and must be supplied in the diet.
These essential (indispensable) amino acids are: arginine ,histidine ,isoleucine, leucine ,lysine , methionine,phenylalanine,threonine,tryptophan and valine. There are other amino acids that fish can synthesize.These amino acids are termed non - essential (dispensable) amino acids and consist of: alanine,asparagine ,aspartic acid ,cysteine ,cystine ,glutamic acid,glutamine , glycine,hudroxyproline ,proline serine, and tyrosine. Non essential amino acids can be synthesized by transfer of an amino acid group to a - keto acids which can be derived from non -protein sources,such as glucose.
When a particular essential amino acid is deficient in a diet , it is referred as a limiting amino acid because it limits the synthesis of protein.
All of the amino acids needed to synthesize a protein must be available or no synthesis can occur.
This is why protein quality is important in fish nutrition. Protein synthesis involves a series of reaction that are specific for each protein. Each protein is manufactured according to the code derived from deoxyribonucleic acid (DNA) , a chromosomal component of the cell.
Messenger ribonucleic acid (mRNA) transcribes the information from DNA and brings it to the cytoplasm of the cell as a single strand; transfer RNA(tRNA) cary the specific amino acid to the ribosome , where the interact with mRNA; ribosomal RNA(rRNA) is a component of the ribosomes in the cell and is the site of protein synthesis.
As the DNA message is decoded ,tRNA brings the proper amino acids to be conjugated. The amino (NH2) portion of one amino acid will combine with the carboxyl(COOH) portion of an other amino acid , forming a peptide linkage and releasing water (H2O). This process of joining amino acids is continued until the specific molecule(protein) encoded by the DNA is produced.Each sequence of amino acids is a different protein and these different proteins accomplish different functions in the body.
Proteins occur in nature in a number of forms that possess unique chemical properties and can be devided into simple proteins and conjugated proteins.
Simple proteins consist only amino acids only, or their derivatives,while conjugated proteins are joined to other non-protein substances.
Simple proteins consist of the albuminoids,albumin,globulins,hormones and blood proteins, contractile proteins such as actin-myosin and keratins.
Conjugated proteins include the chromoproteins(combination of a protein and a pigmented substance), lecithoproteins(combination of a protein and lecithin), lipoproteins(combination of a lipid and a protein) ,metaloproteins(combination of a metal and a protein), nucleoproteins(combination of a nucleic acid and a protein) and phosphoproteins(combination of phosphorus and a protein); the phosphorus is in a form other than phospholipid or nucleic acid).
Protein requirement is somewhat of a misnomer in that it is actually the requirement for essential amino acids that is important for the proper growth ,development and health of fish. While crude protein values are important to a nutritionist formulating a fish diet, they may not allow the nutritionist to determine the quality of the protein.
Gelatin is a high-protein source ;however ,it is virtually devoid of tryptophan. Feather meal , while high in protein,is very poorly digested and utilized by fish ,resulting in reduced growth if incorporated at high levels in the diets.
Thus , it is important to know the amino acid composition and digestibility of a protein source, as well as its total protein content,when formulating a diet.
There are some measurements and mathematical equationsthat one can utilize to quantify protein quality.One such method is to determine the apparent protein utilization(APU). APU is the amount of protein gain of fish feed an experimantal diet,devided by the amount of protein fed.
In other means of determining protein quality is the protein efficiency ratio(PER),which is defined as the total weight gain of fish devided by the total protein intake over the period of the feeding trial. PER assumes that all protein is used for gtrowth and no allowance for maintenance (turnover) is made.
A third method ,net protein utilization (NPU) or net protein value, attempts to take into account protein used for maintenance.This is accomplished by the inclusion of a treatment group of fish that receive diets containing no dietary protein. The NPU is the protein gain of a group of fish fed the experimental diet minus the protein loss of a similar group fed a protein -free diet, divided by the weight of the proteinconsumed.
Most proteins are highly digestible . In fish prossessing stomach ,pepsins and hydrochloric acid(HCL) initiate proteolysis. Once in the intestine ,luminal enzymes , such as trypsins, chymotrypsins and carbo xypeptidase A and B , further break down the protein into amino acids. Amino acids are absorbed across the brush border of the intestines in fish. In agastric fish (fish without stomachs) proteolysis begins directly in the anterior intestine.
There are a number of factors that affect the protein requirement of fish. One is the size of the fish ; small fish require more protein than larger fish. As mentioned previously ,protein quality is important.A third factor is water temperature; when water temperature is at or near of fish's optimum growth.A fourth factor is feeding rate; if fish are fed all they consume , the lower the protein level required compared with fish fed a restricted (not all the diet a fish will consume) amount of diet.
A fifth factor is the presence of natural food items in the culture system. Fish stocked in ponds at low density may have access to more natural foods than those stocke at high density or in a raceway. Low dietary protein level or poorer-quality dietary protein may be used for fish with access to natural foods as compared with fish cultured where natural foods are absent or limited Lastly ,the dietary energy level affects the dietary protein requirements of fish.
If the non protein energy level of a diet is low , the fish will utilize protein to meet metabolic energy needs. This is inefficient and financially wasteful. If the energy level is too high , it may suppress food intake and the fish will not consume enough diet to meet ptotein requirements. This might reduce fish growth.
