A peptide is an amino chain responsible for signalling different responses in the body. These amino chains already exist in the body and that is the good thing about peptides. There are different classes of peptides according to the number sequence that I will have explained in the below article. Certain sequences are considered a PROTEIN such as HGH (Human Growth Hormone) that is 191aa. Insulin is actually an amino sequence considered a PROTEIN as well. Below is a statement from Wikipedia for reference. Notice below mentions a "Peptide Fragment". This can be exampled to the product "HGH frag" that is the isolated amino chain "176-191" that is the chain responsible for the diuretic effects of Human Growth Hormone (HGH).
Peptides are short polymers of amino acids linked by peptide bonds. They have the same peptide bonds as those in proteins, but are commonly shorter in length. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond. There can also be tripeptides, tetrapeptides, pentapeptides, etc. Peptides have an amino end and a carboxyl end, unless they are Cyclic_peptides. A polypeptide is a single linear chain of amino acids bonded together by peptide bonds. Protein molecules consist of one or more polypeptides put together typically in a biologically functional way and sometimes have non-peptide groups attached, which can be called prosthetic groups or cofactors.
One definition is that those chains that are short enough to be made synthetically from the constituent amino acids are called peptides rather than proteins. However, with the advent of better synthetic techniques, peptides as long as hundreds of amino acids can be made, including full proteins like ubiquitin. Native chemical ligation has given access to even longer proteins, so this convention seems to be outdated.
Another definition places an informal dividing line at approximately 50 amino acids in length (some people claim shorter lengths). This definition is somewhat arbitrary. Long peptides, such as the amyloid beta peptide linked to Alzheimer's disease, can be considered proteins; and small proteins, such as insulin, can be considered peptides.
Here are the major classes of peptides, according to how they are produced:
Milk peptides Milk peptides are formed from milk proteins by enzymatic breakdown by digestive enzymes or by the proteinases formed by lactobacilli during the fermentation of milk. Several milk peptides have been shown to have antihypertensive effects in animal and in clinical studies (see also Lactotripeptides). Ribosomal peptides Ribosomal peptides are synthesized by translation of mRNA. They are often subjected to proteolysis to generate the mature form. These function, typically in higher organisms, as hormones and signaling molecules. Some organisms produce peptides as antibiotics, such as microcins. Since they are translated, the amino acid residues involved are restricted to those utilized by the ribosome. However, these peptides frequently have posttranslational modifications, such as phosphorylation, hydroxylation, sulfonation, palmitylation, glycosylation and disulfide formation. In general, they are linear, although lariat structures have been observed. More exotic manipulations do occur, such as racemization of L-amino acids to D-amino acids in platypus venom. Nonribosomal peptides These peptides are assembled by enzymes that are specific to each peptide, rather than by the ribosome. The most common non-ribosomal peptide is glutathione, which is a component of the antioxidant defenses of most aerobic organisms. Other nonribosomal peptides are most common in unicellular organisms, plants, and fungi and are synthesized by modular enzyme complexes called nonribosomal peptide synthetases. These complexes are often laid out in a similar fashion, and they can contain many different modules to perform a diverse set of chemical manipulations on the developing product. These peptides are often cyclic and can have highly-complex cyclic structures, although linear nonribosomal peptides are also common. Since the system is closely related to the machinery for building fatty acids and polyketides, hybrid compounds are often found. The presence of oxazoles or thiazoles often indicates that the compound was synthesized in this fashion. Peptones See also Tryptone Peptones are derived from animal milk or meat digested by proteolytic digestion. In addition to containing small peptides, the resulting spray-dried material includes fats, metals, salts, vitamins and many other biological compounds. Peptone is used in nutrient media for growing bacteria and fungi. Peptide fragments Peptide fragments refer to fragments of proteins that are used to identify or quantify the source protein. Often these are the products of enzymatic degradation performed in the laboratory on a controlled sample, but can also be forensic or paleontological samples that have been degraded by natural effectsReference: