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Are large biological molecules, or macromolecules, consisting of one or more chains of amino acid residues.
Proteins perform a vast array of functions within living organisms, including catalyzing metabolic reactions,
replicating DNA, responding to stimuli, and transporting molecules from one location to another. Proteins differ
from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of
their genes, and which usually results in folding of the protein into a specific three-dimensional structure that
determines its activity.
All proteinogenic amino acids possess common structural features, including an α-carbon to which an amino
group, a carboxyl group, and a variable side chain are bonded. Only proline differs from this basic structure as it
contains an unusual ring to the N-end amine group, which forces the CO–NH amide moiety into a fixed
The principal structural components of body soft tissues are proteins large biological molecules consisting of
one or more chains of amino acids.
The enzymes that regulate most body processes are also proteins. The body can synthesize many of the amino
acids needed for protein syntheses, but some amino acids must be obtained from the proteins in the diet. The
dietary essential amino acids for humans are threonine, valine, leucine, isoleucine, methionine, lysine, histidine,
and tryptophan. Two others can only be formed from essential amino acids: tryosine from phenylalanine, and
cystine from methionine. Human dietary protein requirements are quite modest. An adult man of average
weight is estimated to need about sixty-three grams of protein per day, while an average woman is estimated to
need about fifty grams. The protein must supply the essential amino acids required by humans and sufficient
total nitrogen to allow syntheses of the other amino acids required for protein synthesis.
Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide
sequence of their genes, and which usually results in folding of the protein into a specific three-dimensional
structure that determines its activity. the genetic code specifies 20 standard amino acids Many proteins are
enzymes that catalyze biochemical reactions and are vital to metabolism.
Protein should account for 10% to 20% of the calories consumed each day. Protein is essential to the structure
of red blood cells, for the proper functioning of antibodies resisting infection, for the regulation of enzymes and
hormones, for growth, and for the repair of body tissue.
Protein sources:
• Amino acids are the building blocks of protein and are found in a variety of foods. Meat, milk, cheese, and egg
are complete proteins that have all the essential amino acids. Other sources of protein include whole grains, rice,
corn, beans, legumes, oatmeal, peas, and peanut butter. For vegetarians, vegans and/or those who do not eat
meat, fish, eggs, or dairy products, it is important to eat a variety of these other foods in order to get enough
Protein quality, from a nutrition perspective, is a term used to describe how well a protein from food matches
the body's requirements and, therefore, how useful the protein is for our body. This is determined by looking at
the building blocks which make up the protein; called Amino Acids.
There are 20 primary amino acids, 9 of which our body cannot make on its own. These are considered
"essential" and so must come from our diet. A protein's quality is determined by asking the following questions:
1. Are all the 9 essential amino acids present in the protein?
2. Are the ratios of essential amino acids in the protein ideal?
The first question is answered fairly easily with a "yes". In fact, pretty much every source of protein has at least
some of all the essential amino acids. However, if the protein is deficient in 1 or more essential amino acid then
it is considered incomplete. For your reference, grains are typically low in the amino acid lysine, while legumes
are low in methionine. Animal products, on the other hand, are high in all the essential amino acids and are
usually considered complete. (the exception being gelatin)
Answering the second question is a bit more complex. Basically, the body needs a certain proportion of
essential amino acids in the protein in order for it to be used most effectively. If all 9 are present yet 1 or more
are not very abundant then the protein is of lower quality because the body will not be able to use the protein to
its fullest potential.
The "gold standard" used when determining protein quality is the egg (typically chicken egg). Eggs have all 9
essential amino acids in its protein and the ratios of these amino acids are very similar to the ideal protein
needed by the body. You simply can't find better protein in your diet than egg protein! As a result, egg protein is
considered the #1 quality protein and all other proteins from all other food sources are compared against this
Nitrogen balance is the measure of nitrogen output subtracted from nitrogen input. Blood urea nitrogen can
be used in estimating nitrogen balance, as can the urea concentration in urine.
A positive value is often found during periods of growth, tissue repair or pregnancy. This means that the intake
of nitrogen into the body is greater than the loss of nitrogen from the body, so there is an increase in the total
body pool of protein.
A negative value can be associated with burns, fevers, wasting diseases and other serious injuries and during
periods of fasting. This means that the amount of nitrogen excreted from the body is greater than the amount of
nitrogen ingested. A negative Nitrogen balance can be used as part of a clinical evaluation of malnutrition.
The best-known role of proteins in the cell is as enzymes, which catalyze chemical reactions. Enzymes are
usually highly specific and accelerate only one or a few chemical reactions. Enzymes carry out most of the
reactions involved in metabolism, as well as manipulating DNA in processes such as DNA replication, DNA
repair, and transcription. Some enzymes act on other proteins to add or remove chemical groups in a process
known as posttranslational modification. About 4,000 reactions are known to be catalyzed by enzymes.
The molecules bound and acted upon by enzymes are called substrates. Although enzymes can consist of
hundreds of amino acids, it is usually only a small fraction of the residues that come in contact with the
substrate, and an even smaller fraction—three to four residues on average—that are directly involved in
catalysis. The region of the enzyme that binds the substrate and contains the catalytic residues is known as the
active site.