PROTEINS

Recall: Nutrients in the human body have three general functions:

1. They can supply energy for processes such as growth, movement, electrical signalling, metabolism

2. They can regulate body processes such as metabolism, growth, membrane transport, cellular communication

3. They can provide the building blocks for making the structures of our cells and our bodies

`Proteins in our diet serve all three functions

Proteins

Although cells of the body prefer to use carbohydrates and lipids as sources of energy, many cells can also use proteins as fuel molecules if necessary.

That means that extra dietary protein can be converted to carbohydrate or lipid molecules and stored for future energy needs.

1. Proteins can supply energy:

They help form “receptors” which allow cells to respond to hormones

Proteins

2. Proteins can regulate cellular processes:


They form “channels” in the membrane which allow things to get into and out of cells

Proteins



They form “channels” in the membrane which allow things to get into and out of cells

They form “junctions” on membranes which allow cells to communicate with each other.

Proteins


They form enzymes which catalyze cellular reactions

Proteins



Proteins

They slide past each other to makemuscles contract



Proteins

They slide past each other to makemuscles contract

They fom the cytoskeleton which holds organelles in place and moves them around in the cell


Proteins


They form antibodies allover the body, proteins in theblood, and many hormones.

Proteins such as collagen form tendons and ligaments

Proteins

3. Proteins can make structures our bodies need:

Proteins such as collagen form tendons and ligaments

Proteins

Collagen and another protein called elastin form the dermis is the skin.

The protein keratin forms hair and the top layer of the epidermis.

3. Proteins can make structures our bodies need:

Proteins All of these proteins in the body must constantly be replaced.

That means your diet must contain the building blocks of proteins, called amino acids

Food sourcesMeat, Chicken, FishMilk & cheeseEggs Beans, peas, grains, nuts, seeds, vegetables

Proteins

Amino Acids

Amino Acids Amino acids bond together by a process called dehydration synthesis.


Bonding two amino acids together forms a dipeptide


Bonding two amino acids together forms a dipeptide

Bonding many amino acids together forms a polypeptide or protein

Amino Acids While proteins are long, unbranching chains of amino acids, they do not remain straight.Different side-groups cause the chain to bend and coil.

In fact, the three dimensional stucture of a protein is what gives it a specific function.

Amino Acids Changing the three-dimensional shape of a protein is called denaturing, and changes the function of a protein, often permanently.

Proteins can be denatured many ways: heat alcohol acids or alkalis digestive enzymes (“proteases”) etc.

Protein DigestionProtein digestion begins in the stomach.

Hydochloric acid denatures the protein,unfolding it so proteases can reach the amino acids to break the peptide bonds between them.

Along with the hydrochloric acid, one of these proteases, pepsin, begins protein digestion in the stomach.

Protein Digestion

It is then continued in the small intestine by proteases secreted by the pancreas and cells of the intestine.

Protein Digestion

Protein Digestion & Amino Acids

Individual amino acids are absorbed into the blood vessels in the mucosa of the small intestine.

They are carried first to the liver, which can remove excess amino acids, after which they enter the general circulation and become available to cells all around the body.

Amino Acids All cells of the body have the ability to take those amino acids out of the blood and use them to build the proteins they need.

Amino Acids All cells of the body also have the ability to break down proteins to release the individual amino acids back into the blood.

Amino Acids Thus, there is a pool of amino acids available for building all of the proteins the body needs.

Amino Acids Thus, there is a pool of amino acids available for building all of the proteins the body needs.

Some amino acids can be used to make other molecules such as hormones, nucleotides, or neurotransmitters.

Cells of the liver can break amino acids apart, separating the amino groups (nitrogen atom and two hydrogen atoms) from the rest of the amino acid molecules, called their “carbon skeleton”, which can be used to form lipids or carbohydrates which can be used to produce or store energy.

Amino Acids

The amino groups are hooked together to form urea

Amino Acids

That urea can then safely travel to the kidneys, where it is removed and excreted in the urine.

Your body needs to take in new amino acids from your diet, but this must be balanced by the excretion of nitrogen in the form of urea.

Amino Acids

Your body needs to take in new amino acids from your diet, but this must be balanced by the excretion of nitrogen in the form of urea.

If your liver can’t form enough urea, the nitrogen produced by the breakdown of amino acids can be toxic to the body.

This is called “nitrogen balance”: Your intake of nitrogen (in the form of proteins) must equalYour output of nitrogen (in the form of urea in the urine)

Amino Acids

Nitrogen equilibriumNitrogen intake = nitrogen outputHealthy adults

Positive nitrogen balanceNitrogen intake > nitrogen outputNecessary during growth; recovery from illness

Negative nitrogen balanceNitrogen intake < nitrogen outputResults from injury and illness

Nitrogen Balance

Proteins in the DietRecommended protein intake:

Adult RDA = 0.8 gram per kilogram body weightMore in children, pregnancy, recovery from injury, etc



Thus: A 50 kg adult (110 pounds) should eat 50 x .8 = 40 grams of protein per day




A 70 kg adult (154 pounds) should eat 70 x .8 = 56 grams of protein per day






In the United States: protein intake > protein needs

Excess protein can strain kidney function

There are links between high-protein diets and Osteoporosis Obesity Heart disease Some forms of cancer

Proteins in the Diet

At the other extreme, it is possible to have too little protein in your diet.

This can occur in diets which are not properly designed, but it is most often associated with extreme poverty, and occurs together with insufficient intake of fuel molecules such as carbohydrates or lipids.

Kwashiorkor and Marsmas


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Proteins in the DietNot all dietary proteins are equally useful for the body.


TyrosineSerineProlineValineGlycineTryptophanGlutamineThreonineGlutamic acidPhenylalanin

e

CysteineMethionineAspartic AcidLysineAsparagineLeucineArginineIsoleucineAlanineHistidineNonessential AA

Essential AA Not all dietary proteins are equally useful for the body.

Of the 20 amino acids which are used to make proteins in the human body, only 11 of them can be synthesized. The remaining 9 must be obtained through the diet.

Proteins in the DietProtein quality:

Complete proteins supply all of the essential amino acids(e.g. animal proteins; soy proteins)



Incomplete proteins are low in one or more essential amino acid( e.g. most plant proteins)



Incomplete proteins are low in one or more essential amino acid( e.g. most plant proteins)

This can create problems for people on vegetarian or vegan diets, or for people on special diets low in meats, milk, eggs, etc.

Protein quality:You can ensure that you get all essential amino acids by combinig two or more incomplete proteins which are missing different ones.

These are called complementary proteins


A healthy diet should include a variety of proteins from meats, eggs, dairy, soy products, legumes and other vegetables.

A vegetarian diet must include complementary proteins such as those in - rice and beans

- whole grains and vegetables - corn and beans

to ensure that all essential amino acids are available

Total protein intake should be moderated


PROTEINS

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