All proteins contain nitrogen in their basic structure in addition to the carbon and water(= hydrogen and oxygen) which are typically found in polysaccharides. Proteins arepolymer forms of the nitrogen containing amino acids. We will characterize their specificarea of activity by looking at their properties, and will compare this to similar processes innature to give us a view of their role in nature.
Protein metabolism and nitrogen
The nitrogen balance
The catabolism of proteins in organisms is not an expedient way to provide energy to theorganism since the catabolism of larger amounts of protein would result in a structuralbreakdown of chiefly muscle proteins. This becomes visible in the nitrogen balance oforganisms. Under physiological conditions the amount of nitrogen excreted must equalthe amount taken up in nutrition.
Nitrogen metabolism in nature
The nitrogen that is in amino acids and proteins ultimately comes from the air, whichcontains 80% N2. Yet it cannot be taken up directly from the air by most living organisms.It first enters the soil through nitrogen fixation by special bacteria that form the rootnodules of legumes. Through this process N2 is first converted to ammonia andsubsequently incorporated in -ketoglutarate, a citric acid cycle intermediate, to form theamino acid glutamate. The amine group of glutamate is transferred in transamination
Fig. 4.1 The cycle of nitrogen metabolism in nature
processes to further citric acidcycle intermediates and othermetabolites of carbohydratesand lipids (acetyl-CoA andacetoacetyl-CoA). Complicatedconversion processes canultimately lead to the formationof all 20 amino acids.Plants take up the organicnitrogen-containing compoundsof bacteria in the soil into theirorganism.Higher organisms get their nitrogen from the plants through their food. Their wasteproducts contribute nitrogen to the soil again.Denitrification reactions in bacteria in the soil render the nitrogen back into theatmosphere.In humans, 10 of the 20 amino acids cannot be synthesized in sufficient quantity,especially in growing children, and must be taken up in the diet to prevent structuralbreakdown.
The urea cycle in organisms
processes to further citric acidcycle intermediates and othermetabolites of carbohydratesand lipids (acetyl-CoA andacetoacetyl-CoA). Complicatedconversion processes canultimately lead to the formationof all 20 amino acids.Plants take up the organicnitrogen-containing compoundsof bacteria in the soil into theirorganism.Higher organisms get their nitrogen from the plants through their food. Their wasteproducts contribute nitrogen to the soil again.Denitrification reactions in bacteria in the soil render the nitrogen back into theatmosphere.In humans, 10 of the 20 amino acids cannot be synthesized in sufficient quantity,especially in growing children, and must be taken up in the diet to prevent structuralbreakdown.
Fig.4.2 The urea cycle, the nitrogen cycle in organisms
with the citric acid cycle, via oxaloacetateand fumarate.The secretion of urea requires the presenceof water, therefore many mammals excrete itin their urine as a waste product. Sufficientquantities of water are needed to be able toexcrete it properly.Nitrogen metabolism is a controlledmechanism with limits on the supply andthe excretion sides. It is controlled by negative feedback systems. When the amount of endproduct reaches a certain level it inhibits its own synthesis. Feedback systems function ascontrol cycles. They are characteristic for nitrogen metabolism because of the latter’sinherent tight supply and demand limits.
Protein structure as the basis for protein function
The specificity of protein structure
Protein primary structure consists ofvarying combinations of 20 differentamino acids in long polypeptide chains(section 2.1.2.). Protein conformation ismore differentiated and more complicatedthan carbohydrate structure. Proteins havethree possible levels of conformationbeyond their primary structure of aminoacid chains (polypeptide chains).
Secondary structure involves the formation of -helical forms and -pleated sheets (seealso section 2.1.2.). Helices and planar forms in separate compounds also exist incarbohydrates. In protein they exist in the same compound. Tertiary structure is the overallfolding of the protein on itself, which results in the typical three-dimensional arrangementthat allows its function. Quaternary structure exists when several polypeptide chains(subunits) together form an interacting molecule such as hemoglobin.The variety of amino acids creates the potential for differentiated polypeptide chains,which allows for many different proteins. The proteins in an organism are very varied andthey can perform a large number of functions. Proteins, in contradistinction tocarbohydrates, are also highly specific to the function they fulfill. The alteration of one ormore amino acids in the polypeptide chain can give them a different function (as forinstance in myoglobin, in relation to the hemoglobin and chains) or render themdysfunctional (as in hemoglobin S in sickle-cell anemia).Protein conformation can be fibrous or globular. Membrane proteins are situated in themembranes of cells. They can have different forms, varying with their function.Glycoproteins are proteins with carbohydrate residues.
Fibrous proteins
The overall shape of fibrous proteins is along rod. Fibrous proteins are non-solublein water.
Fig. 4.4 Collagen (from Campbell,1999)
triple helix. Hydrogen bonds hold the three strands together. Collagen is locatedextracellularly in bone and connective tissue. The polypeptide chains consist mainly of arelatively simple triad of three amino acids: glycine, proline and hydroxyproline. Every thirdamino acid is glycine, the second is proline or hydroxyproline, and the first amino acid ismostly also proline or hydroxyproline or can be another amino acid. Collagen has animportant structural function. It is the main fiber in connective tissue: the tissue betweenorgans and other tissues in which cells and organ parts are embedded.• Another example of fibrous conformation in proteins is keratin in wool and hair. It ismostly -helical. The fibers of fibroin, the main protein in silk, has largely -sheets.• The muscle proteins, actin and myosin, are also mainly fibrous proteins. They are theprincipal constituents of muscle fibers, but do also occur in other kinds of cells. Theseproteins are always located intracellularly. Their structure is relatively simple, albeit morecomplex than that of collagen. Myosin has a globular head that is instrumental in musclecontraction. It provides the power stroke for the contraction by interacting with actin. Inaction, the interlocking of actin and myosin components intensifies and the musclecontracts, thus moving the body in part or as a whole and altering the relation of theorgan or organism to its surroundings. These proteins are structural proteins in the restingstate as well as functional proteins when in action. Muscle fibers also affect movementwithin cells.Fibrous proteins provide structural elements to the animal and human organism.
Globular proteins
Globular proteins are compact functional proteins. The overall shape of a globular proteinis spherical, as the name indicates. Their tertiary and quaternary structures are complex.Globular proteins are water-soluble.
Fig. 4.5 Myoglobin, a globular protein (from Campbell, 1999)
Membrane proteins
Membrane proteins are an important component of membranes. 20 - 80 % of themembrane weight of animal and human cells consists of protein. These proteins have astructural function, but their major role is functional. They form channels through themembrane that allow the passage of specific compounds under certain conditions, they effect active transport of certain compounds across the membrane, and they function asreceptors on the membrane’s inner and outer surface for compounds such asneurotransmitters. They can also be enzymes themselves
Glycoproteins
Glycoproteins play a role as antibodies in immune recognition and as antigenicdeterminants in human cell membranes. The use of glycoproteins for the typing andmatching of blood groups and grafts is exemplary of the specificity of proteins inorganisms. Even though close matches can be found, for instance in twins, the perfectmatch is only found in proteins within an individual organism. Proteins emphasize thesingularity of organisms and play an important role in recognizing the distinction betweenself and non-self, which is the function of the immune system.
Amino acids
Protein breakdown in the metabolism does not specificallyyield energy-carrying substances, even though aminoacids yield energy when they are broken down. Aminoacids are important as metabolites that can be used bythe organism in anabolic processes to build up its ownproteins. The energy for this process comes from thecatabolism of carbohydrates.
Amino acid activity
Many amino acids are themselves biologically active in the organism (such as glutamateor glycine) or with small structural changes (such as the monoamine serotonin, which isformed from tryptophan, and the catecholamines, which are derived from tyrosine) or a
peptides (small amino acid chains). They are active as neurotransmitters in nervous tissue,where they connect nerve cells functionally by transmitting the electrical impulsechemically from the axon to the receptors on the axon or cell body of the next cell. Somehave an important function as bile salt (glycine in glycocholic acid), in methylationreactions (methionine), or in inflammation (histamine from histidine). Thyroxine is aderivative of tyrosine, and functions as a hormonal substance important for the rate ofmetabolism. Oxytocin, vasopressin and insulin are peptide hormones, which effectcontraction of smooth muscles in the uterus, contraction of smooth muscles in the bloodvessels, and carbohydrate metabolism respectively.Many of these compounds are also known to influence consciousness. Serotonin andhistamine released from a bee sting provoke a strong sensation of pain as well as localinflammation. In schizophrenia we find increased levels of serotonin and catecholamines,including dopamine. In endogenous depressions, a lack of serotonin and catecholaminemetabolism is found. The stimulating effect of coffee on consciousness is due to itsstimulating influence on monoamine metabolism. Stimulants of consciousness such ascocaine and LSD mimic the central nervous system action of catecholamines andserotonin respectively. The rate of monoamine metabolism varies with the rhythm ofsleeping and waking in the healthy organism. Serotonin and dopamine are secreted in thearea of the brainstem that is active during waking (see Elsas, 1994).
The amino acid leucine is only ketogenic, meaning it can only be broken down to acetyl-CoA or acetoacetyl-CoA and its breakdown may lead to the formation of ketone bodies orfatty acids. It can be used in the citric acid cycle but cannot be converted to glucose. Isoleucine,lysine, phenolalanine, tryptophan and tyrosine are both ketogenic andglucogenic.Amino acids also contribute to the synthesis of nucleic acids and the pyrrole ring ofhemoglobin.
Summary and conclusion
Membrane proteins play a role in the transmission of signals and compounds across
membranes. Glycoproteins play a role in immune recognition. The function of the immune
system is to recognize the distinction between self and non-self. As such glycoproteins
emphasize the singularity of organisms and promote individualization.
Many protein monomers - the amino acids, and their derivatives - are biologically active in
the organism. They facilitate the conduction in the nervous system, metabolic processes,
and they affect smooth muscle contraction.
Characterization
The typical difference between plants and animals is that animals have the ability to
move themselves with the help of muscle activity and conduction in the nervous system.
This makes animals more individualized than plants. They form a diverse array of inner
organs as a result, which have specialized functions in their organism. Plants form only
external organs, such as flowers. Individualization also necessitates new ways to relate to
the environment to avoid isolation. Muscles, nerves, and senses serve animals in
sustaining a relation to their environment. Animals characteristically learn from
behavioral feedback.
The characteristics we have indicated for animals are related to characteristics of proteinsand amino acids as shown in table 4.1. The diversity in structures and functions of aminoacids and proteins can be summarized as one comprehensive idea: amino acids andproteins serve to enhance the connections within organisms and of organisms with theirenvironment.Connective tissue is the structural prototype and enzymes are the functional prototype ofthis.Conclusion: Amino acids and proteins have functions in organisms that are related to thecharacteristic functions in animals. Amino acids and proteins are ”animal-like.” Theircomprehensive characteristic is that they enhance connections.
Protein metabolism and nitrogen
The nitrogen balance
The catabolism of proteins in organisms is not an expedient way to provide energy to theorganism since the catabolism of larger amounts of protein would result in a structuralbreakdown of chiefly muscle proteins. This becomes visible in the nitrogen balance oforganisms. Under physiological conditions the amount of nitrogen excreted must equalthe amount taken up in nutrition.
Nitrogen metabolism in nature
The nitrogen that is in amino acids and proteins ultimately comes from the air, whichcontains 80% N2. Yet it cannot be taken up directly from the air by most living organisms.It first enters the soil through nitrogen fixation by special bacteria that form the rootnodules of legumes. Through this process N2 is first converted to ammonia andsubsequently incorporated in -ketoglutarate, a citric acid cycle intermediate, to form theamino acid glutamate. The amine group of glutamate is transferred in transamination
Fig. 4.1 The cycle of nitrogen metabolism in natureprocesses to further citric acidcycle intermediates and othermetabolites of carbohydratesand lipids (acetyl-CoA andacetoacetyl-CoA). Complicatedconversion processes canultimately lead to the formationof all 20 amino acids.Plants take up the organicnitrogen-containing compoundsof bacteria in the soil into theirorganism.Higher organisms get their nitrogen from the plants through their food. Their wasteproducts contribute nitrogen to the soil again.Denitrification reactions in bacteria in the soil render the nitrogen back into theatmosphere.In humans, 10 of the 20 amino acids cannot be synthesized in sufficient quantity,especially in growing children, and must be taken up in the diet to prevent structuralbreakdown.
The urea cycle in organisms
processes to further citric acidcycle intermediates and othermetabolites of carbohydratesand lipids (acetyl-CoA andacetoacetyl-CoA). Complicatedconversion processes canultimately lead to the formationof all 20 amino acids.Plants take up the organicnitrogen-containing compoundsof bacteria in the soil into theirorganism.Higher organisms get their nitrogen from the plants through their food. Their wasteproducts contribute nitrogen to the soil again.Denitrification reactions in bacteria in the soil render the nitrogen back into theatmosphere.In humans, 10 of the 20 amino acids cannot be synthesized in sufficient quantity,especially in growing children, and must be taken up in the diet to prevent structuralbreakdown.
Fig.4.2 The urea cycle, the nitrogen cycle in organisms
with the citric acid cycle, via oxaloacetateand fumarate.The secretion of urea requires the presenceof water, therefore many mammals excrete itin their urine as a waste product. Sufficientquantities of water are needed to be able toexcrete it properly.Nitrogen metabolism is a controlledmechanism with limits on the supply andthe excretion sides. It is controlled by negative feedback systems. When the amount of endproduct reaches a certain level it inhibits its own synthesis. Feedback systems function ascontrol cycles. They are characteristic for nitrogen metabolism because of the latter’sinherent tight supply and demand limits.
Protein structure as the basis for protein function
The specificity of protein structure
Protein primary structure consists ofvarying combinations of 20 differentamino acids in long polypeptide chains(section 2.1.2.). Protein conformation ismore differentiated and more complicatedthan carbohydrate structure. Proteins havethree possible levels of conformationbeyond their primary structure of aminoacid chains (polypeptide chains).
Secondary structure involves the formation of -helical forms and -pleated sheets (seealso section 2.1.2.). Helices and planar forms in separate compounds also exist incarbohydrates. In protein they exist in the same compound. Tertiary structure is the overallfolding of the protein on itself, which results in the typical three-dimensional arrangementthat allows its function. Quaternary structure exists when several polypeptide chains(subunits) together form an interacting molecule such as hemoglobin.The variety of amino acids creates the potential for differentiated polypeptide chains,which allows for many different proteins. The proteins in an organism are very varied andthey can perform a large number of functions. Proteins, in contradistinction tocarbohydrates, are also highly specific to the function they fulfill. The alteration of one ormore amino acids in the polypeptide chain can give them a different function (as forinstance in myoglobin, in relation to the hemoglobin and chains) or render themdysfunctional (as in hemoglobin S in sickle-cell anemia).Protein conformation can be fibrous or globular. Membrane proteins are situated in themembranes of cells. They can have different forms, varying with their function.Glycoproteins are proteins with carbohydrate residues.
Fibrous proteins
The overall shape of fibrous proteins is along rod. Fibrous proteins are non-solublein water.
Examples of typical fibrous proteins:
• Collagen, is the prototype of fibrousproteins and the most frequently occurringprotein in vertebrates. Collagen consists ofthree helical polypeptide chains wrappedaround each other to form a superimposedFig. 4.4 Collagen (from Campbell,1999)triple helix. Hydrogen bonds hold the three strands together. Collagen is locatedextracellularly in bone and connective tissue. The polypeptide chains consist mainly of arelatively simple triad of three amino acids: glycine, proline and hydroxyproline. Every thirdamino acid is glycine, the second is proline or hydroxyproline, and the first amino acid ismostly also proline or hydroxyproline or can be another amino acid. Collagen has animportant structural function. It is the main fiber in connective tissue: the tissue betweenorgans and other tissues in which cells and organ parts are embedded.• Another example of fibrous conformation in proteins is keratin in wool and hair. It ismostly -helical. The fibers of fibroin, the main protein in silk, has largely -sheets.• The muscle proteins, actin and myosin, are also mainly fibrous proteins. They are theprincipal constituents of muscle fibers, but do also occur in other kinds of cells. Theseproteins are always located intracellularly. Their structure is relatively simple, albeit morecomplex than that of collagen. Myosin has a globular head that is instrumental in musclecontraction. It provides the power stroke for the contraction by interacting with actin. Inaction, the interlocking of actin and myosin components intensifies and the musclecontracts, thus moving the body in part or as a whole and altering the relation of theorgan or organism to its surroundings. These proteins are structural proteins in the restingstate as well as functional proteins when in action. Muscle fibers also affect movementwithin cells.Fibrous proteins provide structural elements to the animal and human organism.
Globular proteins
Globular proteins are compact functional proteins. The overall shape of a globular proteinis spherical, as the name indicates. Their tertiary and quaternary structures are complex.Globular proteins are water-soluble.
Examples of globular proteins
• Most enzymes are globular proteins and are principally only functional. They catalyzereaction processes between molecules. In the lock and key model of enzyme functionthey bind substrate and catalyze a specific reaction, then let go of the reactionproducts. Their configuration changes reversibly with their activity. Otherwise they areunchanged by the process they catalyze.Enzymes catalyze numerous metabolicreactions throughout the organism.Reactions catalyzed by enzymes willproceed up to 1014 times faster than noncatalyzedreactions. Each reaction processhas a specific enzyme that catalyzes it. Thisdiversity of form in proteins becomespossible through the configuration of theirprimary structure with 20 different aminoacids, which subsequently enables theformation of their specific secondary,tertiary, and possibly quaternary structures.The amino acid sequence has to be exactfor the protein to be biologically active.• Other examples of globular proteinsinclude myoglobin and hemoglobin.Fig. 4.5 Myoglobin, a globular protein (from Campbell, 1999)Membrane proteins
Membrane proteins are an important component of membranes. 20 - 80 % of themembrane weight of animal and human cells consists of protein. These proteins have astructural function, but their major role is functional. They form channels through themembrane that allow the passage of specific compounds under certain conditions, they effect active transport of certain compounds across the membrane, and they function asreceptors on the membrane’s inner and outer surface for compounds such asneurotransmitters. They can also be enzymes themselves
Glycoproteins
Glycoproteins play a role as antibodies in immune recognition and as antigenicdeterminants in human cell membranes. The use of glycoproteins for the typing andmatching of blood groups and grafts is exemplary of the specificity of proteins inorganisms. Even though close matches can be found, for instance in twins, the perfectmatch is only found in proteins within an individual organism. Proteins emphasize thesingularity of organisms and play an important role in recognizing the distinction betweenself and non-self, which is the function of the immune system.
Amino acids
Protein breakdown in the metabolism does not specificallyyield energy-carrying substances, even though aminoacids yield energy when they are broken down. Aminoacids are important as metabolites that can be used bythe organism in anabolic processes to build up its ownproteins. The energy for this process comes from thecatabolism of carbohydrates.
Amino acid activity
Many amino acids are themselves biologically active in the organism (such as glutamateor glycine) or with small structural changes (such as the monoamine serotonin, which isformed from tryptophan, and the catecholamines, which are derived from tyrosine) or a
peptides (small amino acid chains). They are active as neurotransmitters in nervous tissue,where they connect nerve cells functionally by transmitting the electrical impulsechemically from the axon to the receptors on the axon or cell body of the next cell. Somehave an important function as bile salt (glycine in glycocholic acid), in methylationreactions (methionine), or in inflammation (histamine from histidine). Thyroxine is aderivative of tyrosine, and functions as a hormonal substance important for the rate ofmetabolism. Oxytocin, vasopressin and insulin are peptide hormones, which effectcontraction of smooth muscles in the uterus, contraction of smooth muscles in the bloodvessels, and carbohydrate metabolism respectively.Many of these compounds are also known to influence consciousness. Serotonin andhistamine released from a bee sting provoke a strong sensation of pain as well as localinflammation. In schizophrenia we find increased levels of serotonin and catecholamines,including dopamine. In endogenous depressions, a lack of serotonin and catecholaminemetabolism is found. The stimulating effect of coffee on consciousness is due to itsstimulating influence on monoamine metabolism. Stimulants of consciousness such ascocaine and LSD mimic the central nervous system action of catecholamines andserotonin respectively. The rate of monoamine metabolism varies with the rhythm ofsleeping and waking in the healthy organism. Serotonin and dopamine are secreted in thearea of the brainstem that is active during waking (see Elsas, 1994).
Amino acid metabolism
Most amino acids are glucogenic. Their degradation after de-amination yields pyruvate oroxaloacetate. This gives the possibility for them to be converted to glucose throughgluconeogenesis and enter carbohydrate metabolism, or they may be used up in the citricacid cycle. Amino acids can therefore provide energy for the organism like thecarbohydrates, but they do so less efficiently. When body energy requirements needprotein degradation in larger quantity, as in hunger states or malnutrition, this happens atthe expense of other tasks of proteins and amino acids in the organism, both structural aswell as functional (section 4.1.1.).The amino acid leucine is only ketogenic, meaning it can only be broken down to acetyl-CoA or acetoacetyl-CoA and its breakdown may lead to the formation of ketone bodies orfatty acids. It can be used in the citric acid cycle but cannot be converted to glucose. Isoleucine,lysine, phenolalanine, tryptophan and tyrosine are both ketogenic andglucogenic.Amino acids also contribute to the synthesis of nucleic acids and the pyrrole ring ofhemoglobin.
Summary and conclusion
Nitrogen
Nitrogen is fixed from the atmosphere by soil bacteria and taken up by leguminous plants.When animals and humans eat the plants, the nitrogen-containing substances enter theirorganism. The nitrogen becomes part of the urea cycle in organisms, which has links toboth anabolic and catabolic processes as well as to the citric acid cycle. Once the nitrogenis excreted as urea it enters the larger cycle in nature again and can be either taken up byplants again or denitrified by soil bacteria to become part of the nitrogen in the air in aneven larger cycle. Nitrogen metabolism in organisms is controlled by extensive negativefeedback mechanisms to support the tight nitrogen balance. Nitrogen intake has to matchexcretion to prevent structural breakdown.The breakdown of protein in the digestive tract yields amino acids, which are important asmetabolites for the organism to build up its own proteins in anabolic processes. Theenergy for these processes comes from the catabolism of carbohydrates. When proteinsare broken down for energy in the organism as happens in hunger states, this meansstructural as well as functional breakdown of the organism. Proteins have a structuralfunction in animals and humans, and they cannot be used as the regular energy source.We found that carbohydrates take on the structural function in plants, and also have amajor role in the structural components of bacteria and invertebrates. In the latter two the structural element contains amine or amino acid derivatives. They may be seen astransitions to the structural proteins. This makes proteins characteristic in animalstructure.Conformation of proteins
The conformation of proteins is fibrous or globular. Proteins usually have 20 differentamino acids in varying sequences in their polypeptide backbone. The polypeptide chainmay consist of several hundred amino acids. But a change as small as one amino acid mayrender the protein dysfunctional. Protein conformation is as diverse as it is specific. Theprimary structure of proteins, the amino acid sequence, determines their ability to formthe secondary and tertiary (and sometimes quaternary) structure that is required to bebiologically active in organisms. Amino acids and proteins have a large array of functionsand structures, but these are as specific as they are diverse. Carbohydrates have thespecific function of energy storage and supply. In section 2.1.2. we found that proteinshave the whole array of bonds, where specialized covalent bonds are typical forcarbohydrates.The most abundant protein in higher organisms is collagen, the fibrous protein inconnective tissue and bone. It has mostly just three different amino acids in its triple helixconformation. As the main fiber of connective tissue it provides the structure in whichorgans and cells are embedded. Another form of fibrous protein occurs in muscle. Musclefibers at rest contribute to the organism’s structure. In action, they enable movement ofthe organism and its parts. Fibrous proteins are insoluble in water and are mainlystructural proteins.Enzymes are the examples of globular proteins. Most globular proteins are enzymes. Theyaffect the relations between reaction substrates by catalyzing reactions. Theirconfiguration changes only temporarily and reversibly with their activity. Enzymes catalyzenumerous metabolic reactions throughout the organism, and each reaction process has aspecific enzyme that catalyzes it. The diversity and specificity of proteins becomesexemplary in enzymes. Globular proteins are water-soluble and principally only functional.Membrane proteins play a role in the transmission of signals and compounds across
membranes. Glycoproteins play a role in immune recognition. The function of the immune
system is to recognize the distinction between self and non-self. As such glycoproteins
emphasize the singularity of organisms and promote individualization.
Many protein monomers - the amino acids, and their derivatives - are biologically active in
the organism. They facilitate the conduction in the nervous system, metabolic processes,
and they affect smooth muscle contraction.
Characterization
The typical difference between plants and animals is that animals have the ability to
move themselves with the help of muscle activity and conduction in the nervous system.
This makes animals more individualized than plants. They form a diverse array of inner
organs as a result, which have specialized functions in their organism. Plants form only
external organs, such as flowers. Individualization also necessitates new ways to relate to
the environment to avoid isolation. Muscles, nerves, and senses serve animals in
sustaining a relation to their environment. Animals characteristically learn from
behavioral feedback.
The characteristics we have indicated for animals are related to characteristics of proteinsand amino acids as shown in table 4.1. The diversity in structures and functions of aminoacids and proteins can be summarized as one comprehensive idea: amino acids andproteins serve to enhance the connections within organisms and of organisms with theirenvironment.Connective tissue is the structural prototype and enzymes are the functional prototype ofthis.Conclusion: Amino acids and proteins have functions in organisms that are related to thecharacteristic functions in animals. Amino acids and proteins are ”animal-like.” Theircomprehensive characteristic is that they enhance connections.
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