The living matter is composed of mainly carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. These elements together constitute about 90% of the dry weight of the human body
Other functionally important elements found in cells
Ca
K
Na
Cl
Mg
Fe
Cu
Co
I
Zn
F
Mo
Se
Chemical composition of the human body
Water is the solvent of life and contributes to more than 60% of the weight, followed by protein (mostly in muscle) and lipid (mostly in adipose tissue), carbohydrate content is rather low in the form of glycogen
Water
The most abundant chemical compound in living human cells, accounting for 65 percent to 90 percent of each cell, also present between cells e.g. in blood and cerebrospinal fluid
Fat
The percentage varies from person to person, but even an obese person has more water than fat
Protein
In a lean male, the percentages of protein and water are comparable, about 16 percent by mass, found in muscles, hair, fingernails, and skin
Minerals
Account for about 6 percent of the body, include salts and metals like sodium, chlorine, calcium, potassium, and iron
Carbohydrates
Although used as an energy source, only account for about 1% of body mass
Carbon is the most predominant and versatile element of life, possessing the ability to form stable covalent bonds and C-C chains of unlimited length, about 90% of compounds found in living systems contain carbon
Most elements are found within compounds, water and minerals are inorganic compounds, organic compounds include fat, protein, carbohydrates, and nucleic acids
Proteins
The most versatile macromolecules in living systems, serving crucial functions in essentially all biological processes including as catalysts, transport, structural support, immune protection, movement, nerve impulses, growth and differentiation
Origin of the word 'protein'
Derived from the Greek word proteios, meaning holding the first place, suggested by Berzelius and Mulder to refer to the high molecular weight nitrogen-rich and most abundant substances present in animals and plants
Proteins may also contain other elements such as P, Fe, Cu, I, Mg, Mn, Zn
Proteins
Polymers of amino acids, formed by the incorporation of 20 standard amino acids during synthesis, some of which undergo post-translational modifications
Central dogma of molecular biology
The flow of genetic information from DNA to RNA to proteins, first proposed by Francis Crick
Amino acids
Organic compounds containing amino and carboxyl functional groups, exist in ionized form in biological systems
Optical isomerism of amino acids
Amino acids (except glycine) have four distinct groups attached to the alpha carbon, resulting in left-handed (L) and right-handed (D) optical isomers, proteins are composed of L-alpha amino acids
The preference for L-amino acids in proteins is likely due to their slightly higher solubility compared to racemic mixtures
Classification of amino acids based on polarity
Non-polar (hydrophobic)
Polar with no charge
Polar with positive charge
Polar with negative charge
Nutritional classification of amino acids
Essential (indispensable)
Non-essential (dispensable)
Semi-essential
Physical properties of amino acids
Soluble in water, insoluble in organic solvents, high melting points, varying tastes (sweet, tasteless, bitter)
Aspartame
An artificial sweetener 200 times sweeter than sucrose, a methyl ester of the aspartic acid/phenylalanine dipeptide, most stable at pH 4.3, deemed safe for human consumption by regulatory agencies
Chemical properties of amino acids
Undergo decarboxylation to form amines, dicarboxylic amino acids react with ammonia to form amides, act as ampholytes (can donate or accept protons)
Aspartame
Most stable at pH 4.3, where its half-life is nearly 300 days
Aspartame has been deemed safe for human consumption by over 100 regulatory agencies in their respective countries, including the FDA
Amino acids
General reactions are mostly due to the presence of two functional groups: carboxyl (COOH) group and amino (NH2) group
Decarboxylation
Amino acids undergo decarboxylation to produce corresponding amines
Reaction with ammonia
The carboxyl group of dicarboxylic amino acids reacts with NH3 to form amide
Amino acids as ampholytes
Amino acids contain both acidic (COOH) and basic (NH2) groups, they can donate a proton or accept a proton
Zwitterion or dipolar ion
A hybrid molecule containing positive and negative ionic groups
Amino acids rarely exist in a neutral form with free carboxylic (COOH) and free amino (NH2) groups
Isoelectric point (pI)
The pH at which a molecule exists as a zwitterion or dipolar ion and carries no net charge
Isoelectric point (pI)
The negatively and positively charged molecular species are present in equal concentrations
The molecule is electrically neutral
Neutral side chain amino acids
Characterized by two pKas: pKa1 for the carboxylic acid and pKa2 for the amine
The isoelectric point is the average of these two pKas
Acidic side chain amino acids
The isoelectric point is at a lower pH because the acidic side chain introduces an "extra" negative charge
Basic side chain amino acids
The isoelectric point is at a higher pH because the basic side chain introduces an "extra" positive charge
Electrophoresis
Used to show the distribution of charged species in a sample by observing the movement of solute molecules in an electric field
Peptide bond
Formed when the amino group of an amino acid combines with the carboxyl group of another amino acid
Peptides containing more than 10 amino acids are referred to as polypeptides