All Chapters
Ch.1 - Introduction to Biology
Ch.2 - Chemistry
Ch.3 - Water
Ch.4 - Carbon
Ch.5 - Biological Molecules
Ch.6 - Cells
Ch.7 - The Membrane
Ch.8 - Energy and Metabolism
Ch.9 - Respiration
Ch.10 - Photosynthesis
Ch.11 - Cell Signaling
Ch.12 - Cell Division
Ch.13 - Meiosis
Ch.14 - Mendelian Genetics
Ch.15 - Chromosomal Theory of Inheritance
Ch.16 - DNA Synthesis
Ch.17 - Gene Expression
Ch.18 - Regulation of Expression
Ch.19 - Viruses
Ch.20 - Biotechnology
Ch.21 - Genomics
Ch.22 - Development
Ch.23 - Evolution by Natural Selection
Ch.24 - Evolution of Populations
Ch.25 - Speciation
Ch.26 - History of Life on Earth
Ch.27 - Phylogeny
Ch.28 - Prokaryotes
Ch.29 - Protists
Ch.30 - Plants
Ch.31 - Fungi
Ch.32 - Overview of Animals
Ch.33 - Invertebrates
Ch.34 - Vertebrates
Ch.35 - Plant Anatomy
Ch.36 - Vascular Plant Transport
Ch.37 - Soil
Ch.38 - Plant Reproduction
Ch.39 - Plant Sensation and Response
Ch.40 - Animal Form and Function
Ch.41 - Digestive System
Ch.42 - Circulatory System
Ch.43 - Immune System
Ch.44 - Osmoregulation and Excretion
Ch.45 - Endocrine System
Ch.46 - Animal Reproduction
Ch.47 - Nervous System
Ch.48 - Sensory Systems
Ch.49 - Muscle Systems
Ch.50 - Ecology
Ch.51 - Animal Behavior
Ch.52 - Population Ecology
Ch.53 - Community Ecology
Ch.54 - Ecosystems
Ch.55 - Conservation Biology
Sections
Properties of Water
Acids and Bases
Additional Practice
Calculating pH

Solution: In our discussion of the acid-base properties of individual amino acids, we indicated that most amino acids will be electrically neutral over a very broad range of pH values, so that the pI of an individual amino acid is a nearly meaningless quantity. By contrast, a full-length protein will be electrically neutral over a relatively narrow pH range — typically less than one pH unit. Thus the pI of a protein is a well-defined quantity. Why is this?

Problem

In our discussion of the acid-base properties of individual amino acids, we indicated that most amino acids will be electrically neutral over a very broad range of pH values, so that the pI of an individual amino acid is a nearly meaningless quantity. By contrast, a full-length protein will be electrically neutral over a relatively narrow pH range — typically less than one pH unit. Thus the pI of a protein is a well-defined quantity. Why is this?

Solution

Remember that the isoelectric point (pI) of a/an amino acid/polypeptide/protein is the pH values at which the net charge of the molecule is zero. To clarify a common misconception, pI is not just the average of the pKa values of the molecule's side chains, along with the pKa of the amino group in the N terminal and the carboxyl group in the C terminal.

In comparison with amino acids, proteins are usually made up of hundreds to several thousands of amino acids. And since its building blocks can only come from 20 different amino acids, repetition of amino acids in a sequence cannot be avoided.

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